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How Massive Is The Oort Cloud _ Science For Sleep · Boring Science

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  Speaker 1: The solar system ends somewhere. That much feels obvious. You
• 0:05
  Speaker 1: learn the planets in order. You picture the sun at
• 0:08
  Speaker 1: the center, and somewhere at the outer edge of all
• 0:11
  Speaker 1: of it, there is a boundary, a line, a place
• 0:16
  Speaker 1: where our solar system stops and the rest of the
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  Speaker 1: universe begins. Most people put that line somewhere around Pluto.
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  Speaker 1: Some put it at Neptune. since Pluto lost its planet status,
• 0:30
  Speaker 1: and Neptune is the last real world out there. Either way,
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  Speaker 1: the picture is the same. Eight planets, a few belts
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  Speaker 1: of debris, and then nothing. Just empty, cold interstellar space
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  Speaker 1: stretching away in every direction forever. That picture is wrong.
• 0:49
  Speaker 1: Not slightly wrong. Not off by a margin of error. Completely,
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  Speaker 1: fundamentally wrong. in a way that changes what the solar
• 0:59
  Speaker 1: system actually is. Here is what makes this strange. The
• 1:03
  Speaker 1: confidence is not irrational. Every diagram ever printed in a
• 1:09
  Speaker 1: textbook puts the edge near the planets. Every model hanging
• 1:14
  Speaker 1: in a classroom stops at Neptune or Pluto. The image
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  Speaker 1: of a tidy, bounded solar system has been reinforced so
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  Speaker 1: consistently and for so long that it feels like established fact.
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  Speaker 1: It is not a guess. It feels like something we measured.
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  Speaker 1: And in a narrow sense, we did measure the distances
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  Speaker 1: to the planets. Those numbers are accurate. Neptune really is
• 1:42
  Speaker 1: about 30 times farther from the Sun than Earth is.
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  Speaker 1: Pluto really does orbit out at around 40 times that distance.
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  Speaker 1: These are not fabrications. The problem is not the numbers.
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  Speaker 1: The problem is the assumption that the planets mark the
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  Speaker 1: edge of anything meaningful. They do not. The planets are
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  Speaker 1: the innermost portion of the solar system. Everything we think
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  Speaker 1: of as our cosmic neighborhood, every world ever visited by
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  Speaker 1: a spacecraft, every object ever photographed up close, occupies only
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  Speaker 1: a tiny fraction of the Sun's true gravitational reach. Beyond Neptune,
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  Speaker 1: beyond Pluto, beyond the scattered field of icy debris that
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  Speaker 1: extends past Pluto's orbit, there is more. Far more. So
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  Speaker 1: much more that describing it in the usual terms almost
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  Speaker 1: fails to communicate the reality. If you find comfort in
• 2:46
  Speaker 1: these gentle explorations of space and science, I'd be grateful
• 2:51
  Speaker 1: if you'd consider subscribing and giving this video a like.
• 2:55
  Speaker 1: I'd love to know where you're watching from and what
• 2:58
  Speaker 1: time it is for you. Feel free to drop your
• 3:01
  Speaker 1: location and time in the comments below. Picture a city.
• 3:11
  Speaker 1: You know the downtown well. You know the streets, the neighbourhoods,
• 3:16
  Speaker 1: the boundaries of the city limits. Now imagine being told
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  Speaker 1: that the city actually extends hundreds of miles beyond what
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  Speaker 1: you thought were its borders, into territory you have never visited,
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  Speaker 1: never seen on a map, and never heard named. The
• 3:34
  Speaker 1: downtown you knew is real. The neighbourhoods are real. But
• 3:39
  Speaker 1: you are looking at a tiny corner of the full picture.
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  Speaker 1: and calling it the whole thing. That is roughly the
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  Speaker 1: situation with the solar system and what lies beyond the planets.
• 3:51
  Speaker 1: The reason most people have never heard the full version
• 3:55
  Speaker 1: is partly historical and partly practical. For most of the
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  Speaker 1: history of astronomy, telescopes could not see far enough. The
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  Speaker 1: outer planets were the limit of direct observation. So the
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  Speaker 1: outer planets became the limit of the mental model. What
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  Speaker 1: could not be seen was not discussed in textbooks written
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  Speaker 1: for students. Later, when evidence emerged for something much larger,
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  Speaker 1: the textbooks did not catch up. The classroom picture stayed
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  Speaker 1: frozen while science moved on. What science moved on to
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  Speaker 1: is something enormous, a structure so large that the numbers
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  Speaker 1: describing it require a different unit of measurement than the
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  Speaker 1: one used for planetary distances. Something that surrounds the entire
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  Speaker 1: solar system in every direction. Not as a flat disk,
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  Speaker 1: but as a full sphere. Something containing potentially trillions of objects,
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  Speaker 1: preserved unchanged since the solar system formed. So distant from
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  Speaker 1: the Sun that light itself takes over a year to
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  Speaker 1: cross from the Sun to its outer boundary. Something that
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  Speaker 1: has been sitting there for billions of years, occasionally sending
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  Speaker 1: a fragment falling inward toward the sun, a fragment that,
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  Speaker 1: when it finally arrives, glows with a tail and crosses
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  Speaker 1: the sky above us, and we call it a comet,
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  Speaker 1: and we watch it, and almost none of us think
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  Speaker 1: about where it came from. That structure has a name,
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  Speaker 1: and the fact that most people have never heard it
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  Speaker 1: is one of the more remarkable gaps between what science
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  Speaker 1: knows and what the rest of us were taught. The
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  Speaker 1: solar system does not end anywhere near where you think
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  Speaker 1: it does. Most people, if you ask them where the
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  Speaker 1: solar system ends, will say something about Pluto. Some will
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  Speaker 1: say Neptune. A few will mention the asteroid belt, that
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  Speaker 1: ring of rocky debris sitting between Mars and Jupiter. as
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  Speaker 1: though it marks some kind of dividing line between the
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  Speaker 1: known and the unknown. These answers are not wrong exactly.
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  Speaker 1: They are just describing something much smaller than the actual
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  Speaker 1: solar system, the way you might describe a country by
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  Speaker 1: only naming its capital city. Start with the asteroid belt,
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  Speaker 1: because it is the first boundary people reach for. The
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  Speaker 1: asteroid belt sits roughly 2.2 to 3.2 astronomical units from
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  Speaker 1: the sun, One AU, remember, is the distance from Earth
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  Speaker 1: to the Sun, about 93 million miles. The asteroid belt
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  Speaker 1: is not a wall, it is not even particularly dense.
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  Speaker 1: The total mass of everything in it, every rock and
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  Speaker 1: chunk of metal and loose rubble, adds up to roughly
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  Speaker 1: 4% of the Moon. Spacecraft have crossed it multiple times
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  Speaker 1: without incident. It is a region, not a barrier. Nothing
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  Speaker 1: about it signals the end of anything. It is, by
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  Speaker 1: any reasonable measure, still very much inside the solar system's
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  Speaker 1: inner neighborhood. Then there is Neptune, the eighth planet, the
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  Speaker 1: last planet. At roughly 30 AU from the Sun, Neptune
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  Speaker 1: sits so far out that sunlight takes more than four
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  Speaker 1: hours to reach it. It is the boundary that makes
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  Speaker 1: intuitive sense. the last of the giant worlds, the outermost
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  Speaker 1: of the planets. For a long time, Neptune felt like
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  Speaker 1: the frontier. Beyond it was just darkness. But the planets
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  Speaker 1: were never the edge of the solar system. They are
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  Speaker 1: just the visible part, the part that caught the light
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  Speaker 1: and grew large enough to see. The solar system has
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  Speaker 1: always extended well past the last planet. We just have
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  Speaker 1: not fully worked out how far. Pluto complicated this. Discovered
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  Speaker 1: in 1930 and classified as a planet for 76 years,
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  Speaker 1: Pluto seemed to push the boundary outward to about 40
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  Speaker 1: AU at its closest approach to the Sun and nearly
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  Speaker 1: 50 AU at its farthest. For decades, Pluto was the
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  Speaker 1: edge by default, the last world, the lonely outpost. Then
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  Speaker 1: astronomers started finding other objects beyond Neptune's orbit. Hundreds of them,
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  Speaker 1: then thousands. Pluto was not the last planet. It was
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  Speaker 1: one of the first known members of an entirely different region.
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  Speaker 1: The Kuiper Belt. A disc-shaped zone of icy bodies stretching
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  Speaker 1: from roughly 30 AU out, out to about 50 AU,
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  Speaker 1: sitting flat against the plane of the solar system. populated
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  Speaker 1: with objects that never quite managed to become planets. Pluto
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  Speaker 1: is a Kuiper belt object, one of the larger ones,
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  Speaker 1: but not exceptional among its actual neighbours. The discovery of
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  Speaker 1: the Kuiper belt pushed a recognised boundary of the solar
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  Speaker 1: system outward again, but it still was not the end.
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  Speaker 1: Beyond the Kuiper belt, the Sun keeps blowing. This is
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  Speaker 1: not a metaphor. the Sun constantly releases a stream of
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  Speaker 1: charged particles, electrons, protons, the raw output of its nuclear furnace,
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  Speaker 1: in every direction at once. This is the solar wind.
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  Speaker 1: It travels outward at hundreds of miles per second, filling
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  Speaker 1: an enormous bubble of space with solar material. Inside this bubble,
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  Speaker 1: the Sun's output physically dominates. Outside it, interstellar space takes over.
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  Speaker 1: The boundary between these two regions is called the heliopause.
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  Speaker 1: It sits somewhere around 120 AU from the Sun, though
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  Speaker 1: the exact distance varies with the Sun's activity cycle and
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  Speaker 1: the pressure of the interstellar medium pushing back against it.
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  Speaker 1: The heliopause became the boundary that serious astronomers pointed to.
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  Speaker 1: It had something the others lacked, a physical mechanism, an
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  Speaker 1: actual transition from one regime to another. Inside the heliopause,
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  Speaker 1: you are inside the heliosphere, the domain of the sun's
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  Speaker 1: outward breath. Outside, you are technically in interstellar space. Headlines
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  Speaker 1: ran with it. The solar system, many people read, had
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  Speaker 1: been officially exited. But here is where that framing quietly
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  Speaker 1: falls apart. Voyager 1 crossed the heliopause. It did not
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  Speaker 1: escape the Sun's gravity. Not even close. The solar wind stops.
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  Speaker 1: Gravity does not stop. These are two entirely different things,
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  Speaker 1: and conflating them is where most popular descriptions of the
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  Speaker 1: solar system's edge go wrong. The Sun's gravitational influence, its
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  Speaker 1: actual reach, the domain where its gravity is stronger than
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  Speaker 1: competing forces, extends incomparably further than the heliopause. The wind
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  Speaker 1: is a feature of the Sun's current activity. Gravity is
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  Speaker 1: a feature of its mass, and mass does not stop
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  Speaker 1: at 120 AU. Mass reaches outward for tens of thousands
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  Speaker 1: of AU. Objects that far out are still gravitationally bound
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  Speaker 1: to the sun. They still orbit it. They are still,
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  Speaker 1: in every meaningful physical sense, part of the solar system.
• 11:57
  Speaker 1: Think about what that means. Voyager 1 has now travelled
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  Speaker 1: for nearly 50 years and reached about 160 AUU. It
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  Speaker 1: crossed what many people call the boundary of the solar system.
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  Speaker 1: and it has barely left. It is still deep inside
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  Speaker 1: the Sun's gravitational domain. The actual outer limit of the
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  Speaker 1: Sun's gravitational influence is not 120 AU. It is not
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  Speaker 1: 160 AU. It is not even close to those numbers.
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  Speaker 1: It is somewhere between 100,000 and 200,000 AU out. where
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  Speaker 1: the galaxy's own tidal forces begin to overpower the Sun's grip.
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  Speaker 1: That is a very different number. That is a number
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  Speaker 1: that changes everything about how you picture the solar system's size.
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  Speaker 1: Everything inside that gravitational boundary, every object still orbiting the Sun,
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  Speaker 1: still bound to it, still falling through space in paths
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  Speaker 1: the Sun controls, belongs to the solar system. And stretching
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  Speaker 1: through that enormous volume, surrounding the known planets the way
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  Speaker 1: an ocean surrounds a tiny island, is a structure that
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  Speaker 1: most people have never heard of. The Heliopause was never
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  Speaker 1: the edge. Neptune was never the edge. Pluto was never
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  Speaker 1: the edge. Beyond all of it, past every line we
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  Speaker 1: had learned to name as a boundary, there was something
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  Speaker 1: else entirely. In 1950, A Dutch astronomer named Jan Oort
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  Speaker 1: sat down with the orbital data of 19 known comets
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  Speaker 1: and noticed something that did not fit. This was not
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  Speaker 1: a glamorous moment in science. There were no major telescopes
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  Speaker 1: pointed at some distant mystery, no signal coming in from
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  Speaker 1: deep space. Oort was doing what astronomers of that era
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  Speaker 1: often did, working through numbers on paper. tracing the mathematics
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  Speaker 1: of known objects backward through time, trying to understand where
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  Speaker 1: they had originally come from before the Sun's gravity bent
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  Speaker 1: their paths inward. It was patient, methodical work, the kind
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  Speaker 1: of work that rarely produces anything dramatic. This time, it did.
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  Speaker 1: Long-period comets are different from the short-period kind. Short-period comets,
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  Speaker 1: like Halley's, return every few decades on predictable schedules. Long-period
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  Speaker 1: comets are stranger. They arrive from unexpected directions, swing around
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  Speaker 1: the Sun in enormous arcs, and then vanish back into
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  Speaker 1: the dark on orbits that take thousands or millions of
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  Speaker 1: years to complete. Some appear to be visiting the inner
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  Speaker 1: solar system for the first time. Their paths are extreme.
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  Speaker 1: stretched so far out that the far end of the
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  Speaker 1: orbit sits at distances that made no sense against the
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  Speaker 1: known boundaries of the solar system at the time. Oort
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  Speaker 1: was not the first person to wonder about this. In 1932,
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  Speaker 1: an Estonian astronomer named Ernst Erpik had suggested that long-period
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  Speaker 1: comets might originate from a distant reservoir of icy objects
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  Speaker 1: surrounding the solar system. The idea was reasonable. Erpik sketched
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  Speaker 1: it out, gestured at the possibility, and moved on. He
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  Speaker 1: did not build the mathematical case. He did not commit
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  Speaker 1: to specific distances or a specific structure. The suggestion sat
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  Speaker 1: there unfinished. Ort picked it up and finished it. What
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  Speaker 1: he did was deceptively simple. He took the known orbits
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  Speaker 1: of 19 long-period comets and worked backward. Before a comet
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  Speaker 1: arrives near the Sun, the Sun's gravity has already begun
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  Speaker 1: pulling on it, accelerating it, warping its path. To find
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  Speaker 1: where a comet actually came from, you have to reverse
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  Speaker 1: that process, account for the gravitational influence, and trace the
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  Speaker 1: orbit back to its original shape, before the Sun started
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  Speaker 1: reshaping it. Oort did this for all 19. He calculated
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  Speaker 1: what each comet's orbit looked like before it ever entered
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  Speaker 1: the inner solar system. The results stopped him. Every single orbit,
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  Speaker 1: traced back to its origin point, converged on the same
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  Speaker 1: general region of space. Not the same precise point. These
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  Speaker 1: were 19 different comets arriving from different directions, on different trajectories.
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  Speaker 1: but the distances were strikingly consistent. The far ends of
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  Speaker 1: their original orbits clustered around a shell somewhere between 20,000
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  Speaker 1: and 100,000 astronomical units from the Sun. That is not
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  Speaker 1: a number that has any obvious meaning to the human mind.
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  Speaker 1: But consider this. Neptune, the most distant planet, sits at
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  Speaker 1: roughly 30 AUO, Pluto averages around 40. The region Oort
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  Speaker 1: was pointing to sat 500 to 2,500 times further out
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  Speaker 1: than Neptune. It was somewhere no astronomer had seriously imagined looking.
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  Speaker 1: No single comet told the story. That is the part
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  Speaker 1: worth holding on to. One comet coming from a very
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  Speaker 1: distant orbit could have been a coincidence, an oddity, an anomaly.
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  Speaker 1: two could have been dismissed. Nineteen, all pointing to the
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  Speaker 1: same distant region, was a pattern, and patterns in astronomy
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  Speaker 1: are the closest thing to proof that inference can offer
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  Speaker 1: when direct observation is impossible. Oort published his findings in 1950,
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  Speaker 1: and his conclusion was bold. He proposed that the solar
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  Speaker 1: system is surrounded by an enormous spherical cloud of icy
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  Speaker 1: bodies a reservoir of material extending to extraordinary distances in
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  Speaker 1: every direction. Not a disk, not a ring, a sphere,
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  Speaker 1: wrapping around the entire solar system from all sides, dense
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  Speaker 1: enough to contain billions or trillions of small icy objects,
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  Speaker 1: but so remote and so dim that none of it
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  Speaker 1: had ever been seen. Comets in this picture. were not
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  Speaker 1: random wanderers from interstellar space. They were objects knocked loose
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  Speaker 1: from this cloud by some external disturbance sent falling inward
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  Speaker 1: on their long arcing paths toward the Sun. The proposal
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  Speaker 1: was striking for what it required you to accept. Oort
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  Speaker 1: was asking the scientific community to believe in a structure
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  Speaker 1: of almost incomprehensible scale that no telescope had ever detected.
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  Speaker 1: based on the orbital histories of 19 objects. There was
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  Speaker 1: no image. There was no direct detection. There was only
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  Speaker 1: the mathematics of those 19 paths, converging on a region
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  Speaker 1: of space so distant it had never appeared in any
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  Speaker 1: observation ever made. Astronomers accepted it anyway, not immediately and
• 19:45
  Speaker 1: not without scepticism, but the logic was difficult to escape.
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  Speaker 1: If the cloud was not there, where were the comets
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  Speaker 1: coming from? Their orbits demanded an explanation. Oort had provided one,
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  Speaker 1: and no competing explanation held up under the same scrutiny.
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  Speaker 1: The math was clean, the pattern was real. and the cloud,
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  Speaker 1: whatever it looked like in detail, had to be out there.
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  Speaker 1: Oort spent the rest of his career as one of
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  Speaker 1: the most respected astronomers of the 20th century. He mapped
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  Speaker 1: the rotation of the Milky Way. He contributed foundational work
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  Speaker 1: to radio astronomy. He was, by any measure, a scientist
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  Speaker 1: of remarkable range. But the thing that carries his name
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  Speaker 1: into every conversation about the solar system's outer edge is
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  Speaker 1: those 19 comets and the afternoon he spent tracing their
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  Speaker 1: origins backward through space. He had never seen a single
• 20:46
  Speaker 1: object in the cloud he had just described, and he
• 20:50
  Speaker 1: never would. The first number you need is one astronomical unit,
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  Speaker 1: which is the average distance from the Earth to the Sun.
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  Speaker 1: That distance is 93 million miles. Written out, it looks
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  Speaker 1: like this. 93 million. It is a number large enough
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  Speaker 1: that most people cannot hold it in their heads in
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  Speaker 1: any meaningful way. The fastest passenger jet ever built would
• 21:18
  Speaker 1: take roughly 20 years to cover it. Light, traveling at 186,000
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  Speaker 1: miles per second, needs just over eight minutes. One astronomical unit,
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  Speaker 1: Earth to Sun, that is the baseline. Now build from there.
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  Speaker 1: Mars sits about 1.5 AUU out. Jupiter, the largest planet
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  Speaker 1: in the solar system, orbits at roughly 5 AUU. Saturn,
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  Speaker 1: with its rings, is at about 9.5 AUU. Uranus is
• 21:52
  Speaker 1: around 19 AU. Neptune, the outermost planet, orbits at roughly
• 21:58
  Speaker 1: 30 AU from the Sun. That is 30 times the
• 22:02
  Speaker 1: Earth to Sun distance. about 2.8 billion miles. The Voyager
• 22:08
  Speaker 1: probes took decades to get there, and they were moving fast.
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  Speaker 1: Beyond Neptune, Pluto averages around 40 AU along its elliptical path.
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  Speaker 1: The Kuiper belt, the disk of icy debris that is
• 22:22
  Speaker 1: home to Pluto and hundreds of similar worlds, extends out
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  Speaker 1: to maybe 50 AUa. In school diagrams, this is where
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  Speaker 1: the solar system usually stops. The picture ends around Pluto,
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  Speaker 1: sometimes around the Kuiper belt, and the rest is labelled
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  Speaker 1: interstellar space. Here is where that picture falls apart completely.
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  Speaker 1: The inner edge of the Oort cloud begins somewhere around
• 22:48
  Speaker 1: 2000 AU from the Sun. Not 50. Not 100. 2000.
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  Speaker 1: That is 40 times further from the Sun than Pluto.
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  Speaker 1: It is 40 times farther than the edge of everything
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  Speaker 1: shown in nearly every diagram of the solar system ever printed.
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  Speaker 1: And that is the inner edge, the near side, the
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  Speaker 1: closest point where the Oort cloud even begins. The outer
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  Speaker 1: edge of the cloud extends somewhere between 100,000 and 200,000
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  Speaker 1: AU out, 100,000 astronomical units. At the upper end of estimates,
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  Speaker 1: nearly 200,000 times the distance from here to the Sun,
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  Speaker 1: the numbers have stopped making intuitive sense. That is expected.
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  Speaker 1: The human brain evolved to estimate distances measured in feet
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  Speaker 1: and miles, maybe across a continent if pressed. It did
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  Speaker 1: not evolve to reason about 18 trillion miles without some
• 23:51
  Speaker 1: kind of anchor. So here is an anchor. Imagine the
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  Speaker 1: Sun reduced to a marble. A small glass marble, roughly
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  Speaker 1: half an inch across, sitting on a table in front
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  Speaker 1: of you. On that same scale, Earth would be a
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  Speaker 1: grain of sand. Tiny, barely visible. And that grain of
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  Speaker 1: sand would be sitting about 30 feet from the marble.
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  Speaker 1: That is the Earth-Sun distance at this scale. 30 feet.
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  Speaker 1: Now find Neptune on this model. That grain of sand
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  Speaker 1: representing Neptune sits about 900 feet from the marble sun,
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  Speaker 1: almost three football fields away. Pluto is even farther, somewhere
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  Speaker 1: around 1,100 feet out. The Kuiper belt ends at a
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  Speaker 1: little over a quarter of a mile from your marble.
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  Speaker 1: Now find the inner edge of the Oort cloud. On
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  Speaker 1: this same scale, you would need to travel roughly 60
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  Speaker 1: miles from the marble to reach it. 60 miles, not feet,
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  Speaker 1: not yards, miles. The outer edge of the Oort cloud
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  Speaker 1: at 100,000 AU sits approximately 3,000 miles away from the
• 25:08
  Speaker 1: marble on your table. If you placed that marble in
• 25:12
  Speaker 1: New York City, the outer Oort cloud would extend past
• 25:16
  Speaker 1: Los Angeles in one direction and deep into the Atlantic
• 25:20
  Speaker 1: Ocean in another, It would be a sphere with a
• 25:23
  Speaker 1: radius of 3,000 miles, surrounding the marble from all directions.
• 25:30
  Speaker 1: And the marble is the Sun, and the planets, all
• 25:34
  Speaker 1: of them, Mercury through Neptune. Everything we have ever sent
• 25:38
  Speaker 1: a spacecraft to, everything we have ever named and mapped
• 25:43
  Speaker 1: and studied, all of it fits inside a circle smaller
• 25:47
  Speaker 1: than a mile in radius around that marble. The solar
• 25:51
  Speaker 1: system that fills every textbook, every classroom poster, every NASA
• 25:56
  Speaker 1: mission ever flown, occupies a tiny dot at the center
• 26:01
  Speaker 1: of something almost unimaginably larger. Think about light for a moment.
• 26:08
  Speaker 1: Light crosses the Earth-Sun distance in about eight minutes. It
• 26:12
  Speaker 1: reaches Neptune in roughly four hours, Pluto in about five
• 26:17
  Speaker 1: and a half hours. Light is not slow. It is
• 26:20
  Speaker 1: the fastest thing in the universe. And yet light leaving
• 26:24
  Speaker 1: the Sun right now, travelling outward at 186,000 miles every
• 26:30
  Speaker 1: second without pause, will not reach the outer edge of
• 26:34
  Speaker 1: the Oort cloud for more than a year. Some estimates
• 26:38
  Speaker 1: push that to a year and a half. The Sun's
• 26:40
  Speaker 1: own light takes longer to cross its gravitational domain than
• 26:45
  Speaker 1: it takes Earth to complete one full orbit of the Sun.
• 26:48
  Speaker 1: That is the actual size of our solar system. And
• 26:52
  Speaker 1: here is the fact that land's hardest of all, everything
• 26:57
  Speaker 1: inside the Kuiper Belt, the Sun, all eight planets, all
• 27:02
  Speaker 1: the moons, all the asteroids, all the spacecraft we have
• 27:06
  Speaker 1: ever launched, occupies only the innermost fraction of the Sun's
• 27:11
  Speaker 1: full gravitational reach. Roughly 0.05% of the distance to the
• 27:18
  Speaker 1: outer Oort cloud. Not 5%, not even 1%, 0.05%. The
• 27:26
  Speaker 1: planets are not the solar system. They are a small
• 27:29
  Speaker 1: cluster of objects near the center of the solar system.
• 27:33
  Speaker 1: The way a campfire is not the forest, but just
• 27:36
  Speaker 1: a bright point inside one. The Oort cloud is not
• 27:40
  Speaker 1: a thin shell at the edge of things. It is
• 27:43
  Speaker 1: the bulk of the solar system by volume. It dwarfs
• 27:47
  Speaker 1: everything inside it by a factor that the numbers struggle
• 27:51
  Speaker 1: to express. If the solar system were an ocean, the
• 27:56
  Speaker 1: planets would be the shallow water near the shore. The
• 27:59
  Speaker 1: Oort cloud would be everything else, all the open water,
• 28:04
  Speaker 1: most of the depth, virtually all of the extent. This
• 28:08
  Speaker 1: is what Jan Oort described from 19 comet orbits in 1950,
• 28:13
  Speaker 1: working with paper calculations and basic telescopic data. He looked
• 28:18
  Speaker 1: at the echoes and inferred the source. A cloud so
• 28:22
  Speaker 1: vast that the planets are a footnote inside it. A
• 28:26
  Speaker 1: cloud so enormous that the light travelling outward from its
• 28:30
  Speaker 1: centre takes over a year just to arrive. At those distances,
• 28:35
  Speaker 1: the word solar system stops meaning what you thought it meant.
• 28:40
  Speaker 1: The Oort cloud is not one uniform thing. Most illustrations
• 28:45
  Speaker 1: show it as a single fuzzy sphere wrapped around the
• 28:49
  Speaker 1: solar system, a smooth shell of debris sitting at some
• 28:53
  Speaker 1: enormous distance from the Sun. That picture is not wrong exactly,
• 28:58
  Speaker 1: but it is incomplete. The actual structure divides into two
• 29:03
  Speaker 1: distinct regions. and the difference between them matters more than
• 29:08
  Speaker 1: it might first seem. They behave differently, they respond to
• 29:13
  Speaker 1: different forces, and their relationship to each other is what
• 29:17
  Speaker 1: keeps the whole system functioning the way it does. The
• 29:21
  Speaker 1: inner region has a name most people have never heard.
• 29:25
  Speaker 1: Astronomers call it the Hills Cloud, after the researcher Jack Hills,
• 29:31
  Speaker 1: who characterized it in detail in 1981, It begins somewhere
• 29:36
  Speaker 1: around 2000 AU from the Sun and extends outward to
• 29:40
  Speaker 1: perhaps 20,000 AU. That range alone is staggering. Remember that
• 29:46
  Speaker 1: Neptune sits at about 30 AU and Pluto averages roughly
• 29:51
  Speaker 1: 40 AU. The Hills cloud begins where the outer solar
• 29:56
  Speaker 1: system already feels impossibly remote. and it stretches from there
• 30:02
  Speaker 1: across a distance 20 times larger than anything we have
• 30:06
  Speaker 1: ever sent a spacecraft near. But the crucial thing about
• 30:10
  Speaker 1: the Hills Cloud is its shape. It is not a sphere.
• 30:14
  Speaker 1: It is a disk, roughly flattened into the same plane
• 30:18
  Speaker 1: that the planets orbit in, the same ecliptic that has
• 30:22
  Speaker 1: organized the solar system since it formed. This is not
• 30:26
  Speaker 1: a coincidence. the inner cloud still feels the gravitational influence
• 30:31
  Speaker 1: of the giant planets. Jupiter and Saturn, even at those distances,
• 30:37
  Speaker 1: exert enough pull to keep the cloud's inner population organised
• 30:42
  Speaker 1: along a preferred plane, rather than scattered in all directions.
• 30:47
  Speaker 1: The Hill's cloud is also denser than what lies beyond it.
• 30:51
  Speaker 1: The objects are more tightly grouped, relatively speaking. and the
• 30:56
  Speaker 1: region holds a larger concentration of material than the outer
• 31:00
  Speaker 1: cloud per unit of volume. It is, in a sense,
• 31:04
  Speaker 1: the compressed inner layer of a much larger structure. Think
• 31:09
  Speaker 1: of it like sediment at the bottom of a very
• 31:11
  Speaker 1: deep ocean. More material, more structure, organised by the forces
• 31:17
  Speaker 1: acting nearby. Less dramatic from a distance than what lies
• 31:22
  Speaker 1: further out, but more substantial when examined up close. Then
• 31:26
  Speaker 1: there is the outer cloud. This is the structure most
• 31:30
  Speaker 1: people mean when they say the Oort cloud, and this
• 31:34
  Speaker 1: is where the real strangeness begins. The outer cloud starts
• 31:38
  Speaker 1: somewhere around 20,000 AU and extends to 100,000 AU or further.
• 31:46
  Speaker 1: Unlike the Hills cloud, it is not a disk. It
• 31:49
  Speaker 1: is a full sphere, surrounding the solar system from every
• 31:54
  Speaker 1: direction simultaneously, above and below the ecliptic plane, in every orientation.
• 32:02
  Speaker 1: If you could somehow see it from outside the solar system,
• 32:06
  Speaker 1: it would look like a halo, a vast, dim, spherical
• 32:10
  Speaker 1: shell of frozen debris enclosing everything inside it the way
• 32:15
  Speaker 1: an eggshell encloses its contents. The reason for this difference
• 32:19
  Speaker 1: in shape comes down to forces. The outer cloud is
• 32:24
  Speaker 1: so far from the giant planets that their gravity barely
• 32:28
  Speaker 1: registers at those distances. The force that has shaped the
• 32:32
  Speaker 1: outer cloud over billions of years is not Jupiter, not Saturn,
• 32:38
  Speaker 1: not anything inside the solar system. It is the galaxy,
• 32:43
  Speaker 1: the gravitational tide of the Milky Way. The combined pull
• 32:47
  Speaker 1: of hundreds of billions of stars organized across a disk
• 32:52
  Speaker 1: tens of thousands of light-years wide exerts a differential force
• 32:57
  Speaker 1: on objects at 50,000 or 100,000 AU that has gradually
• 33:03
  Speaker 1: reshuffled their orbital orientations. Over billions of years, this galactic
• 33:09
  Speaker 1: tide has randomized the inclinations of outer cloud objects tilting
• 33:15
  Speaker 1: their orbits in every direction until the flat disk structure
• 33:19
  Speaker 1: was completely erased and replaced with a sphere. That process
• 33:24
  Speaker 1: is why the outer cloud looks the way it does today.
• 33:28
  Speaker 1: Not an accident, not a coincidence, a consequence of four
• 33:33
  Speaker 1: and a half billion years of slow gravitational sculpting by
• 33:37
  Speaker 1: the galaxy itself. This spherical shape is also the answer
• 33:42
  Speaker 1: to a question that puzzled early comet observers for a
• 33:45
  Speaker 1: long time. Long period comets do not arrive from one direction.
• 33:51
  Speaker 1: They do not cluster around the ecliptic plane the way
• 33:54
  Speaker 1: the planets do, the way short period comets tend to.
• 33:59
  Speaker 1: They arrive from everywhere, from above the plane, from below it,
• 34:03
  Speaker 1: from directions that have no obvious relationship to the flat
• 34:07
  Speaker 1: disk of the solar system at all. The outer cloud
• 34:11
  Speaker 1: is Y. When something disturbs an object in the outer cloud,
• 34:15
  Speaker 1: that object can fall inward from any direction, and when
• 34:19
  Speaker 1: it reaches the inner solar system and brightens into a comet,
• 34:24
  Speaker 1: it arrives at an angle that reflects its origin point
• 34:27
  Speaker 1: in a spherical shell, not a flat disk. The sky
• 34:32
  Speaker 1: is isotropic for long-period comets because the reservoir sending them
• 34:37
  Speaker 1: is isotropic. The two regions are not independent of each other.
• 34:42
  Speaker 1: They form a system, and the connection between them is
• 34:45
  Speaker 1: what keeps both regions populated over geological time. The outer cloud,
• 34:51
  Speaker 1: being so loosely bound to the Sun, loses objects steadily.
• 34:55
  Speaker 1: Passing stars eject some into interstellar space. The galactic tide
• 35:01
  Speaker 1: strips others away. Every comet that falls inward and gets
• 35:05
  Speaker 1: destroyed or ejected by a planet's gravity is another object
• 35:09
  Speaker 1: permanently removed from the reservoir. If the outer cloud were
• 35:14
  Speaker 1: a closed system with no replenishment, it would have depleted
• 35:18
  Speaker 1: itself long ago. But it is not a closed system.
• 35:22
  Speaker 1: The inner hill's cloud acts as a supply source. Objects
• 35:26
  Speaker 1: from the inner cloud can be perturbed outward into the
• 35:30
  Speaker 1: outer cloud, gradually replenishing the population that the outer cloud
• 35:35
  Speaker 1: is continuously losing. The flow runs inward too, of course,
• 35:39
  Speaker 1: with some inner cloud objects being nudged into trajectories that
• 35:43
  Speaker 1: eventually reach the Sun. But the dominant direction of transfer
• 35:48
  Speaker 1: on long timescales is from the inner cloud outward, topping
• 35:53
  Speaker 1: off the outer cloud as the outer cloud drains. This
• 35:56
  Speaker 1: is a remarkably organized system for something sitting at the
• 36:00
  Speaker 1: edge of everything. Two regions, different shapes, different densities, different
• 36:07
  Speaker 1: governing forces, but coupled together and interdependent in a way
• 36:11
  Speaker 1: that has persisted for billions of years. The inner cloud
• 36:16
  Speaker 1: is the reserve. The outer cloud is the frontier. And
• 36:21
  Speaker 1: together they hold something close to the complete outer history
• 36:25
  Speaker 1: of the solar system in frozen form. Four and a
• 36:29
  Speaker 1: half billion years ago, none of this existed. There was
• 36:34
  Speaker 1: no Oort cloud. There was no outer shell of frozen
• 36:38
  Speaker 1: debris sitting in silence tens of thousands of AU from
• 36:42
  Speaker 1: the sun. There was not even a sun yet. Not fully.
• 36:46
  Speaker 1: Just a dense collapsing knot of gas and dust spinning
• 36:50
  Speaker 1: faster as it contracted, heating at its core, pulling in
• 36:55
  Speaker 1: more material from the surrounding cloud. Around it, a flat
• 37:00
  Speaker 1: disk of leftover gas and solids began to take shape.
• 37:04
  Speaker 1: The protoplanetary disk, the raw material for everything the solar
• 37:10
  Speaker 1: system would become. The objects now sitting in the Oort
• 37:13
  Speaker 1: cloud were in there somewhere, part of that disk, nowhere
• 37:18
  Speaker 1: near the outer solar system. They were close in, warm
• 37:23
  Speaker 1: in relative terms, moving fast in tight orbits, slowly clumping
• 37:28
  Speaker 1: together with other material the way small things do when
• 37:31
  Speaker 1: gravity has enough time. This is the part that surprises
• 37:35
  Speaker 1: most people when they first hear it. The Oort cloud
• 37:39
  Speaker 1: was not built from material that formed out there, The
• 37:43
  Speaker 1: icy bodies now frozen at 20,000 or 50,000 AU from
• 37:47
  Speaker 1: the Sun did not form at those distances. They formed
• 37:52
  Speaker 1: much closer, in the same zone that was building planets.
• 37:56
  Speaker 1: The protoplanetary disk was the origin of everything. The planets,
• 38:01
  Speaker 1: the moons, the asteroids, the comets and the Oort cloud
• 38:05
  Speaker 1: objects alike. They all started in the same rough neighbourhood.
• 38:10
  Speaker 1: What happened next is what separated them. The early solar
• 38:14
  Speaker 1: system was not a calm place. In the first tens
• 38:18
  Speaker 1: of millions of years, the giant planets were growing. Jupiter
• 38:23
  Speaker 1: was the dominant force. It accumulated mass faster than anything
• 38:28
  Speaker 1: else in the disk, pulling in gas and rock and
• 38:32
  Speaker 1: ice at a rate that made it, within a few
• 38:34
  Speaker 1: million years, the largest object orbiting the young sun by
• 38:39
  Speaker 1: an enormous margin. Saturn followed. Their gravity transformed the solar
• 38:46
  Speaker 1: system from a relatively orderly disk into something more violent.
• 38:51
  Speaker 1: Objects that passed too close to Jupiter did not simply
• 38:55
  Speaker 1: stay in their original orbits. They got flung. Some fell
• 39:01
  Speaker 1: inward toward the Sun and were destroyed. Some hit the
• 39:05
  Speaker 1: early rocky planets, contributing to the bombardment that scarred those
• 39:10
  Speaker 1: surfaces and, in some cases, may have delivered water and
• 39:15
  Speaker 1: organic compounds to the inner planets. Some were ejected outward
• 39:19
  Speaker 1: on hyperbolic trajectories and left the solar system entirely, thrown
• 39:24
  Speaker 1: into interstellar space, never to return. And some were thrown
• 39:29
  Speaker 1: out but not quite far enough to escape these were
• 39:33
  Speaker 1: the ones that mattered for the Oort cloud Jupiter and
• 39:37
  Speaker 1: Saturn and to a lesser extent Uranus and Neptune scattered
• 39:41
  Speaker 1: smaller objects outward on extremely elliptical orbits these objects would
• 39:47
  Speaker 1: swing far from the Sun slow down at the outer
• 39:50
  Speaker 1: end of their orbit and then fall back in again.
• 39:54
  Speaker 1: In those early trajectories, they were vulnerable. If they swung
• 39:59
  Speaker 1: back close enough to a giant planet again, they could
• 40:02
  Speaker 1: be scattered further or captured or ejected completely. But the
• 40:08
  Speaker 1: Sun's gravity and the gentle influence of passing stars in
• 40:12
  Speaker 1: the Sun's birth cluster gradually altered the inner portions of
• 40:16
  Speaker 1: those orbits. lifting what's called the perihelion, the closest approach point,
• 40:23
  Speaker 1: away from the giant planets. Once the perihelion was high
• 40:27
  Speaker 1: enough that Jupiter could no longer reach them, the objects
• 40:31
  Speaker 1: were effectively safe. They settled into wide, slow, stable orbits.
• 40:37
  Speaker 1: They stopped interacting with the inner solar system. They became
• 40:41
  Speaker 1: the Oort cloud. The bulk of the material that ended
• 40:45
  Speaker 1: up there came from a specific region of the early disk.
• 40:49
  Speaker 1: Models consistently point to the zone between roughly 15 and
• 40:54
  Speaker 1: 35 AGU from the Sun, which is the territory where
• 40:58
  Speaker 1: Uranus and Neptune formed. This was cold enough for volatile
• 41:03
  Speaker 1: ices to condense. Water ice, methane, ammonia, carbon monoxide, And
• 41:10
  Speaker 1: the objects that formed there were loaded with these materials.
• 41:14
  Speaker 1: They were also small enough to be moved by the
• 41:17
  Speaker 1: giants rather than to resist them. Objects from closer in
• 41:21
  Speaker 1: tended to hit things or fall into the sun. Objects
• 41:25
  Speaker 1: from this outer range of the disk were in the
• 41:28
  Speaker 1: right place to be launched outward without being destroyed. Most
• 41:32
  Speaker 1: of the frozen bodies now drifting in the Oort cloud
• 41:36
  Speaker 1: trace their origins to that specific band of the early
• 41:39
  Speaker 1: solar system. The timeline matters here. This process was not slow,
• 41:45
  Speaker 1: not on geological scales. The main phase of Oort cloud
• 41:50
  Speaker 1: formation happened somewhere between 20 and 50 million years after
• 41:55
  Speaker 1: the solar system began forming. Within the first 50 million years,
• 42:01
  Speaker 1: Most of the scattering events that sent objects to their
• 42:04
  Speaker 1: current distant orbits had already happened. That sounds like a
• 42:09
  Speaker 1: long time. But compared to the 4.6 billion years the
• 42:14
  Speaker 1: solar system has existed, it is a brief opening act.
• 42:19
  Speaker 1: By the time the planets had settled into something resembling
• 42:22
  Speaker 1: their current configuration, the Oort cloud was largely in place.
• 42:27
  Speaker 1: The heavy bombardment continued for a while longer, The inner
• 42:31
  Speaker 1: solar system stayed violent for hundreds of millions of years more,
• 42:36
  Speaker 1: but the distant reservoir was already built. There is something
• 42:40
  Speaker 1: worth sitting with in that timeline. The Oort cloud is
• 42:44
  Speaker 1: not a product of the mature solar system. It is
• 42:48
  Speaker 1: a product of the young one. It was assembled during
• 42:51
  Speaker 1: the period of maximum chaos, when the giant planets were
• 42:55
  Speaker 1: still migrating, when the disk was still being cleared, when
• 43:00
  Speaker 1: collisions were happening constantly and the whole system was sorting
• 43:04
  Speaker 1: itself out into something stable. Everything thrown outward in that period,
• 43:09
  Speaker 1: every icy body that survived the scattering without being destroyed
• 43:14
  Speaker 1: or ejected, ended up in those distant orbits where it
• 43:17
  Speaker 1: has sat, essentially untouched, ever since. The deep freeze of
• 43:23
  Speaker 1: space at those distances means there has been no chemistry,
• 43:27
  Speaker 1: no geology, no change. These objects are the same now
• 43:32
  Speaker 1: as they were when they arrived. They are 4.5 billion
• 43:37
  Speaker 1: year old relics of the solar system's most chaotic period,
• 43:42
  Speaker 1: preserved perfectly in the cold. In that sense, the Oort
• 43:46
  Speaker 1: cloud is a record. Not a perfect one, because a
• 43:50
  Speaker 1: lot of material was lost, ejected into interstellar space or
• 43:55
  Speaker 1: destroyed in the early bombardment. But what remains is a
• 44:00
  Speaker 1: snapshot of conditions that no longer exist anywhere else in
• 44:04
  Speaker 1: the solar system. The inner planets have been melted, differentiated, battered,
• 44:11
  Speaker 1: reshaped by volcanism and tectonics. The asteroids have been heated
• 44:17
  Speaker 1: and altered by impacts and solar radiation. The Oort cloud
• 44:22
  Speaker 1: objects have not. They have been sitting out there since
• 44:26
  Speaker 1: the solar system was young, preserved in the dark, unchanged
• 44:31
  Speaker 1: since Jupiter first flung them outward. Everything that would later
• 44:36
  Speaker 1: arrive in the inner solar system as a comet was
• 44:39
  Speaker 1: already out there, locked into its slow orbit, waiting. If
• 44:45
  Speaker 1: you could somehow reach the Oort cloud and hold one
• 44:49
  Speaker 1: of its objects, it would not look like much. Most
• 44:53
  Speaker 1: of what lives out there is small, a few kilometres
• 44:56
  Speaker 1: across maybe. Some objects push toward 100 kilometers in diameter.
• 45:02
  Speaker 1: But those are the exceptions. The typical Oort cloud body
• 45:06
  Speaker 1: is roughly the size of a small city, irregular in shape,
• 45:11
  Speaker 1: dark in color, and covered in a surface that has
• 45:14
  Speaker 1: not been touched by sunlight in any meaningful way for
• 45:18
  Speaker 1: billions of years. It would look, at first glance, like
• 45:22
  Speaker 1: a dirty rock. But look closer, and the chemistry tells
• 45:27
  Speaker 1: a different story. These objects are not made of rock
• 45:31
  Speaker 1: in the way Earth is made of rock. They are
• 45:33
  Speaker 1: built from ice. Not just water ice, but that is
• 45:38
  Speaker 1: there too. Methane ice, ethane, ammonia, carbon monoxide frozen solid,
• 45:46
  Speaker 1: hydrogen cyanide locked into the surface. The full list of
• 45:51
  Speaker 1: volatiles reads like a catalogue of the most dangerous chemicals
• 45:55
  Speaker 1: you can imagine. frozen together into a body smaller than
• 46:00
  Speaker 1: most counties and sitting in darkness at temperatures close to
• 46:04
  Speaker 1: absolute zero. These are the same materials that give comets
• 46:09
  Speaker 1: their brilliant tails when they fall inward toward the Sun.
• 46:13
  Speaker 1: When heat finally reaches one of these objects after a
• 46:16
  Speaker 1: long inward journey it does not melt so much as
• 46:20
  Speaker 1: it erupts. The ices sublimate directly into gas and and
• 46:24
  Speaker 1: that gas carries dust outward into space, and the result
• 46:29
  Speaker 1: is what we see from Earth as a comet's tail.
• 46:32
  Speaker 1: Everything that makes a comet spectacular when it approaches the
• 46:37
  Speaker 1: Sun was already sitting there, frozen in the Oort cloud, waiting.
• 46:43
  Speaker 1: The remarkable thing is how well preserved they are. These
• 46:48
  Speaker 1: objects formed 4.6 billion years ago, in the same disk
• 46:53
  Speaker 1: of gas and dust that built the planets. And they
• 46:56
  Speaker 1: have been sitting in deep freeze ever since. Nothing has
• 47:01
  Speaker 1: changed them. No heat, no radiation worth mentioning at those distances,
• 47:07
  Speaker 1: no geological process, no impact large enough to significantly alter
• 47:13
  Speaker 1: their bulk chemistry. They are essentially unchanged samples of the
• 47:17
  Speaker 1: original solar system. The planets have been cooked, compressed, chemically
• 47:23
  Speaker 1: transformed by billions of years of volcanism and bombardment and
• 47:28
  Speaker 1: atmospheric chemistry, the Oort cloud objects have not. They represent
• 47:34
  Speaker 1: a snapshot of what the raw material of our solar
• 47:37
  Speaker 1: system looked like before any of that happened. Scientists who
• 47:42
  Speaker 1: study comets falling inward from the outer cloud are, in
• 47:46
  Speaker 1: a very real sense, reading the composition of the solar
• 47:51
  Speaker 1: nebula as it existed before the planets existed. Now here
• 47:56
  Speaker 1: is where the picture gets genuinely strange. There are, by
• 48:00
  Speaker 1: most estimates, somewhere between 100 billion and several trillion of
• 48:06
  Speaker 1: these objects out there. The range in those estimates is
• 48:10
  Speaker 1: itself remarkable, and it tells you something important about how
• 48:14
  Speaker 1: little we know. Even pinning down whether the cloud contains
• 48:19
  Speaker 1: a hundred billion or a hundred trillion objects is beyond
• 48:23
  Speaker 1: current ability. But take the conservative number. Say there are
• 48:28
  Speaker 1: one trillion objects larger than a kilometre across. A trillion
• 48:33
  Speaker 1: is one thousand billion. That is more objects than there
• 48:38
  Speaker 1: are stars in most galaxies. The Oort cloud, by sheer
• 48:43
  Speaker 1: count of bodies is one of the most populated regions
• 48:47
  Speaker 1: of the entire solar system. And yet it is, in practice,
• 48:52
  Speaker 1: almost perfectly empty. This is the thing that breaks the intuition.
• 48:57
  Speaker 1: The cloud occupies such an enormous volume of space that
• 49:01
  Speaker 1: even trillions of objects spread across it produce something closer
• 49:05
  Speaker 1: to a hard vacuum than a crowd. The average distance
• 49:09
  Speaker 1: between neighbouring Oort cloud objects has been estimated at tens
• 49:13
  Speaker 1: of millions of kilometres. To put that in terms that
• 49:17
  Speaker 1: have some grip on the human mind, tens of millions
• 49:21
  Speaker 1: of kilometres is roughly the distance between Earth and Mars
• 49:25
  Speaker 1: at their closest approach. That is the gap between individual objects. Mars,
• 49:31
  Speaker 1: when it is nearest to us, still takes anywhere from
• 49:35
  Speaker 1: three to seven months to reach with current spacecraft. And
• 49:40
  Speaker 1: in the Oort cloud, that is the typical space between
• 49:43
  Speaker 1: two neighbours. Imagine standing on the surface of one of
• 49:48
  Speaker 1: these objects. You're on a dark, frozen body a few
• 49:52
  Speaker 1: kilometres across. The surface is quiet. There is no atmosphere,
• 49:58
  Speaker 1: no wind, no sound of any kind. You look up.
• 50:02
  Speaker 1: The stars are there, just as sharp and numerous as
• 50:06
  Speaker 1: they would be from anywhere in deep space. And among
• 50:10
  Speaker 1: those stars, you look for the Sun. You find it.
• 50:14
  Speaker 1: It does not look like the Sun you know. It
• 50:17
  Speaker 1: looks like a bright star. Brilliant, yes. Measurably brighter than
• 50:23
  Speaker 1: everything else around it. But a point. Not a disk.
• 50:28
  Speaker 1: Not a source of warmth you could feel. Just a
• 50:31
  Speaker 1: slightly more intense dot among billions of other dots. You
• 50:37
  Speaker 1: look for neighbouring objects in the cloud. You see nothing.
• 50:41
  Speaker 1: Not because there are no neighbours, but because even if
• 50:45
  Speaker 1: one sits tens of millions of kilometres away, it is
• 50:49
  Speaker 1: too small and too dark to resolve with the naked
• 50:53
  Speaker 1: eye against the backdrop of space. This is the thing
• 50:57
  Speaker 1: that illustrations always get wrong. Every diagram, every artist's rendering
• 51:03
  Speaker 1: of the Oort cloud, shows it as a haze of objects,
• 51:07
  Speaker 1: a visible swarm, a cloud in the way the word
• 51:11
  Speaker 1: cloud suggests something you could see. In reality, you could
• 51:15
  Speaker 1: stand inside it and have no idea it existed. The
• 51:20
  Speaker 1: objects are real, the trillions of bodies are real, but
• 51:24
  Speaker 1: the distances between them are so vast that the cloud is, experientially,
• 51:31
  Speaker 1: indistinguishable from empty space. It is the loneliest place in
• 51:36
  Speaker 1: the solar system. Whatever sits out there has been sitting
• 51:40
  Speaker 1: in that isolation since before the Earth had oceans. What
• 51:44
  Speaker 1: you would be holding is one of the oldest unaltered
• 51:47
  Speaker 1: pieces of the early solar system that exists anywhere and
• 51:52
  Speaker 1: there would be nothing else in sight. The Oort cloud
• 51:56
  Speaker 1: does not exist in isolation from the rest of the galaxy.
• 52:00
  Speaker 1: That might seem like an obvious statement, Nothing exists in
• 52:05
  Speaker 1: true isolation, but for the Oort cloud it means something
• 52:09
  Speaker 1: specific and strange. At the distances we are talking about,
• 52:14
  Speaker 1: somewhere between 50,000 and 100,000 astronomical units from the Sun,
• 52:21
  Speaker 1: the Sun's gravitational grip has weakened to something barely distinguishable
• 52:26
  Speaker 1: from nothing. And when the Sun's pull drops that low,
• 52:31
  Speaker 1: something else begins to matter. The Milky Way itself starts
• 52:35
  Speaker 1: pulling back. This is called the galactic tide. Not a
• 52:40
  Speaker 1: tide in the ocean sense, though the physics are related.
• 52:44
  Speaker 1: A tide in the gravitational sense is what happens when
• 52:49
  Speaker 1: one massive object pulls differently on two points at different
• 52:53
  Speaker 1: distances from it. The moon pulls slightly harder on the
• 52:57
  Speaker 1: side of Earth facing it than on the side facing away.
• 53:01
  Speaker 1: That difference, that gradient across Earth's diameter, is what raises
• 53:06
  Speaker 1: the oceans. The galactic tide works the same way, just
• 53:11
  Speaker 1: at a scale that is almost impossible to hold in
• 53:14
  Speaker 1: your mind. The Milky Way contains hundreds of billions of stars,
• 53:20
  Speaker 1: enormous amounts of dark matter, vast reservoirs of gas and dust.
• 53:26
  Speaker 1: Its total gravitational influence is staggering. and the Sun sitting
• 53:32
  Speaker 1: out in one of the galaxy's spiral arms, roughly 26.
• 53:37
  Speaker 1: 000 light years from the galactic centre is not immune
• 53:42
  Speaker 1: to any of it. Here is the key detail. The
• 53:46
  Speaker 1: Sun and its outermost objects are not at the same
• 53:50
  Speaker 1: distance from the galactic centre. They cannot be. The Oort
• 53:54
  Speaker 1: cloud extends outward in every direction. Objects on the side
• 53:59
  Speaker 1: of the cloud closest to the galactic centre are slightly
• 54:02
  Speaker 1: closer to it than the Sun is, while objects on
• 54:05
  Speaker 1: the far side are slightly farther. That tiny positional difference
• 54:10
  Speaker 1: translates into a tiny difference in gravitational acceleration. Objects closer
• 54:16
  Speaker 1: to the galactic centre are being pulled toward it fractionally harder.
• 54:21
  Speaker 1: Objects farther away are being pulled fractionally less. The Sun
• 54:26
  Speaker 1: sits in the middle, experiencing its own pull. And the
• 54:30
  Speaker 1: result of all these fractional differences, acting continuously over millions
• 54:36
  Speaker 1: and billions of years, is that orbits get twisted, stretched, tilted.
• 54:43
  Speaker 1: Objects that might have been orbiting in the flat plane
• 54:46
  Speaker 1: of the solar system gradually have their orbital inclinations randomized
• 54:51
  Speaker 1: by the galaxy's uneven tug. This is why the outer
• 54:55
  Speaker 1: Oort cloud is a sphere, It is not shaped like
• 54:59
  Speaker 1: a disk, the way the planets are, or even like
• 55:02
  Speaker 1: the slightly flattened inner hills cloud. It is a full
• 55:06
  Speaker 1: sphere surrounding the solar system in every direction. And the
• 55:10
  Speaker 1: reason for that shape is the galactic tide. Over billions
• 55:14
  Speaker 1: of years, the differential pull of the galaxy has worked
• 55:18
  Speaker 1: on orbital inclinations the way wind works on a pile
• 55:22
  Speaker 1: of leaves, scattering them in every direction. randomizing orientations that
• 55:29
  Speaker 1: might otherwise have stayed flat. The spherical shape of the
• 55:33
  Speaker 1: outer cloud is not an accident of formation. It is
• 55:38
  Speaker 1: the direct signature of galactic gravity written into the structure
• 55:43
  Speaker 1: of our own solar system. What you can do with
• 55:46
  Speaker 1: that realization is significant. When astronomers look at long period
• 55:52
  Speaker 1: comets arriving from the Oort cloud, and notice that they
• 55:56
  Speaker 1: come from every direction in the sky, not concentrated near
• 56:00
  Speaker 1: the plane of the solar system, but distributed across the
• 56:04
  Speaker 1: full celestial sphere. That pattern is direct evidence of the
• 56:09
  Speaker 1: galactic tide at work. The randomized arrival directions of comets
• 56:15
  Speaker 1: are the observable fingerprint of a force operating at galactic scales.
• 56:21
  Speaker 1: sculpting our solar system's outer edge from a distance of
• 56:25
  Speaker 1: tens of thousands of light years. But the galactic tide
• 56:30
  Speaker 1: does not just reshape orbits over long time scales and
• 56:33
  Speaker 1: leave them stable. It keeps working. And for some objects
• 56:38
  Speaker 1: in the outer cloud, the continuous gentle distortion of the
• 56:43
  Speaker 1: galactic tide is enough to tip an orbit across a threshold.
• 56:48
  Speaker 1: An object that has been circling the Sun in a
• 56:50
  Speaker 1: roughly stable path for billions of years can, through the
• 56:54
  Speaker 1: slow accumulation of galactic perturbations, find its orbit stretched just
• 57:00
  Speaker 1: enough that its closest approach to the Sun drops from
• 57:04
  Speaker 1: tens of thousands of AU into the realm of the
• 57:08
  Speaker 1: inner solar system. It does not happen quickly. The process
• 57:12
  Speaker 1: can take millions of years of continuous gravitational nudging, before
• 57:18
  Speaker 1: the orbit shifts far enough. But it happens. The galactic
• 57:23
  Speaker 1: tide is one of the primary engines that sends comets
• 57:27
  Speaker 1: falling inward from the Oort cloud. Every long period comet
• 57:32
  Speaker 1: arriving in the inner solar system likely owes some part
• 57:35
  Speaker 1: of its journey to this force. There is something almost
• 57:39
  Speaker 1: vertiginous about this once you sit with it. The comet
• 57:43
  Speaker 1: you might see crossing the sky over the course of
• 57:46
  Speaker 1: a few weeks bright enough to see with the naked eye,
• 57:50
  Speaker 1: began its fall because the Milky Way pulled on it,
• 57:54
  Speaker 1: not the Sun, not a passing star, the galaxy. A
• 57:59
  Speaker 1: structure 100,000 light years across has been exerting a slow
• 58:04
  Speaker 1: and patient influence on objects at the edge of our
• 58:07
  Speaker 1: solar system. Those objects eventually fell inward and became the
• 58:12
  Speaker 1: comets that ancient civilizations watched with terror and that modern
• 58:17
  Speaker 1: astronomers track with spectrographs. The Milky Way reaches all the
• 58:23
  Speaker 1: way into our local experience of the sky. This also
• 58:27
  Speaker 1: means the Oort cloud cannot be understood purely as a
• 58:31
  Speaker 1: solar phenomenon. It is embedded in a galactic context. The
• 58:36
  Speaker 1: shape of the cloud, the stability of individual orbits within it,
• 58:41
  Speaker 1: the rate at which comets are fed into the inner
• 58:44
  Speaker 1: solar system, All of these depend not just on the
• 58:48
  Speaker 1: Sun's gravity, but on the structure and mass distribution of
• 58:52
  Speaker 1: the galaxy around us. Change the position of the solar
• 58:56
  Speaker 1: system within the Milky Way. Move it closer to the
• 59:00
  Speaker 1: denser galactic disk, or nearer the centre. The galactic tide strengthens.
• 59:06
  Speaker 1: The Oort cloud would lose objects faster. Comet delivery to
• 59:11
  Speaker 1: the inner solar system would increase. The solar system's outer
• 59:16
  Speaker 1: edge is not a fixed or self-contained thing. It responds
• 59:20
  Speaker 1: to where the solar system happens to be sitting within
• 59:23
  Speaker 1: the largest structure of the galaxy at any given moment.
• 59:27
  Speaker 1: The galaxy is not just the background against which the
• 59:31
  Speaker 1: solar system sits, it is an active force shaping what
• 59:35
  Speaker 1: the solar system's outer edge looks like. Getting an object
• 59:40
  Speaker 1: from the Oort cloud into the inner solar system is
• 59:44
  Speaker 1: not a quick or simple process. There is no single trigger,
• 59:48
  Speaker 1: no one event that reliably sends an icy body falling inward. Instead,
• 59:55
  Speaker 1: There are three mechanisms, each operating on different time scales,
• 1:00:00
  Speaker 1: each capable of nudging something out of a stable orbit
• 1:00:04
  Speaker 1: it has held for billions of years, and setting it
• 1:00:07
  Speaker 1: on a path toward the Sun. The galactic tide, which
• 1:00:11
  Speaker 1: was introduced in the previous section, is the most constant
• 1:00:16
  Speaker 1: of these forces. It operates on everything in the outer
• 1:00:20
  Speaker 1: cloud simultaneously, always pulling, always reshaping orbits in subtle ways.
• 1:00:27
  Speaker 1: Over millions of years, that constant differential pull can accumulate
• 1:00:33
  Speaker 1: enough to make certain orbits unstable. An object that was
• 1:00:37
  Speaker 1: perfectly balanced in its distant loop gradually finds its path distorted.
• 1:00:43
  Speaker 1: The far end of the orbit gets stretched. The near
• 1:00:47
  Speaker 1: end dips slightly closer to the Sun. The geometry tips
• 1:00:52
  Speaker 1: far enough that the object begins a genuine inward fall.
• 1:00:57
  Speaker 1: This is not dramatic. Nothing explodes. Nothing collides. The object
• 1:01:03
  Speaker 1: simply reaches a threshold where its orbit no longer keeps
• 1:01:07
  Speaker 1: it safely in the outer cloud, and it begins a long,
• 1:01:11
  Speaker 1: slow descent. The second mechanism is a passing star. On average,
• 1:01:17
  Speaker 1: a star wanders within a few light years of the sun,
• 1:01:21
  Speaker 1: roughly every 50,000 years. That sounds infrequent. Over the full
• 1:01:27
  Speaker 1: four and a half billion year history of the solar system,
• 1:01:31
  Speaker 1: it represents tens of thousands of individual encounters. Each one
• 1:01:36
  Speaker 1: is a gravitational event. Even a star passing two or
• 1:01:40
  Speaker 1: three light years away exerts a measurable pull on Oort
• 1:01:45
  Speaker 1: cloud objects, especially those in the outermost. most loosely bound
• 1:01:50
  Speaker 1: regions of the cloud. The effect is not uniform. Some
• 1:01:54
  Speaker 1: objects get nudged outward, and if the nudge is strong enough,
• 1:01:59
  Speaker 1: they escape the Sun's gravity entirely and drift away into
• 1:02:03
  Speaker 1: interstellar space, lost forever. Others get nudged inward just enough
• 1:02:09
  Speaker 1: that their orbit tips towards the inner solar system. The
• 1:02:13
  Speaker 1: difference between those two outcomes can be a very small
• 1:02:17
  Speaker 1: change in velocity. We are talking about a gravitational push,
• 1:02:22
  Speaker 1: so gentle it would be undetectable over a human lifetime,
• 1:02:26
  Speaker 1: but enough, over the right geometry, to redirect a comet's
• 1:02:31
  Speaker 1: entire future. The third mechanism is the least common, but
• 1:02:36
  Speaker 1: potentially the most powerful. Giant molecular clouds are exactly what
• 1:02:42
  Speaker 1: they sound like, vast regions of dense gas and dust
• 1:02:47
  Speaker 1: scattered throughout the galaxy. some of them containing millions of
• 1:02:51
  Speaker 1: times the mass of the Sun. The solar system passes
• 1:02:55
  Speaker 1: through or near these clouds occasionally during its long orbit
• 1:02:59
  Speaker 1: around the galactic centre. When it does, the added gravitational
• 1:03:04
  Speaker 1: influence can disturb the outer Oort cloud on a massive scale,
• 1:03:09
  Speaker 1: sending large numbers of objects inward at roughly the same time.
• 1:03:14
  Speaker 1: Some researchers have linked periodic spikes in the comet impact
• 1:03:18
  Speaker 1: record on Earth to these encounters. The evidence is not conclusive,
• 1:03:23
  Speaker 1: but the physics is real. A close passage with a
• 1:03:26
  Speaker 1: molecular cloud is the Oort cloud equivalent of shaking a
• 1:03:30
  Speaker 1: snow globe. Now consider what happens after one of these
• 1:03:34
  Speaker 1: mechanisms does its work. An object gets perturbed. Its orbit changes.
• 1:03:40
  Speaker 1: It begins to fall inward, but it does not arrive quickly.
• 1:03:44
  Speaker 1: The orbit of a typical Oort cloud object is so
• 1:03:47
  Speaker 1: enormous that even after being nudged, the journey toward the
• 1:03:51
  Speaker 1: Sun takes an almost incomprehensible amount of time. A comet
• 1:03:56
  Speaker 1: originating from the outer cloud at, say, 80,000 AU, falling
• 1:04:02
  Speaker 1: inward on a highly elliptical orbit, might take several million
• 1:04:07
  Speaker 1: years just to complete one pass through the inner solar system.
• 1:04:12
  Speaker 1: Most of that time is spent far from the Sun,
• 1:04:15
  Speaker 1: moving slowly at the outer reaches of the orbit, covering
• 1:04:19
  Speaker 1: the vast distance at a crawl. The object spends only
• 1:04:23
  Speaker 1: a tiny fraction of its total orbital period anywhere near
• 1:04:27
  Speaker 1: the planets. For most of that journey, nothing visible happens.
• 1:04:33
  Speaker 1: The object is a frozen chunk of ice and rock, dark, silent,
• 1:04:39
  Speaker 1: and completely indistinguishable from any other piece of debris in space.
• 1:04:44
  Speaker 1: It carries the same chemical composition it had when it
• 1:04:47
  Speaker 1: formed four and a half billion years ago. Water ice, methane,
• 1:04:53
  Speaker 1: carbon monoxide, ammonia, hydrogen cyanide. None of that has changed.
• 1:05:00
  Speaker 1: The outer solar system is cold enough to preserve these
• 1:05:03
  Speaker 1: materials almost indefinitely. The object has been in deep freeze
• 1:05:08
  Speaker 1: since the early solar system, and the fall inward does
• 1:05:12
  Speaker 1: not immediately change that. For millions of years of travel,
• 1:05:17
  Speaker 1: it remains frozen and invisible. Then it crosses a threshold.
• 1:05:22
  Speaker 1: Somewhere around the orbit of Jupiter, the Sun's heat finally
• 1:05:26
  Speaker 1: begins to matter. At 5 AU, solar radiation is 25
• 1:05:31
  Speaker 1: times weaker than it is at Earth. But it is enough.
• 1:05:36
  Speaker 1: The ices near the surface begin to sublimate, converting directly
• 1:05:40
  Speaker 1: from solid to gas without passing through liquid form. Gases escape,
• 1:05:46
  Speaker 1: carrying dust and small particles with them. A cloud of
• 1:05:50
  Speaker 1: gas and dust begins to form around the nucleus. This
• 1:05:54
  Speaker 1: is the coma, the fuzzy atmosphere that gives comets their
• 1:05:59
  Speaker 1: distinctive blurred appearance through a telescope. As the comet continues inward,
• 1:06:04
  Speaker 1: the solar wind, constant stream of charged particles flowing out
• 1:06:10
  Speaker 1: from the Sun pushes the escaping gas and dust away
• 1:06:14
  Speaker 1: from the nucleus, always away from the Sun regardless of
• 1:06:18
  Speaker 1: the direction of travel. This is the tail, or rather
• 1:06:22
  Speaker 1: the tails, because most comets develop two, one made of
• 1:06:26
  Speaker 1: ionized gas pointing directly away from the Sun, one made
• 1:06:30
  Speaker 1: of dust curving gently along the orbital path. A bright
• 1:06:35
  Speaker 1: naked eye comet, the kind that gets reported in news
• 1:06:39
  Speaker 1: articles and draws people outside at night to look up,
• 1:06:42
  Speaker 1: may be visible for a few weeks, sometimes a few months.
• 1:06:48
  Speaker 1: Then it passes through the inner solar system, rounds the
• 1:06:52
  Speaker 1: sun and heads outward again on the far leg of
• 1:06:56
  Speaker 1: its orbit, fading as the distance grows and the ices
• 1:07:01
  Speaker 1: stop sublimating. It may not return for millions of years,
• 1:07:07
  Speaker 1: or it may be captured by a planet's gravity and
• 1:07:10
  Speaker 1: forced into a shorter orbit, or it may be ejected entirely.
• 1:07:15
  Speaker 1: The outcome depends on every gravitational encounter it has along
• 1:07:20
  Speaker 1: the way, and those are effectively impossible to predict with precision.
• 1:07:25
  Speaker 1: What is certain is the timescale. The mechanics of Oort
• 1:07:29
  Speaker 1: cloud perturbation, the long orbital fall, The slow approach, the
• 1:07:35
  Speaker 1: brief bright passage through the inner solar system, all of
• 1:07:39
  Speaker 1: it runs on a clock measured in millions of years.
• 1:07:43
  Speaker 1: By the time a long period comet is bright enough
• 1:07:46
  Speaker 1: to see with the naked eye from Earth, the journey
• 1:07:50
  Speaker 1: that sent it here may have begun millions of years ago.
• 1:07:54
  Speaker 1: The only direct evidence that the Oort cloud exists is
• 1:07:58
  Speaker 1: the comets it occasionally sends inward. That is worth sitting
• 1:08:02
  Speaker 1: with for a moment. Astronomers have spent decades building detailed
• 1:08:07
  Speaker 1: models of this structure, mapping its two regions, calculating its mass,
• 1:08:13
  Speaker 1: estimating its population in the trillions, describing how galactic tides
• 1:08:19
  Speaker 1: sculpt its outer shell, and yet not a single object
• 1:08:23
  Speaker 1: has ever been confirmed to actually reside there. No telescope
• 1:08:27
  Speaker 1: has resolved an Oort cloud body sitting in its orbit.
• 1:08:32
  Speaker 1: No spacecraft has flown near one. The entire edifice of
• 1:08:37
  Speaker 1: what we know about the outer solar system's most enormous
• 1:08:41
  Speaker 1: feature rests on a specific kind of indirect reasoning. We
• 1:08:46
  Speaker 1: have never seen the cloud. We have only seen what
• 1:08:50
  Speaker 1: falls out of it. This is not a small caveat.
• 1:08:54
  Speaker 1: This is the central fact about the Oort cloud. Everything.
• 1:08:59
  Speaker 1: The shape. the size, the population estimates, the two region
• 1:09:04
  Speaker 1: structure is inferred. The cloud is, in a very real sense,
• 1:09:10
  Speaker 1: a conclusion rather than an observation. The comets that reach
• 1:09:14
  Speaker 1: the inner solar system are the only physical data points
• 1:09:18
  Speaker 1: we have. And working backward from those comets to the
• 1:09:22
  Speaker 1: reservoir that produced them is one of the most careful
• 1:09:25
  Speaker 1: and consequential pieces of detective work in the history of astronomy.
• 1:09:30
  Speaker 1: Here is how it works. When a long period comet
• 1:09:34
  Speaker 1: swings through the inner solar system, astronomers can measure its
• 1:09:39
  Speaker 1: current trajectory with considerable precision. But the trajectory we observe
• 1:09:45
  Speaker 1: today has been distorted. As the comet fell inward over
• 1:09:49
  Speaker 1: millions of years, the gravitational pull of Jupiter, Saturn and
• 1:09:54
  Speaker 1: the other giant planets steadily bent its path, nudging it
• 1:09:59
  Speaker 1: away from whatever original trajectory sent it falling sunward in
• 1:10:03
  Speaker 1: the first place. To find where the comet actually came from,
• 1:10:08
  Speaker 1: you'd have to run the physics backward, strip out every
• 1:10:12
  Speaker 1: gravitational interaction it experienced on the way in, compute the
• 1:10:17
  Speaker 1: orbit it would have had before any planet touched it.
• 1:10:21
  Speaker 1: That original orbit, called the original osculating orbit, tells you
• 1:10:26
  Speaker 1: where the comet was before everything changed. When Jan Ortt
• 1:10:31
  Speaker 1: did this for 19 known comets in 1950, the result
• 1:10:35
  Speaker 1: was striking. The original orbits did not scatter randomly across space.
• 1:10:41
  Speaker 1: They clustered. The farthest points of those orbits, the Ophelia,
• 1:10:47
  Speaker 1: the places where each comet had been slowest and most
• 1:10:50
  Speaker 1: distant before beginning its inward fall, grouped together in a
• 1:10:54
  Speaker 1: zone somewhere between 20,000 and 150,000 AU from the Sun.
• 1:11:00
  Speaker 1: Not near any planet. Not in the Kuiper Belt. Not
• 1:11:05
  Speaker 1: somewhere close and easy to explain. Impossibly far out. in
• 1:11:10
  Speaker 1: a region that no instrument of that era had any
• 1:11:13
  Speaker 1: hope of directly examining. And the Ophelia came from all
• 1:11:17
  Speaker 1: directions in the sky, not clustered along the plane of
• 1:11:21
  Speaker 1: the solar system the way short-period comets are, but distributed
• 1:11:26
  Speaker 1: across every angle of the celestial sphere. That told Oort
• 1:11:31
  Speaker 1: the source had to be roughly spherical, not a disk,
• 1:11:35
  Speaker 1: a shell. 19 comets is not a large sample. Oort
• 1:11:41
  Speaker 1: knew this. But the pattern was clean enough, and the
• 1:11:44
  Speaker 1: alternative explanations weak enough, that the hypothesis held. The cloud
• 1:11:50
  Speaker 1: had to be there. The modern dataset is far larger.
• 1:11:54
  Speaker 1: Hundreds of long-period comets have now had their original orbits computed.
• 1:12:00
  Speaker 1: The pattern Oort found in 19 cases survives intact across
• 1:12:04
  Speaker 1: all of them. The Ophelia distribution remains sharply peaked. Objects
• 1:12:09
  Speaker 1: cluster between roughly 25,000 and 50,000 AU in numbers that
• 1:12:15
  Speaker 1: cannot be explained by chance or by any other known mechanism.
• 1:12:20
  Speaker 1: The statistical signal is overwhelming. Run the numbers with any
• 1:12:24
  Speaker 1: reasonable assumption about how comets behave and the same answer
• 1:12:29
  Speaker 1: comes back every time. There is a vast spherical reservoir
• 1:12:34
  Speaker 1: out there at those distances. and it is continuously leaking
• 1:12:38
  Speaker 1: objects inward at roughly the rate we observe, nothing else fits.
• 1:12:44
  Speaker 1: That phrase matters. Science rarely deals in certainties, but the
• 1:12:50
  Speaker 1: Oort cloud sits in a narrow category where the alternative
• 1:12:54
  Speaker 1: is not uncertainty, but incoherence. If the cloud did not exist,
• 1:13:00
  Speaker 1: long-period comets could not exist in the numbers and distribution
• 1:13:05
  Speaker 1: we observe. You cannot get comets arriving from random directions
• 1:13:09
  Speaker 1: in the sky on orbits that trace back to 40,000
• 1:13:13
  Speaker 1: AUU without a source at 40,000 AUU. The comets demand
• 1:13:19
  Speaker 1: the cloud the way a shadow demands an object casting it.
• 1:13:24
  Speaker 1: This kind of reasoning has worked before. In the 1840s,
• 1:13:29
  Speaker 1: astronomers noticed that Uranus was not quite following the orbit
• 1:13:33
  Speaker 1: Newton's laws predicted for it. Something was pulling on it.
• 1:13:38
  Speaker 1: The perturbations were small but consistent, and two mathematicians, Urbain
• 1:13:43
  Speaker 1: Le Verrier in France and John Couch Adams in England,
• 1:13:48
  Speaker 1: independently calculated where an unseen planet would have to be
• 1:13:52
  Speaker 1: to produce exactly those deviations. When astronomers pointed a telescope
• 1:13:58
  Speaker 1: at the predicted location in September of 1846, Neptune was there,
• 1:14:05
  Speaker 1: almost exactly where the math said it would be. The
• 1:14:08
  Speaker 1: planet had been found by inference before it was found
• 1:14:11
  Speaker 1: by observation. The Oort cloud was found the same way,
• 1:14:16
  Speaker 1: except the confirming observation has never come. We have the inference,
• 1:14:21
  Speaker 1: we have the math, we have hundreds of comets all
• 1:14:25
  Speaker 1: pointing at the same answer. What we do not have
• 1:14:29
  Speaker 1: is the image of a single object sitting in its
• 1:14:32
  Speaker 1: orbit at 50,000 AU. confirming what the equations say. That
• 1:14:38
  Speaker 1: gap between inference and direct observation is not a weakness
• 1:14:42
  Speaker 1: in the science. The math is solid, the comet data
• 1:14:46
  Speaker 1: is real, and the conclusion is about as secure as
• 1:14:49
  Speaker 1: conclusions get in astronomy without a direct image. But it
• 1:14:53
  Speaker 1: does mean the Oort cloud remains in a strange epistemic category.
• 1:14:58
  Speaker 1: We know it is there the way you can know
• 1:15:00
  Speaker 1: a room is full of people in the dark. Not
• 1:15:03
  Speaker 1: because you see them, but because you can hear them breathing.
• 1:15:08
  Speaker 1: Every long period comet that passes through the inner solar
• 1:15:11
  Speaker 1: system is a message from the outer edge. And for decades,
• 1:15:15
  Speaker 1: astronomers were reading the messages without quite understanding where they
• 1:15:19
  Speaker 1: had been sent from. The most famous comet in recorded
• 1:15:23
  Speaker 1: human history almost certainly did not come from the Oort cloud.
• 1:15:29
  Speaker 1: Halley's Comet returns every 76 years. Ancient Babylonians recorded it.
• 1:15:36
  Speaker 1: Chinese astronomers tracked it for centuries. Edmund Halley himself recognized
• 1:15:42
  Speaker 1: in 1705 that the bright comets of 1531, 1607, 1682
• 1:15:43
  Speaker 1: were the same object coming back around. That regularity is
• 1:15:47
  Speaker 1: exactly the problem. A 76-year orbital period puts Halley firmly
• 1:16:01
  Speaker 1: in the category of short-period comets, objects that loop through
• 1:16:05
  Speaker 1: the inner solar system frequently enough to be caught, studied
• 1:16:10
  Speaker 1: and predicted. Oort cloud comets don't do that. They arrive
• 1:16:15
  Speaker 1: on orbits so enormous that a single trip around the
• 1:16:18
  Speaker 1: Sun takes hundreds of thousands or millions of years. Halley's
• 1:16:23
  Speaker 1: most likely origin is the Kuiper belt, the disk of
• 1:16:27
  Speaker 1: icy bodies just beyond Neptune. Though some astronomers think it
• 1:16:32
  Speaker 1: may have started as a genuine Oort cloud object, whose
• 1:16:36
  Speaker 1: orbit was gradually shortened over millions of years by repeated
• 1:16:41
  Speaker 1: gravitational nudges from the giant planets. Either way, it is
• 1:16:46
  Speaker 1: not what Oort was describing when he proposed his cloud.
• 1:16:50
  Speaker 1: Halley is a familiar visitor. True Oort cloud comets are
• 1:16:54
  Speaker 1: not visitors. They are strangers arriving for the first time.
• 1:16:59
  Speaker 1: Comet West, which appeared in early 1976, was something different entirely.
• 1:17:07
  Speaker 1: It became one of the brightest comets of the 20th century,
• 1:17:11
  Speaker 1: visible to the naked eye even in morning twilight, with
• 1:17:15
  Speaker 1: a broad fan-shaped tail that observers described as spectacular. When
• 1:17:21
  Speaker 1: astronomers computed its original orbit, Tracing its path backward through
• 1:17:26
  Speaker 1: the gravitational influence of the planets to reconstruct where it
• 1:17:31
  Speaker 1: had been before, the numbers were striking. Comet West had
• 1:17:36
  Speaker 1: an orbital period measured not in years or centuries, but
• 1:17:40
  Speaker 1: in millions of years. It had almost certainly never passed
• 1:17:45
  Speaker 1: through the inner solar system before. It came from the
• 1:17:48
  Speaker 1: deep outer cloud. fell inward on a trajectory that had
• 1:17:53
  Speaker 1: been undisturbed for billions of years, swung around the Sun
• 1:17:58
  Speaker 1: and departed again. During its brief pass, it broke into
• 1:18:02
  Speaker 1: four pieces as tidal stresses from the Sun tore it apart.
• 1:18:07
  Speaker 1: The fragmentation gave astronomers an unexpected look at fresh interior material,
• 1:18:13
  Speaker 1: chemically pristine ice that had never been exposed to solar
• 1:18:17
  Speaker 1: radiation before that single approach. Comet West didn't just put
• 1:18:22
  Speaker 1: on a show. It cracked itself open and let scientists
• 1:18:26
  Speaker 1: read what was inside. Comet Hale-Bopp is another entry in
• 1:18:31
  Speaker 1: the genuine Oort Cloud column. Discovered in 1995 by two
• 1:18:36
  Speaker 1: amateur astronomers working independently on the same night, it became
• 1:18:41
  Speaker 1: visible to the naked eye in 1996 and stayed that
• 1:18:43
  Speaker 1: way until late 1997, 18 months of naked eye visibility.
• 1:18:52
  Speaker 1: Most comets are spectacular for a few weeks at most.
• 1:18:56
  Speaker 1: Hale-Bopp held the sky for a year and a half,
• 1:18:59
  Speaker 1: which tells you something about its size. Estimates put its
• 1:19:03
  Speaker 1: nucleus at roughly 60 kilometers across, one of the largest
• 1:19:07
  Speaker 1: cometary nuclei ever measured. Its previous orbital period, calculated from
• 1:19:13
  Speaker 1: its inbound trajectory, was somewhere around 4,000 years. That means
• 1:19:18
  Speaker 1: it had visited the inner solar system at least once
• 1:19:22
  Speaker 1: before in recorded human prehistory. Its outbound trajectory after the
• 1:19:27
  Speaker 1: 1997 approach was altered by planetary gravity, shortening its future
• 1:19:34
  Speaker 1: period to around 2,500 years. Hale-Bopp sits in an interesting
• 1:19:41
  Speaker 1: middle category, not a first-time visitor like Comet West, but
• 1:19:46
  Speaker 1: far from a short-period comet like Halley. Its chemical composition
• 1:19:51
  Speaker 1: was studied extensively and found to include water ice, carbon monoxide,
• 1:19:57
  Speaker 1: dust silicates, a range of organic molecules, essentially a sample
• 1:20:02
  Speaker 1: of the early outer solar system preserved in deep freeze
• 1:20:06
  Speaker 1: for billions of years. Then there is Comet ISON, which
• 1:20:11
  Speaker 1: arrived in 2013 carrying enormous expectations and delivered something more complicated.
• 1:20:18
  Speaker 1: Discovered in 2012 by two astronomers using a Russian telescope,
• 1:20:24
  Speaker 1: ISON was immediately recognized as a dynamically new comet, almost
• 1:20:29
  Speaker 1: certainly making its first pass through the inner solar system
• 1:20:33
  Speaker 1: after falling inward from the Oort cloud. Its orbital calculations
• 1:20:39
  Speaker 1: pointed to an origin at roughly 58,000 AU Hu. First-time
• 1:20:44
  Speaker 1: visitors tend to arrive with high concentrations of volatile ices
• 1:20:49
  Speaker 1: on their surfaces, because those ices have never been burned
• 1:20:53
  Speaker 1: off by solar heat before. Eisen was predicted to be
• 1:20:57
  Speaker 1: extraordinarily bright as those fresh ices vaporized. What actually happened
• 1:21:03
  Speaker 1: was that ISON approached the Sun to within about 1.1
• 1:21:07
  Speaker 1: million kilometres on Thanksgiving Day, 2013, and did not survive.
• 1:21:14
  Speaker 1: The nucleus disintegrated. ISON was destroyed before it could complete
• 1:21:20
  Speaker 1: its first solar orbit. The remnants scattered and faded. What
• 1:21:25
  Speaker 1: ISON gave astronomers before it fell apart was still valuable.
• 1:21:30
  Speaker 1: Spectroscopic data on a chemically fresh object that had never
• 1:21:34
  Speaker 1: been this close to a star before. Carrying material unchanged
• 1:21:40
  Speaker 1: since the early solar system. This is the category that
• 1:21:43
  Speaker 1: astronomers care most about. Dynamically new comets, objects falling inward
• 1:21:50
  Speaker 1: for the first time, or very close to the first time.
• 1:21:54
  Speaker 1: Their surfaces have never been heated, their volatile ices have
• 1:21:58
  Speaker 1: never outgassed. The chemistry sitting on the outer centimetres of
• 1:22:02
  Speaker 1: their nuclei is, in a very real sense, original, unchanged
• 1:22:08
  Speaker 1: since these objects formed 4.6 billion years ago in the
• 1:22:12
  Speaker 1: disk of gas and dust that built the planets. When
• 1:22:16
  Speaker 1: a spectroscope is pointed at a dynamically new comet as
• 1:22:20
  Speaker 1: it begins to warm near the Sun, what comes off
• 1:22:23
  Speaker 1: the surface is a direct chemical signal from the early
• 1:22:28
  Speaker 1: solar system. No other object we can study gives us
• 1:22:32
  Speaker 1: access to that era so cleanly. The bright comets that
• 1:22:36
  Speaker 1: have defined human history, the ones that triggered superstitions, that
• 1:22:41
  Speaker 1: were painted into medieval manuscripts, that drove people outside on
• 1:22:46
  Speaker 1: cold nights to stare upward. Most of the truly dramatic ones,
• 1:22:52
  Speaker 1: the ones arriving with no prior solar warming, were first-time
• 1:22:56
  Speaker 1: visitors from the outer cloud. Each one of them is
• 1:23:00
  Speaker 1: a chunk of the original solar system falling inward for
• 1:23:03
  Speaker 1: the first, or nearly the first, time in billions of years.
• 1:23:09
  Speaker 1: Its surface chemistry is a read on conditions that have
• 1:23:12
  Speaker 1: not existed anywhere else since the planets formed. Before you
• 1:23:17
  Speaker 1: reach the Oort cloud, you pass through something else entirely.
• 1:23:22
  Speaker 1: Between the realm of the planets and the vast frozen
• 1:23:26
  Speaker 1: shell of the outer solar system, There is a third
• 1:23:29
  Speaker 1: region that most people have never heard of and that
• 1:23:33
  Speaker 1: took astronomers most of the 20th century to properly understand.
• 1:23:38
  Speaker 1: It is called the Kuiper Belt. It sits just beyond Neptune,
• 1:23:43
  Speaker 1: beginning at roughly 30 astronomical units from the Sun and
• 1:23:47
  Speaker 1: extending outward to somewhere around 50 AU Su. That puts
• 1:23:52
  Speaker 1: it in a completely different category from anything the inner
• 1:23:55
  Speaker 1: solar system contains. Yet compared to what lies beyond it,
• 1:24:00
  Speaker 1: the Kuiper Belt is practically next door. The Kuiper Belt
• 1:24:04
  Speaker 1: is a disc-shaped ring of icy debris aligned with the
• 1:24:08
  Speaker 1: plane of the solar system, populated by objects that never
• 1:24:12
  Speaker 1: quite managed to become planets. Most of them are small,
• 1:24:17
  Speaker 1: a few kilometres across, frozen remnants of the same protoplanetary
• 1:24:22
  Speaker 1: disc that built Neptune and Uranus. But some are not
• 1:24:26
  Speaker 1: small at all. Pluto is in the Kuiper belt. So
• 1:24:30
  Speaker 1: is Eris, which is slightly more massive than Pluto and
• 1:24:35
  Speaker 1: orbits even farther out, reaching nearly 100 AU at its
• 1:24:39
  Speaker 1: most distant point. Makemake is in the Kuiper belt. Haumea
• 1:24:44
  Speaker 1: is in the Kuiper belt. These are dwarf planets, worlds
• 1:24:48
  Speaker 1: with enough mass to pull themselves into rounded shapes, geologically
• 1:24:54
  Speaker 1: complex in their own quiet way. sitting in the deep
• 1:24:57
  Speaker 1: cold beyond the last of the gas giants. Pluto has
• 1:25:02
  Speaker 1: mountains made of water ice and a thin nitrogen atmosphere
• 1:25:06
  Speaker 1: that collapses and refreezes as it swings through its orbit.
• 1:25:11
  Speaker 1: These are not blank rocks. They are places. The Kuiper
• 1:25:16
  Speaker 1: belt is also where short period comets come from. When
• 1:25:20
  Speaker 1: astronomers talk about comets that return every few decades or
• 1:25:24
  Speaker 1: every few centuries, Those objects trace their origins back to
• 1:25:28
  Speaker 1: this disk. A gravitational nudge from Neptune or one of
• 1:25:33
  Speaker 1: the other giant planets can knock a Kuiper Belt object
• 1:25:36
  Speaker 1: out of its stable orbit and send it falling inward.
• 1:25:40
  Speaker 1: The journey from 40 AU to the inner solar system
• 1:25:44
  Speaker 1: takes thousands of years. But the important distinction is this.
• 1:25:49
  Speaker 1: Kuiper Belt comets come in on flat. well-behaved trajectories aligned
• 1:25:54
  Speaker 1: with the plane of the solar system. Long-period comets arrive
• 1:25:58
  Speaker 1: from every direction, from above, from below, from angles that
• 1:26:03
  Speaker 1: have nothing to do with the ecliptic. That difference in
• 1:26:07
  Speaker 1: geometry is one of the core pieces of evidence for
• 1:26:10
  Speaker 1: the Oort cloud. Something spherical and enormous is feeding those
• 1:26:15
  Speaker 1: long-period orbits, and the Kuiper belt cannot be that source.
• 1:26:20
  Speaker 1: So the picture seemed clean for a while. The Kuiper
• 1:26:24
  Speaker 1: belt handled the short-period comets. The Oort cloud, somewhere far beyond,
• 1:26:30
  Speaker 1: handled the long-period ones. Two separate reservoirs, two separate populations,
• 1:26:37
  Speaker 1: a tidy model. Then, in November of 2003, a small
• 1:26:42
  Speaker 1: team of astronomers studying distant solar system objects found something
• 1:26:47
  Speaker 1: that did not fit into either category. They called it Sedna.
• 1:26:51
  Speaker 1: Sedna is a small reddish world, somewhere around 1,000 kilometers across.
• 1:26:58
  Speaker 1: Its surface is one of the reddest in the solar system,
• 1:27:01
  Speaker 1: likely stained by complex organic molecules called folins, the same
• 1:27:07
  Speaker 1: chemistry that gives Pluto its rusty patches and that forms
• 1:27:11
  Speaker 1: wherever ultraviolet radiation hits methane ice over long timescales. It
• 1:27:18
  Speaker 1: was not Sedna's surface that made astronomers stop cold. It
• 1:27:22
  Speaker 1: was its orbit. At its closest approach to the Sun,
• 1:27:27
  Speaker 1: Sedna comes in to roughly 76 AU. That alone puts
• 1:27:32
  Speaker 1: it well beyond the Kuiper belt. But its orbit is
• 1:27:35
  Speaker 1: enormously elongated, stretching outward to roughly 900 AU at its
• 1:27:40
  Speaker 1: farthest point. Its orbital period is approximately 11,400 years. To
• 1:27:48
  Speaker 1: put that in terms of human time, the last time
• 1:27:51
  Speaker 1: Sedna was anywhere near its closest approach to the Sun,
• 1:27:56
  Speaker 1: agriculture had barely been invented. The wheel did not exist.
• 1:28:01
  Speaker 1: There were no written languages anywhere on Earth. The problem
• 1:28:05
  Speaker 1: Sedna created was immediate and obvious. It was too far
• 1:28:10
  Speaker 1: out to be a Kuiper Belt object. Nothing in the
• 1:28:13
  Speaker 1: Kuiper Belt reaches 76 Agu at its closest approach. The
• 1:28:18
  Speaker 1: belt simply does not extend that far. But it was
• 1:28:22
  Speaker 1: also far too close, far too tightly bound to the sun,
• 1:28:27
  Speaker 1: to be a true Oort cloud object. The inner edge
• 1:28:31
  Speaker 1: of the Oort cloud is generally placed somewhere around 2000
• 1:28:34
  Speaker 1: Ag UA. Sedna's farthest point at 900 Ag UA. barely
• 1:28:41
  Speaker 1: grazes half that distance. It sits in a gap between
• 1:28:45
  Speaker 1: two known regions, too distant for one and not nearly
• 1:28:49
  Speaker 1: distant enough for the other. One astronomer described it as
• 1:28:53
  Speaker 1: finding a city in the middle of the ocean with
• 1:28:56
  Speaker 1: no bridges and no boats. Several explanations were proposed. Some
• 1:29:01
  Speaker 1: researchers argued that Sedna represents the first detected member of
• 1:29:06
  Speaker 1: the inner Oort cloud, also called the Hill's cloud, the denser,
• 1:29:12
  Speaker 1: disc-shaped inner region that sits inside the full spherical shell.
• 1:29:18
  Speaker 1: That would push the inner edge of the Oort cloud
• 1:29:20
  Speaker 1: system closer to the Sun than previously assumed. That would
• 1:29:25
  Speaker 1: be a significant revision. Others argue that Sedna belongs to
• 1:29:30
  Speaker 1: a third structure entirely, a region sometimes called the extended
• 1:29:35
  Speaker 1: scattered disk or the detached disk, a population of objects
• 1:29:40
  Speaker 1: pushed onto highly elongated orbits by gravitational interactions during the
• 1:29:46
  Speaker 1: early solar system. Neither fully Kuiper belt nor truly Oort cloud.
• 1:29:52
  Speaker 1: The most compelling explanation involves the Sun's birth cluster. When
• 1:29:57
  Speaker 1: the Sun formed, it was not alone. It was born
• 1:30:01
  Speaker 1: inside a dense stellar nursery alongside hundreds of other young stars.
• 1:30:07
  Speaker 1: For its first tens of millions of years, the Sun
• 1:30:11
  Speaker 1: had neighbors, close ones. Close enough to exert real gravitational
• 1:30:16
  Speaker 1: influence on the material surrounding it. A star passing within
• 1:30:21
  Speaker 1: a few hundred AU during that early period could have
• 1:30:25
  Speaker 1: pushed a Kuiper Belt object onto an orbit exactly like Sedna's,
• 1:30:30
  Speaker 1: stretched far outward, but not far enough to join the
• 1:30:33
  Speaker 1: true cloud. The birth cluster eventually dispersed, the neighboring stars
• 1:30:39
  Speaker 1: scattered across the galaxy. Sedna was left on its strange
• 1:30:43
  Speaker 1: elongated path, frozen evidence of a time when the Sun
• 1:30:48
  Speaker 1: was not alone. Since Sedna's discovery, A handful of other
• 1:30:53
  Speaker 1: objects with similarly strange extended orbits have been found. They
• 1:30:58
  Speaker 1: are sometimes called Sednoids, and each one deepens the puzzle.
• 1:31:03
  Speaker 1: There may be thousands more out there, too distant and
• 1:31:07
  Speaker 1: too faint for current telescopes to detect. Sedna is out
• 1:31:11
  Speaker 1: there right now completing its 11,400 year orbit. Nobody is
• 1:31:18
  Speaker 1: entirely certain what to call it or what it implies
• 1:31:21
  Speaker 1: about the region between the Kuiper Belt and the true cloud.
• 1:31:25
  Speaker 1: The Oort cloud has neighbours, and its neighbours are not
• 1:31:28
  Speaker 1: entirely harmless. Most people picture the space around our solar
• 1:31:33
  Speaker 1: system as fundamentally empty, a vast, quiet dark, with the
• 1:31:39
  Speaker 1: nearest star sitting 4.2 light-years away like a distant lighthouse. Harmless. Stationary.
• 1:31:47
  Speaker 1: But stars are not stationary. Every star in the galaxy
• 1:31:51
  Speaker 1: is moving, and from time to time one of them
• 1:31:53
  Speaker 1: moves close enough to matter. On average, a star passes
• 1:31:58
  Speaker 1: within a few light years of the Sun roughly every 50,000 years.
• 1:32:03
  Speaker 1: That sounds infrequent. Over the 4.6 billion year history of
• 1:32:09
  Speaker 1: the solar system though, it adds up to somewhere in
• 1:32:12
  Speaker 1: the range of 90,000 such encounters. Tens of thousands of
• 1:32:18
  Speaker 1: passing stars, each one reaching out gravitationally into the outermost
• 1:32:23
  Speaker 1: edges of the Oort cloud, tugging at the loosely bound
• 1:32:27
  Speaker 1: objects sitting there in the dark. The cumulative effect is staggering.
• 1:32:33
  Speaker 1: The Oort cloud, over billions of years, has been continuously jostled,
• 1:32:38
  Speaker 1: prodded and reshaped by the stellar traffic moving through our
• 1:32:42
  Speaker 1: corner of the galaxy. To understand why this matters, you
• 1:32:46
  Speaker 1: have to remember how weakly the Sun holds its outermost objects.
• 1:32:51
  Speaker 1: At 50,000 or 100,000 AU, the Sun's gravity is barely
• 1:32:56
  Speaker 1: a whisper. An object at that distance completes a single
• 1:33:01
  Speaker 1: orbit over millions of years, drifting along on a path
• 1:33:06
  Speaker 1: so wide and slow that almost any outside force can
• 1:33:10
  Speaker 1: perturb it. A passing star does not need to come
• 1:33:14
  Speaker 1: dangerously close to cause serious disruption. Even a star passing
• 1:33:19
  Speaker 1: within one or two light years exerts a gravitational pull
• 1:33:23
  Speaker 1: that is enough to slightly alter the trajectories of Oort
• 1:33:27
  Speaker 1: cloud objects in its path. Some of those objects get
• 1:33:31
  Speaker 1: pushed outward, stripped from the solar system entirely, and sent
• 1:33:36
  Speaker 1: into interstellar space. Others get nudged inward. Their orbits tighten.
• 1:33:42
  Speaker 1: Over thousands or millions of years, they fall toward the Sun,
• 1:33:47
  Speaker 1: picking up speed, and eventually they arrive in the inner
• 1:33:51
  Speaker 1: solar system as long-period comets. A bright comet blazing across
• 1:33:57
  Speaker 1: the sky might owe its entire journey to a star
• 1:34:01
  Speaker 1: that passed within a light year of the someday 10
• 1:34:04
  Speaker 1: million years ago and gave it the slightest push. There
• 1:34:09
  Speaker 1: is one particular star that astronomers are watching. Its name
• 1:34:13
  Speaker 1: is Gliese 710. It is an orange dwarf, about 64%
• 1:34:19
  Speaker 1: the mass of the Sun, currently sitting roughly 62 light
• 1:34:24
  Speaker 1: years away. It is approaching. Models based on precise measurements
• 1:34:30
  Speaker 1: from the Gaia Space Observatory project that Gliese 710 will
• 1:34:35
  Speaker 1: pass within about 0.06 light years of the Sun, in
• 1:34:40
  Speaker 1: approximately 1.3 million years. That is roughly 13,000 AU at
• 1:34:47
  Speaker 1: closest approach, well inside the outer Oort cloud. This will
• 1:34:52
  Speaker 1: be the closest known stellar passage in the history of
• 1:34:55
  Speaker 1: the Solar System. At that distance, Gliese 710's gravitational influence
• 1:35:02
  Speaker 1: on the Oort cloud will not be subtle. Simulations suggest
• 1:35:07
  Speaker 1: it could send a significant shower of comets inward, dramatically
• 1:35:12
  Speaker 1: increasing the number of long-period comets entering the inner solar
• 1:35:16
  Speaker 1: system over the following millions of years. The Earth will
• 1:35:21
  Speaker 1: not be threatened directly, but the comet flux arriving in
• 1:35:25
  Speaker 1: the inner solar system could increase substantially, and some fraction
• 1:35:30
  Speaker 1: of those comets will cross Earth's orbital path. 1.3 million
• 1:35:35
  Speaker 1: years from now is not tomorrow, but on astronomical timescales
• 1:35:40
  Speaker 1: it is close. What makes Gliese 710 especially interesting is
• 1:35:46
  Speaker 1: not just what it will do, it is what it
• 1:35:49
  Speaker 1: tells us about what has already happened. If one star
• 1:35:53
  Speaker 1: we can currently track is going to pass this close,
• 1:35:57
  Speaker 1: Then over the past 4.6 billion years, many other stars
• 1:36:02
  Speaker 1: must have passed at comparable or even shorter distances, each
• 1:36:08
  Speaker 1: one sending its own shower of comets inward, reshaping the
• 1:36:12
  Speaker 1: structure of the cloud in ways that accumulated across deep time.
• 1:36:17
  Speaker 1: The Oort cloud we see today is not the same
• 1:36:21
  Speaker 1: cloud that existed 2 billion years ago, or even 500
• 1:36:24
  Speaker 1: million years ago. It has been continuously sculpted by encounters
• 1:36:31
  Speaker 1: we can only estimate and never directly observe. But the
• 1:36:35
  Speaker 1: most profound stellar influence on the Oort Cloud did not
• 1:36:40
  Speaker 1: come from passing strangers. It came from family. The Sun
• 1:36:45
  Speaker 1: was not born alone. Stars forming clusters, dense nurseries of
• 1:36:51
  Speaker 1: gas and dust that collapse and ignite tens or hundreds
• 1:36:55
  Speaker 1: of new stars in a relatively small region of space.
• 1:37:00
  Speaker 1: In its first tens of millions of years, the Sun
• 1:37:04
  Speaker 1: was surrounded by hundreds of sibling stars, all formed from
• 1:37:09
  Speaker 1: the same parent cloud, all in close proximity. The distances
• 1:37:14
  Speaker 1: between these birth cluster stars were vastly smaller than the
• 1:37:18
  Speaker 1: typical distances between stars in the present-day galaxy. They were
• 1:37:23
  Speaker 1: neighbors in a way that stars almost never are. And
• 1:37:27
  Speaker 1: that closeness had consequences. As the Oort cloud was forming,
• 1:37:32
  Speaker 1: as the giant planets were flinging planetesimals outward into distant orbits,
• 1:37:38
  Speaker 1: those sibling stars were right there. Their gravity was constantly
• 1:37:43
  Speaker 1: tugging on the outer material. Some objects were perturbed into
• 1:37:48
  Speaker 1: even wider orbits than the planets alone could have produced.
• 1:37:53
  Speaker 1: Others were scattered into the spherical distribution that defines the
• 1:37:57
  Speaker 1: outer cloud. The overall shape of the Oort cloud, the
• 1:38:02
  Speaker 1: way it wraps around the solar system from all directions,
• 1:38:06
  Speaker 1: rather than lying flat in the plane of the planets,
• 1:38:10
  Speaker 1: may be partly a product of those early stellar neighbours
• 1:38:14
  Speaker 1: pushing and pulling from every angle before the cluster dispersed.
• 1:38:19
  Speaker 1: Within the first 100 million years or so, The birth
• 1:38:23
  Speaker 1: cluster broke apart. The sibling stars drifted away on their
• 1:38:28
  Speaker 1: individual orbits through the galaxy, spreading across distances so vast
• 1:38:34
  Speaker 1: they are now unidentifiable. The Sun has no known siblings,
• 1:38:40
  Speaker 1: but their influence is still written into the structure of
• 1:38:44
  Speaker 1: what they left behind. This is what makes the Oort
• 1:38:47
  Speaker 1: Cloud's history so strange to sit with. It formed through chaos.
• 1:38:54
  Speaker 1: Planetary giants throwing material outward. Sibling stars disturbing it from outside.
• 1:39:02
  Speaker 1: The galaxy's own tidal forces bending orbits over billions of years.
• 1:39:08
  Speaker 1: And then more passing stars, decade by decade, century by century,
• 1:39:14
  Speaker 1: epoch by epoch. slowly eroding the outer edges while occasionally
• 1:39:20
  Speaker 1: sending a fragment plunging inward to light up the sky.
• 1:39:25
  Speaker 1: The cloud is not a static archive. It is a
• 1:39:29
  Speaker 1: living structure, still responding to forces that have been acting
• 1:39:33
  Speaker 1: on it since before the Earth existed. There is a
• 1:39:37
  Speaker 1: star out there right now, 62 light years away. closing
• 1:39:43
  Speaker 1: the distance at around 51,000 km per hour relative to
• 1:39:48
  Speaker 1: the Sun. In 1.3 million years it will pass through
• 1:39:53
  Speaker 1: the outer Oort cloud. Nobody alive today and nobody alive
• 1:39:58
  Speaker 1: for a thousand generations after today will witness it, but
• 1:40:03
  Speaker 1: the comets it sends inward will eventually arrive. The stellar
• 1:40:08
  Speaker 1: neighborhood has never been as empty or as static as
• 1:40:12
  Speaker 1: it looks on a clear night. Here is a question
• 1:40:15
  Speaker 1: that sounds simple and is not. Where does the solar
• 1:40:19
  Speaker 1: system actually end? Most people have some version of an
• 1:40:24
  Speaker 1: answer already loaded. Pluto maybe, or Neptune if they remember
• 1:40:29
  Speaker 1: that Pluto got reclassified. Some have heard of the Kuiper
• 1:40:33
  Speaker 1: belt and would draw the line somewhere out there. past
• 1:40:37
  Speaker 1: the last recognisable planet, where the disk of icy debris
• 1:40:42
  Speaker 1: finally thins out around 50 AU. That feels like an edge.
• 1:40:47
  Speaker 1: It has the shape of an edge. But none of
• 1:40:50
  Speaker 1: those answers survive contact with what the solar system actually is,
• 1:40:55
  Speaker 1: because they are all describing the same small inner zone
• 1:40:59
  Speaker 1: and mistaking it for the whole thing. So let's take
• 1:41:03
  Speaker 1: the question seriously. There are several different definitions of where
• 1:41:08
  Speaker 1: the solar system ends, and they give different answers. Each
• 1:41:13
  Speaker 1: one is defensible. None of them is clean. The first
• 1:41:17
  Speaker 1: definition is the one Voyager 1 gave us. In 2012,
• 1:41:21
  Speaker 1: after 35 years of travel, Voyager 1 crossed a boundary
• 1:41:26
  Speaker 1: called the heliopause at roughly 121 AU from the Sun.
• 1:41:31
  Speaker 1: The heliopause is where the solar wind stops. The Sun
• 1:41:35
  Speaker 1: constantly blows a stream of charged particles outward in every direction,
• 1:41:40
  Speaker 1: a thin hot wind that pushes against the surrounding interstellar medium.
• 1:41:45
  Speaker 1: At some point, that outward pressure equals the inward pressure
• 1:41:51
  Speaker 1: of particles from the rest of the galaxy. That point
• 1:41:55
  Speaker 1: is the heliopause. Cross it and you are no longer
• 1:42:01
  Speaker 1: inside the Sun's wind. you are in interstellar space. By
• 1:42:06
  Speaker 1: that definition, the solar system ends at around 120 AUW.
• 1:42:13
  Speaker 1: Voyager 1 is currently past it. It is, by that measure,
• 1:42:17
  Speaker 1: already outside the solar system. But here is the problem.
• 1:42:22
  Speaker 1: The solar wind stopping is not the same thing as
• 1:42:25
  Speaker 1: the Sun's gravity stopping. Gravity does not have a heliopause.
• 1:42:30
  Speaker 1: It does not run out of push and equalize against
• 1:42:34
  Speaker 1: some opposing force. It falls off with distance, slowly, but
• 1:42:39
  Speaker 1: it never reaches zero. And the sun's gravitational grip on
• 1:42:44
  Speaker 1: objects extends vastly further than 120 AUW. Pluto at 40
• 1:42:51
  Speaker 1: AU is clearly bound to the sun. Sedna at its
• 1:42:56
  Speaker 1: farthest point, nearly 900 AUW. is still bound to the Sun.
• 1:43:02
  Speaker 1: Objects in the inner Oort cloud at 5000 AU are
• 1:43:06
  Speaker 1: still bound. Objects in the outer Oort cloud at 50,000
• 1:43:10
  Speaker 1: AU are still bound. Barely. But they are still circling
• 1:43:16
  Speaker 1: the Sun on orbits that close, eventually, if left undisturbed.
• 1:43:21
  Speaker 1: This brings you to a second definition. Astronomers call it
• 1:43:25
  Speaker 1: the Hill Sphere. the region around an object where its
• 1:43:29
  Speaker 1: gravity dominates over competing gravitational influences. For the Sun, the
• 1:43:35
  Speaker 1: competing influence at large distances is the Milky Way galaxy itself.
• 1:43:40
  Speaker 1: The galactic tide, as discussed, is not a negligible force
• 1:43:45
  Speaker 1: at 50,000 or 100,000 AU. The galaxy is pulling on
• 1:43:50
  Speaker 1: those outermost objects with a force that begins to rival
• 1:43:54
  Speaker 1: the Sun's own pull. The point at which the galaxy
• 1:43:58
  Speaker 1: winds is roughly where the outer Oort cloud ends. Estimates vary,
• 1:44:04
  Speaker 1: but the Hill Sphere of the Sun sits somewhere between 100,000
• 1:44:08
  Speaker 1: and 200,000 AU. By this definition, the solar system ends
• 1:44:14
  Speaker 1: at the outer edge of the Oort cloud. Everything inside
• 1:44:18
  Speaker 1: is Sun territory. Everything beyond is galactic territory. That sounds
• 1:44:24
  Speaker 1: like a clean answer. It is not. The objects at
• 1:44:28
  Speaker 1: the outer edge of the Oort cloud are barely hanging on.
• 1:44:32
  Speaker 1: Their orbits are so weakly bound to the Sun that
• 1:44:36
  Speaker 1: an encounter with a passing star or a sustained pull
• 1:44:39
  Speaker 1: from the galactic tide can sever the connection entirely. They
• 1:44:44
  Speaker 1: are not stably orbiting in any robust sense. They are drifting, slowly,
• 1:44:51
  Speaker 1: on paths that are genuinely uncertain over long timescales. A
• 1:44:56
  Speaker 1: nudge sends them outward into interstellar space. A nudge in
• 1:45:00
  Speaker 1: the other direction sends them falling inward for millions of years.
• 1:45:06
  Speaker 1: Their membership in the solar system is contingent. They belong
• 1:45:10
  Speaker 1: to the sun right up until they don't, and the
• 1:45:13
  Speaker 1: transition from belonging to not belonging happens gradually, continuously, across
• 1:45:20
  Speaker 1: a vast zone of loosely held objects. This is not
• 1:45:24
  Speaker 1: a line. It is a gradient. Think of it less
• 1:45:28
  Speaker 1: like the edge of a country and more like the
• 1:45:31
  Speaker 1: edge of an atmosphere. Earth's atmosphere does not end at
• 1:45:35
  Speaker 1: a specific altitude. It thins. The density drops. At 100
• 1:45:41
  Speaker 1: kilometres you define something called the Karman line for aeronautical purposes,
• 1:45:47
  Speaker 1: but molecules of air still exist above that. At 500 kilometers,
• 1:45:52
  Speaker 1: there are still enough particles to create measurable drag on spacecraft.
• 1:45:58
  Speaker 1: At 10,000 kilometers, the exosphere bleeds almost imperceptibly into the
• 1:46:04
  Speaker 1: solar wind. There is no moment where atmosphere becomes not atmosphere.
• 1:46:10
  Speaker 1: There is a zone of transition. The solar system has
• 1:46:14
  Speaker 1: the same structure on a scale that spans hundreds of
• 1:46:18
  Speaker 1: thousands of AU. The Sun's influence fades. The galaxy's influence rises.
• 1:46:25
  Speaker 1: Somewhere in that gradient, the math tips from one to
• 1:46:29
  Speaker 1: the other, but the objects in that gradient zone feel both.
• 1:46:33
  Speaker 1: What makes this strange is that we are talking about
• 1:46:37
  Speaker 1: our own solar system, the thing we live inside, and
• 1:46:41
  Speaker 1: we cannot draw a simple line around it. The boundary
• 1:46:45
  Speaker 1: of our local domain is genuinely ambiguous. That is not
• 1:46:50
  Speaker 1: a failure of knowledge. It reflects something real about how
• 1:46:54
  Speaker 1: gravity works and how structures dissolve at their edges. The
• 1:46:59
  Speaker 1: solar system is not a closed container with a wall.
• 1:47:03
  Speaker 1: It is a gravitational well sunk into the fabric of
• 1:47:06
  Speaker 1: the galaxy and the galaxy fills in around it at
• 1:47:10
  Speaker 1: the margins. The outermost Oort cloud objects are both inside
• 1:47:15
  Speaker 1: the solar system and subject to forces that could remove
• 1:47:19
  Speaker 1: them from it at any moment. They are in a
• 1:47:22
  Speaker 1: permanent borderland. Every long-period comet that eventually falls inward came
• 1:47:29
  Speaker 1: from that borderland, nudged loose from its marginal orbit by something,
• 1:47:34
  Speaker 1: a passing star, a galactic ripple, a perturbation that accumulated
• 1:47:40
  Speaker 1: over millions of years until the orbit stopped closing and
• 1:47:44
  Speaker 1: the object began its long fall toward the Sun. Wherever
• 1:47:48
  Speaker 1: you draw the boundary, the honest answer is the same.
• 1:47:52
  Speaker 1: The solar system is bigger and stranger, and less well-defined
• 1:47:57
  Speaker 1: than any textbook diagram has ever suggested. For something containing
• 1:48:03
  Speaker 1: trillions of objects, the Oort cloud has given astronomers almost
• 1:48:08
  Speaker 1: nothing to look at directly. No telescope has ever resolved
• 1:48:12
  Speaker 1: an individual object sitting out there in its proper orbit.
• 1:48:16
  Speaker 1: No spacecraft has passed through it. Every number we have,
• 1:48:21
  Speaker 1: the size estimates, the population counts, the composition guesses, comes
• 1:48:27
  Speaker 1: from inference. From reasoning backward through the comets that fall
• 1:48:32
  Speaker 1: inward and the physics that governs how they move. That
• 1:48:36
  Speaker 1: is an extraordinary situation. We are describing one of the
• 1:48:40
  Speaker 1: largest structures in the solar system without having seen any
• 1:48:44
  Speaker 1: of it. And when you cannot see something, you cannot
• 1:48:49
  Speaker 1: know what you are missing. So the real question is
• 1:48:52
  Speaker 1: not just what is out there. The question is what
• 1:48:56
  Speaker 1: might be out there that we have not accounted for
• 1:48:59
  Speaker 1: at all. Start with Planet 9. This is the most
• 1:49:04
  Speaker 1: discussed possibility in outer solar system research over the last decade.
• 1:49:09
  Speaker 1: It begins with a pattern in the data that nobody
• 1:49:12
  Speaker 1: fully expected. A group of extreme trans-Neptunian objects, small worlds
• 1:49:19
  Speaker 1: with orbits stretching far beyond the Kiker Belt, are not
• 1:49:24
  Speaker 1: distributed randomly. Their orbital paths cluster together in a way
• 1:49:29
  Speaker 1: that statistics say should not happen by chance. The probability
• 1:49:34
  Speaker 1: that this clustering is random noise is low enough that
• 1:49:38
  Speaker 1: many astronomers take it seriously as a signal. Something is
• 1:49:43
  Speaker 1: herding these objects. Something massive enough to gravitationally shepherd a
• 1:49:49
  Speaker 1: population of distant bodies into aligned orbits over billions of years.
• 1:49:55
  Speaker 1: The leading hypothesis is an undiscovered planet, somewhere between 5
• 1:50:01
  Speaker 1: and 10 times the mass of Earth. sitting in a wide,
• 1:50:05
  Speaker 1: eccentric orbit that keeps it far from the Sun for
• 1:50:09
  Speaker 1: most of its journey. Some of the proposed orbits for
• 1:50:12
  Speaker 1: Planet 9 would place it hundreds to thousands of AU
• 1:50:16
  Speaker 1: from the Sun at its farthest point, well inside the
• 1:50:20
  Speaker 1: inner Oort cloud region. It would not be a gas giant.
• 1:50:25
  Speaker 1: It would be something more like a super Earth, dark, cold,
• 1:50:30
  Speaker 1: reflecting almost no sunlight. If it exists, we may have
• 1:50:35
  Speaker 1: already imaged it in archival sky surveys without recognizing what
• 1:50:39
  Speaker 1: we were looking at. The search is ongoing. Nothing has
• 1:50:44
  Speaker 1: been confirmed. But a planet is not the only large
• 1:50:48
  Speaker 1: thing that might be hiding out there. The Oort cloud
• 1:50:52
  Speaker 1: almost certainly contains objects far bigger than the few kilometer
• 1:50:57
  Speaker 1: icy bodies that eventually become comets. Dwarf planet scale objects.
• 1:51:03
  Speaker 1: bodies hundreds of kilometers across, potentially comparable to Pluto or
• 1:51:08
  Speaker 1: Eris in size, could be distributed through the cloud in
• 1:51:12
  Speaker 1: numbers no one has properly estimated. The challenge is simple.
• 1:51:18
  Speaker 1: Objects that far from the Sun reflect almost no light.
• 1:51:22
  Speaker 1: A body 500 kilometers wide sitting at 10.000 AU would
• 1:51:29
  Speaker 1: be so faint that no current telescope could detect it
• 1:51:32
  Speaker 1: in a reasonable survey. We have found Sedna, at 76
• 1:51:38
  Speaker 1: AU at its closest approach. Sedna pushed the limits of
• 1:51:42
  Speaker 1: what was observable at the time of its discovery in 2003.
• 1:51:45
  Speaker 1: The true Oort cloud sits 30 to 40 times further
• 1:51:48
  Speaker 1: than that. Whatever population of large bodies exists, there is
• 1:51:57
  Speaker 1: invisible to us with current technology. The count of dwarf
• 1:52:02
  Speaker 1: planet-scale objects in the Oort cloud is essentially unknown because
• 1:52:06
  Speaker 1: we have never had the means to look. Then there
• 1:52:10
  Speaker 1: is a possibility that cuts even deeper. The Oort cloud
• 1:52:14
  Speaker 1: can lose objects to interstellar space. Galactic tides strip the
• 1:52:19
  Speaker 1: outermost bodies away continuously. A passing star can eject a
• 1:52:25
  Speaker 1: significant number of objects entirely in a single close encounter.
• 1:52:30
  Speaker 1: This process runs in both directions. During the Sun's early history,
• 1:52:35
  Speaker 1: when it was still embedded in the stellar cluster where
• 1:52:38
  Speaker 1: it formed, sibling stars were close enough that gravitational exchanges
• 1:52:44
  Speaker 1: could have transferred material between protoplanetary disks. Objects that condensed
• 1:52:50
  Speaker 1: around a neighboring star could have been captured into wide,
• 1:52:55
  Speaker 1: loosely bound orbits around the Sun during those early close passages.
• 1:53:00
  Speaker 1: The outer Oort cloud, where the Sun's gravity holds objects
• 1:53:04
  Speaker 1: barely at all, is precisely the region where captured interstellar
• 1:53:09
  Speaker 1: material would settle. Some fraction of what we call Oort
• 1:53:14
  Speaker 1: cloud objects may have formed around a completely different star,
• 1:53:19
  Speaker 1: condensed from a completely different protoplanetary disk, then pulled into
• 1:53:24
  Speaker 1: our solar system billions of years ago during an encounter
• 1:53:28
  Speaker 1: that is now ancient history. When a long period comet
• 1:53:33
  Speaker 1: eventually falls inward from the very outer cloud, its surface
• 1:53:38
  Speaker 1: chemistry might carry the signature of a foreign stellar environment
• 1:53:43
  Speaker 1: we would have no way of knowing. Beyond that, The
• 1:53:47
  Speaker 1: population of Oort cloud objects is almost certainly more structurally
• 1:53:52
  Speaker 1: complex than a simple collection of individual icy bodies. Binary objects,
• 1:53:58
  Speaker 1: two bodies orbiting each other as a paired system, are
• 1:54:02
  Speaker 1: known to exist in the Kuiper belt at significant rates.
• 1:54:06
  Speaker 1: There is no reason to assume the Oort cloud is different.
• 1:54:10
  Speaker 1: Objects with small companion moons, loosely bound pairs drifting through
• 1:54:15
  Speaker 1: the cloud together, groups of fragments from ancient collisions still
• 1:54:20
  Speaker 1: travelling on nearly identical orbits. All of these structures are plausible,
• 1:54:26
  Speaker 1: consistent with what we know about small body populations elsewhere.
• 1:54:31
  Speaker 1: Completely unobserved in the Oort cloud itself, the cloud is
• 1:54:35
  Speaker 1: large enough and sparse enough that collisions between objects are
• 1:54:39
  Speaker 1: extraordinarily rare. Once a binary pair forms, or a fragmentary
• 1:54:45
  Speaker 1: cluster disperses into a shared orbital path, there is little
• 1:54:50
  Speaker 1: to disrupt it over billions of years. These structures could
• 1:54:55
  Speaker 1: persist in the outer cloud in essentially pristine form from
• 1:55:00
  Speaker 1: the early solar system. We would have no knowledge of them.
• 1:55:05
  Speaker 1: What makes all of this genuinely unsettling is the scale
• 1:55:09
  Speaker 1: of the uncertainty. The Oort cloud is not a remote
• 1:55:14
  Speaker 1: curiosity at the edge of someone else's solar system. It
• 1:55:18
  Speaker 1: surrounds us. It is part of our solar system. And
• 1:55:22
  Speaker 1: it is one of the least observed, least understood, most
• 1:55:26
  Speaker 1: poorly characterized regions in all of solar astronomy. We have
• 1:55:32
  Speaker 1: mapped the surfaces of planets hundreds of millions of miles
• 1:55:36
  Speaker 1: away in fine detail. We have tracked individual asteroids to
• 1:55:41
  Speaker 1: within meters of their predicted positions, and the vast shell
• 1:55:46
  Speaker 1: that wraps the entire solar system in a sphere extending
• 1:55:51
  Speaker 1: to 100,000 AU remains effectively invisible, populated by objects we
• 1:55:57
  Speaker 1: have never counted, structured in ways we have not verified,
• 1:56:03
  Speaker 1: possibly harboring bodies whose existence we have not yet imagined.
• 1:56:08
  Speaker 1: What else is out there in the dark beyond 10,000 AU?
• 1:56:13
  Speaker 1: In orbits that have never been mapped and around objects
• 1:56:16
  Speaker 1: that have never been seen is a genuinely open question.
• 1:56:21
  Speaker 1: Voyager 1 launched in September of 1977, and by any
• 1:56:26
  Speaker 1: measure that matters to a human being, it has traveled
• 1:56:30
  Speaker 1: an extraordinary distance. It has crossed the orbit of every
• 1:56:35
  Speaker 1: planet in the solar system. It has passed through the heliopause,
• 1:56:39
  Speaker 1: the boundary where the Sun's wind finally gives out against
• 1:56:43
  Speaker 1: the pressure of interstellar space. It is, right now, more
• 1:56:49
  Speaker 1: than 130 astronomical units from the Sun. That is more
• 1:56:54
  Speaker 1: than 130 times the distance from the Earth to the Sun,
• 1:56:59
  Speaker 1: or roughly 12 billion miles of empty space between that
• 1:57:04
  Speaker 1: small metal craft and the star that launched it. No
• 1:57:08
  Speaker 1: human-made object has ever been farther. By any reasonable definition
• 1:57:13
  Speaker 1: of the word, Voyager 1 is the most distant thing
• 1:57:17
  Speaker 1: we have ever sent anywhere, and it has not come
• 1:57:21
  Speaker 1: close to leaving the solar system. This is the number
• 1:57:24
  Speaker 1: that stops most people cold when they first hear it.
• 1:57:29
  Speaker 1: Voyager 1 has been traveling for nearly 50 years. It
• 1:57:33
  Speaker 1: moves at roughly 38,000 miles per hour relative to the Sun.
• 1:57:39
  Speaker 1: At that speed, it will not reach the inner edge
• 1:57:42
  Speaker 1: of the Oort cloud for approximately 300 years. Three centuries
• 1:57:48
  Speaker 1: of continuous travel just to arrive at the beginning. Once
• 1:57:52
  Speaker 1: it gets there, crossing the full depth of the cloud
• 1:57:56
  Speaker 1: will take another 30,000 years. By the time Voyager 1
• 1:58:02
  Speaker 1: exits the far side of the Oort cloud, its nuclear
• 1:58:06
  Speaker 1: power source will have been dead for almost 30,000 years.
• 1:58:11
  Speaker 1: The spacecraft will be cold and silent and completely unreachable,
• 1:58:16
  Speaker 1: drifting through the outer cloud with no way to tell
• 1:58:20
  Speaker 1: us what it sees. This is not a rounding error.
• 1:58:23
  Speaker 1: This is what the solar system actually looks like when
• 1:58:27
  Speaker 1: you account for all of it. It helps to put
• 1:58:30
  Speaker 1: the 300-year figure into human terms. 300 years ago was
• 1:58:35
  Speaker 1: the early 1700s. The steam engine had not been invented yet.
• 1:58:41
  Speaker 1: The United States did not exist as a country. Isaac
• 1:58:45
  Speaker 1: Newton had just recently published his Laws of Motion. And
• 1:58:50
  Speaker 1: most of the world still traveled on horseback. The entire
• 1:58:54
  Speaker 1: span of the Industrial Revolution Every war of the modern era,
• 1:59:01
  Speaker 1: the invention of electricity and flight and satellites and computers,
• 1:59:06
  Speaker 1: all of that human history fits inside the time Voyager
• 1:59:11
  Speaker 1: 1 still needs just to reach the beginning of the
• 1:59:14
  Speaker 1: Oort cloud. The gap between where Voyager is now and
• 1:59:18
  Speaker 1: where the cloud begins is not measured in decades. It
• 1:59:23
  Speaker 1: is measured in centuries of sustained flight, at speeds no
• 1:59:28
  Speaker 1: human has ever directly experienced. What makes this harder to
• 1:59:33
  Speaker 1: absorb is how fast Voyager actually is. 38,000 miles per
• 1:59:39
  Speaker 1: hour is not slow. A commercial aircraft flies at roughly
• 1:59:46
  Speaker 1: 575 miles per hour. Voyager moves at about 66 times
• 1:59:51
  Speaker 1: that speed. If you could drive a car at Voyager's
• 1:59:54
  Speaker 1: velocity you would circle the Earth more than once every minute.
• 2:00:00
  Speaker 1: It crossed the distance from Earth to the Moon in
• 2:00:03
  Speaker 1: roughly six hours. It passed Jupiter in less than two years.
• 2:00:08
  Speaker 1: By any standard built around human experience, this spacecraft is
• 2:00:13
  Speaker 1: extraordinarily fast. And it still needs 300 years to reach
• 2:00:19
  Speaker 1: the inner edge of a structure that is part of
• 2:00:21
  Speaker 1: our own solar system. The reason is scale. The Oort
• 2:00:26
  Speaker 1: cloud does not begin until around 2,000 AU from the Sun.
• 2:00:32
  Speaker 1: Voyager is at 130. The remaining distance is not small.
• 2:00:37
  Speaker 1: If you laid the entire journey Voyager has already completed
• 2:00:41
  Speaker 1: end to end, you would need to repeat it more
• 2:00:44
  Speaker 1: than 14 times just to reach the cloud's inner boundary.
• 2:00:49
  Speaker 1: Everything Voyager has done in nearly 50 years of flight
• 2:00:53
  Speaker 1: is a small fraction of what remains. The planets, the heliosphere,
• 2:00:59
  Speaker 1: the interstellar medium Voyager is now crossing. All of that
• 2:01:03
  Speaker 1: together is still just the innermost portion of the Sun's
• 2:01:07
  Speaker 1: gravitational domain. No propulsion technology currently in existence changes this
• 2:01:13
  Speaker 1: in any meaningful way. The fastest spacecraft ever launched, the
• 2:01:19
  Speaker 1: Parker Solar Probe, reach speeds above 430,000 miles per hour
• 2:01:25
  Speaker 1: during its close solar approaches. That is genuinely remarkable. At
• 2:01:30
  Speaker 1: that speed, the journey to the inner Oort cloud would
• 2:01:34
  Speaker 1: still take decades. Getting through the full extent of the
• 2:01:38
  Speaker 1: cloud would take thousands of years. There is no engine
• 2:01:42
  Speaker 1: humans have built, and no engine currently on a drawing board.
• 2:01:47
  Speaker 1: that would deliver a spacecraft to the Oort Cloud within
• 2:01:50
  Speaker 1: a single human lifetime. The physics required are beyond anything
• 2:01:56
  Speaker 1: we have yet achieved, and the gap between what we
• 2:01:59
  Speaker 1: have and what we would need is not a small one.
• 2:02:03
  Speaker 1: This creates a strange situation. The Oort Cloud is our
• 2:02:07
  Speaker 1: solar system. It is the outer shell of the structure
• 2:02:11
  Speaker 1: the Sun gravitationally owns. It formed from the same material
• 2:02:16
  Speaker 1: that built every planet and moon we have ever studied.
• 2:02:19
  Speaker 1: Comets falling from it have been visible to humans throughout
• 2:02:23
  Speaker 1: all of recorded history. In one sense, the Oort cloud
• 2:02:27
  Speaker 1: is intimately connected to us, the origin point of some
• 2:02:31
  Speaker 1: of the most spectacular objects ever seen in the night sky.
• 2:02:36
  Speaker 1: In another sense, it is completely and perhaps permanently beyond
• 2:02:40
  Speaker 1: our reach. We live inside something we cannot visit. We
• 2:02:45
  Speaker 1: are surrounded by a structure we have never directly observed
• 2:02:49
  Speaker 1: and may never observe within any time scale that feels real.
• 2:02:55
  Speaker 1: This is what separates the Oort cloud from every other
• 2:02:58
  Speaker 1: frontier in solar system exploration. Mars is far, but spacecraft
• 2:03:04
  Speaker 1: reach it in months. The outer planets are distant, but
• 2:03:09
  Speaker 1: Voyager passed them within years of launch. Even the Kuiper belt,
• 2:03:13
  Speaker 1: which once seemed impossibly remote, was visited when New Horizons
• 2:03:18
  Speaker 1: flew through Pluto's neighborhood in 2015. The Oort cloud is different.
• 2:03:25
  Speaker 1: It is not the next step in a sequence of
• 2:03:27
  Speaker 1: increasingly ambitious missions. It is a destination that falls entirely
• 2:03:33
  Speaker 1: outside the range of anything we know how to build.
• 2:03:36
  Speaker 1: The technology required does not yet exist. and the timeline
• 2:03:41
  Speaker 1: for developing it is not clear. Every piece of hardware
• 2:03:45
  Speaker 1: humans have ever launched is still, in cosmological terms, essentially
• 2:03:51
  Speaker 1: next door to the sun. Even the fastest object humanity
• 2:03:56
  Speaker 1: has ever sent into space will not arrive at the
• 2:03:59
  Speaker 1: Oort cloud within the lifetime of anyone who built it,
• 2:04:03
  Speaker 1: or their grandchildren, or any generation that can be concretely imagined.
• 2:04:09
  Speaker 1: In October of 2017, astronomers detected something moving through the
• 2:04:15
  Speaker 1: inner solar system that did not belong to it. The
• 2:04:18
  Speaker 1: object was picked up by the Pan-STARRS telescope in Hawaii,
• 2:04:23
  Speaker 1: and at first it looked like a comet or an
• 2:04:25
  Speaker 1: asteroid on an unusual path. Then the orbital math came back.
• 2:04:31
  Speaker 1: The trajectory was hyperbolic, not slightly hyperbolic, dramatically. unmistakably hyperbolic.
• 2:04:40
  Speaker 1: Whatever this object was, it had not come from the
• 2:04:43
  Speaker 1: solar system. It had come from somewhere else entirely, fallen
• 2:04:48
  Speaker 1: into the Sun's gravitational reach, swung around it, and was
• 2:04:53
  Speaker 1: now heading back out again at a speed nothing bound
• 2:04:57
  Speaker 1: to our Sun could achieve. Astronomers named it Oumuamua, a
• 2:05:02
  Speaker 1: Hawaiian word meaning something like a messenger from afar arriving first.
• 2:05:09
  Speaker 1: The name fit better than anyone fully appreciated at the time.
• 2:05:13
  Speaker 1: Umuamua was strange in almost every measurable way. It was elongated,
• 2:05:20
  Speaker 1: possibly cigar-shaped or flat like a disc, depending on which
• 2:05:24
  Speaker 1: model you trusted. It showed unexpected acceleration as it moved
• 2:05:29
  Speaker 1: away from the sun, more than could be explained by
• 2:05:33
  Speaker 1: gravity alone. It had no visible coma, no tail, none
• 2:05:38
  Speaker 1: of the outgassing behavior you would expect from a comet.
• 2:05:42
  Speaker 1: Explanations multiplied fast. Outgassing of hydrogen. A fragment of a
• 2:05:49
  Speaker 1: shredded planetesimal from another star. Something nobody wanted to say
• 2:05:54
  Speaker 1: out loud, but several serious researchers eventually wrote up in
• 2:05:58
  Speaker 1: peer-reviewed journals. Whatever it was, it was the first confirmed
• 2:06:03
  Speaker 1: visitor from another stellar system, and it passed through too
• 2:06:07
  Speaker 1: quickly for any spacecraft to reach it. It was gone
• 2:06:12
  Speaker 1: before the scientific community had fully registered what it was
• 2:06:15
  Speaker 1: looking at. Then, in 2019, came something clearer. Comet Borisov
• 2:06:23
  Speaker 1: was discovered by an amateur astronomer named Gennady Borisov, and
• 2:06:28
  Speaker 1: its orbit left no room for argument. The trajectory was hyperbolic.
• 2:06:34
  Speaker 1: The excess velocity was large. And unlike Oumuamua, it looked
• 2:06:40
  Speaker 1: like a comet. It had a coma. It had a tail.
• 2:06:45
  Speaker 1: Spectroscopic analysis of the material boiling off its surface showed
• 2:06:49
  Speaker 1: water ice, carbon monoxide, a composition not dramatically different from
• 2:06:55
  Speaker 1: comets born in our own solar system. Borisov had formed
• 2:06:59
  Speaker 1: around another star, travelled across interstellar space, and arrived here
• 2:07:05
  Speaker 1: carrying chemistry that looked remarkably familiar. Two confirmed interstellar objects
• 2:07:11
  Speaker 1: in two years, after centuries of watching the sky and
• 2:07:15
  Speaker 1: finding none. Either the solar system had hit a strange
• 2:07:19
  Speaker 1: streak of luck, or interstellar objects passed through far more
• 2:07:24
  Speaker 1: often than anyone had assumed. and we had simply not
• 2:07:27
  Speaker 1: been watching carefully enough to catch them before. The second
• 2:07:32
  Speaker 1: explanation is almost certainly correct. Models built after these detections
• 2:07:38
  Speaker 1: suggested that interstellar objects the size of Oumuamua should pass
• 2:07:44
  Speaker 1: through the inner solar system roughly once a year. Most
• 2:07:48
  Speaker 1: go undetected because they move fast, they are small, and
• 2:07:53
  Speaker 1: the window to spot them is narrow. The upcoming Vera
• 2:07:57
  Speaker 1: Rubin Observatory with its ability to survey the entire accessible
• 2:08:02
  Speaker 1: sky every few nights, is expected to catch several of them.
• 2:08:07
  Speaker 1: The detections in 2017 and 2019 were not anomalies. They
• 2:08:13
  Speaker 1: were the first glimpses of something that had always been happening.
• 2:08:17
  Speaker 1: This is where the Oort cloud becomes directly relevant. If
• 2:08:21
  Speaker 1: interstellar objects pass through the solar system with reasonable frequency,
• 2:08:27
  Speaker 1: and if the Sun's gravity can tug on them as
• 2:08:29
  Speaker 1: they pass, some fraction of them can be captured. Not many.
• 2:08:35
  Speaker 1: The geometry and the speeds involved make capture difficult. But
• 2:08:40
  Speaker 1: the solar system has been around for four and a
• 2:08:42
  Speaker 1: half billion years. And even a rare event repeated across
• 2:08:47
  Speaker 1: that much time adds up. During the Sun's earliest period,
• 2:08:53
  Speaker 1: when it was still embedded in its birth cluster surrounded
• 2:08:56
  Speaker 1: by hundreds of sibling stars, the stellar separations were far
• 2:09:01
  Speaker 1: smaller than they are today. Objects loosely held by one
• 2:09:06
  Speaker 1: star were within reach of another. Protoplanetary disks overlapped in
• 2:09:11
  Speaker 1: gravitational influence. The exchange of material between young stellar systems
• 2:09:17
  Speaker 1: was not just possible, it was likely. The outer Oort
• 2:09:22
  Speaker 1: cloud is precisely where captured material would end up. Objects
• 2:09:27
  Speaker 1: moving in from interstellar space carry more velocity than objects
• 2:09:31
  Speaker 1: native to the solar system. To be captured, they need
• 2:09:35
  Speaker 1: to lose enough energy to become bound. And that is
• 2:09:39
  Speaker 1: most likely to happen at the outermost edge of the
• 2:09:42
  Speaker 1: Sun's gravitational domain, where the Sun's pull is weakest, and
• 2:09:48
  Speaker 1: where a slight nudge from a passing body or the
• 2:09:51
  Speaker 1: galactic tide might be just enough to tip a fast-moving
• 2:09:56
  Speaker 1: interloper into a loosely bound orbit. The inner solar system
• 2:10:00
  Speaker 1: cannot capture these objects easily. The outer Oort cloud can,
• 2:10:05
  Speaker 1: or could at least, when the Sun was young and
• 2:10:09
  Speaker 1: the stellar neighborhood was crowded. What this means is unsettling
• 2:10:13
  Speaker 1: in a quiet way. When you look at a long
• 2:10:16
  Speaker 1: period comet falling inward from the very outer cloud, you're
• 2:10:20
  Speaker 1: probably looking at something that formed in our own solar
• 2:10:24
  Speaker 1: system four and a half billion years ago, but probably
• 2:10:28
  Speaker 1: is doing a lot of work in that sentence. Some
• 2:10:32
  Speaker 1: fraction of those inbound objects may have formed around a
• 2:10:36
  Speaker 1: different star entirely. Their ice carries the chemical signature of
• 2:10:41
  Speaker 1: a different protoplanetary disk, a different stellar neighborhood, a different history.
• 2:10:48
  Speaker 1: We do not yet have the tools to tell the
• 2:10:51
  Speaker 1: difference reliably. The spectroscopy required to distinguish an Oort cloud
• 2:10:56
  Speaker 1: native from an interstellar capture is at the edge of
• 2:11:00
  Speaker 1: what current technology can do, and long-period comets move fast
• 2:11:05
  Speaker 1: and spend only a short time close enough to study
• 2:11:08
  Speaker 1: in detail. The boundary between the solar system and interstellar
• 2:11:12
  Speaker 1: space has always been treated as a clean line. Interstellar
• 2:11:17
  Speaker 1: space on one side, our solar system on the other.
• 2:11:21
  Speaker 1: What the Oort cloud suggests, and what Oumuamua and Borisov
• 2:11:26
  Speaker 1: made concrete, is that the line was never clean. Objects
• 2:11:31
  Speaker 1: cross it. They have always crossed it. The Sun has
• 2:11:35
  Speaker 1: been exhaling comets into interstellar space for billions of years,
• 2:11:40
  Speaker 1: and interstellar space has been sending material back. If material
• 2:11:45
  Speaker 1: can cross between star systems, then some fraction of what
• 2:11:49
  Speaker 1: we call the Oort cloud may have never originated around
• 2:11:53
  Speaker 1: our Sun at all. The Oort cloud is not a
• 2:11:56
  Speaker 1: permanent fixture. It is eroding right now, has been eroding
• 2:12:01
  Speaker 1: since the day it formed, and will continue eroding until
• 2:12:05
  Speaker 1: there is nothing left to erode. The process is slow
• 2:12:09
  Speaker 1: by human standards. It is not slow by any other
• 2:12:13
  Speaker 1: standard that matters. Start with the losses that are already happening.
• 2:12:19
  Speaker 1: Every time galactic tides nudge an outer cloud object into
• 2:12:23
  Speaker 1: an unstable orbit, That object either falls inward toward the Sun,
• 2:12:29
  Speaker 1: or drifts outward past the point where the Sun's gravity
• 2:12:32
  Speaker 1: can hold it. Either way, it does not come back.
• 2:12:36
  Speaker 1: Every passing star that perturbs a cluster of outer cloud
• 2:12:41
  Speaker 1: objects ejects some fraction of them into interstellar space permanently.
• 2:12:46
  Speaker 1: They become the galaxy's objects now, no longer the Sun's,
• 2:12:51
  Speaker 1: and every comet that falls all the way inward into
• 2:12:54
  Speaker 1: the heat and gravity of the inner solar system, eventually
• 2:12:58
  Speaker 1: meets one of a few endings. It gets shattered by
• 2:13:02
  Speaker 1: tidal forces. It gets absorbed into a planet's atmosphere. It
• 2:13:08
  Speaker 1: sublimes away, orbit by orbit, each pass around the Sun
• 2:13:13
  Speaker 1: boiling off more of its ice until there is nothing,
• 2:13:17
  Speaker 1: left but a scattered trail of debris. Whatever it was,
• 2:13:23
  Speaker 1: It is gone from the reservoir forever. These individual losses
• 2:13:28
  Speaker 1: are small. One object here, a handful there. But the
• 2:13:33
  Speaker 1: cloud contains perhaps a trillion objects, and it has been
• 2:13:37
  Speaker 1: losing them for 4.6 billion years. Models that track this
• 2:13:42
  Speaker 1: erosion suggest the Oort cloud today may hold only a
• 2:13:46
  Speaker 1: fraction of what it originally contained. when the solar system
• 2:13:50
  Speaker 1: first settled into its current configuration. Some estimates put the
• 2:13:55
  Speaker 1: losses as high as 90% of the original population. The
• 2:13:59
  Speaker 1: cloud you would have seen 4 billion years ago was
• 2:14:02
  Speaker 1: vastly denser, vastly more populated. What exists now is already
• 2:14:09
  Speaker 1: the depleted version. It is still enormous by any human measure.
• 2:14:14
  Speaker 1: But it is a smaller thing than it was. The
• 2:14:17
  Speaker 1: pace of loss is not constant. When a star passes close,
• 2:14:22
  Speaker 1: the ejection rate spikes. The approach of Gliese 710, roughly 1.3
• 2:14:29
  Speaker 1: million years from now, will send a wave of perturbations
• 2:14:34
  Speaker 1: through the outer cloud, unlike anything in recent solar history.
• 2:14:38
  Speaker 1: Some fraction of the objects disturbed during that encounter will
• 2:14:42
  Speaker 1: be lost outright. Others will be sent inward as a
• 2:14:46
  Speaker 1: comet shower. a period of elevated comet flux in the
• 2:14:50
  Speaker 1: inner solar system that could last hundreds of thousands of years.
• 2:14:54
  Speaker 1: After the encounter passes, the loss rate drops back down.
• 2:14:59
  Speaker 1: Then another star approaches. Then another molecular cloud crosses a
• 2:15:04
  Speaker 1: close enough path. The erosion is punctuated, uneven, driven by
• 2:15:10
  Speaker 1: events the Oort cloud has no defense against. Now look
• 2:15:14
  Speaker 1: further forward. Not millions of years, but billions. In roughly
• 2:15:20
  Speaker 1: 5 billion years, the Sun will exhaust the hydrogen fuel
• 2:15:25
  Speaker 1: at its core. The fusion that has powered it since
• 2:15:28
  Speaker 1: before the Earth existed will falter. The core will contract,
• 2:15:33
  Speaker 1: and the outer layers will do what outer layers of
• 2:15:36
  Speaker 1: dying stars do. They will expand. The Sun will swell
• 2:15:41
  Speaker 1: into a red giant. growing to a size that will
• 2:15:44
  Speaker 1: swallow Mercury and Venus, and possibly Earth itself. This is
• 2:15:49
  Speaker 1: the same process that made Betelgeuse the enormous bloated thing
• 2:15:54
  Speaker 1: it is today. Our Sun will go through a version
• 2:15:58
  Speaker 1: of the same transformation. When that happens, the Sun will
• 2:16:02
  Speaker 1: not just expand, it will also shed mass. Red giants
• 2:16:08
  Speaker 1: lose enormous quantities of material through stellar winds, bleeding their
• 2:16:13
  Speaker 1: outer layers into space over millions of years. For the
• 2:16:17
  Speaker 1: Oort cloud, this matters enormously. The strength of the Sun's
• 2:16:22
  Speaker 1: gravitational grip depends on its mass. As the Sun sheds mass,
• 2:16:27
  Speaker 1: that grip weakens. Objects that were marginally stable at 80,000
• 2:16:33
  Speaker 1: or 100,000 AU become unstable. Their orbits widen, slow, and
• 2:16:39
  Speaker 1: eventually detach entirely. The outer cloud does not collapse inward
• 2:16:44
  Speaker 1: during this process. It disperses outward, objects drifting away from
• 2:16:50
  Speaker 1: a sun that can no longer hold them, becoming part
• 2:16:53
  Speaker 1: of the general field of interstellar debris. The inner cloud,
• 2:16:58
  Speaker 1: denser and more tightly bound, will fare somewhat better for
• 2:17:02
  Speaker 1: a while. But the sun's red giant phase also means
• 2:17:06
  Speaker 1: a much larger, much hotter stellar surface, radiating energy outward.
• 2:17:12
  Speaker 1: Objects that have been frozen for billions of years, preserved
• 2:17:16
  Speaker 1: intact since the early solar system, will begin to experience
• 2:17:21
  Speaker 1: warmth they have never felt. Some of the innermost Oort
• 2:17:25
  Speaker 1: cloud objects may be vaporized or significantly altered as the
• 2:17:29
  Speaker 1: habitable zone sweeps outward past orbits it was never close
• 2:17:34
  Speaker 1: to before. After the red giant phase, the Sun will
• 2:17:38
  Speaker 1: shed its outer layers entirely in a brief A brilliant
• 2:17:42
  Speaker 1: burst called a planetary nebula. What remains is the core.
• 2:17:47
  Speaker 1: A white dwarf. dense, small and dim. It will contain
• 2:17:53
  Speaker 1: perhaps half or slightly more of the Sun's current mass.
• 2:17:58
  Speaker 1: The gravitational field reorganises around this smaller, dimmer remnant. Whatever
• 2:18:04
  Speaker 1: Oort cloud objects survive the red giant phase, and are
• 2:18:08
  Speaker 1: still loosely orbiting this remnant will find themselves in altered circumstances.
• 2:18:14
  Speaker 1: Some will eventually drift away over further billions of years.
• 2:18:19
  Speaker 1: Some may persist in highly eccentric orbits around a cold
• 2:18:23
  Speaker 1: white dwarf for an almost incomprehensible span of time. But
• 2:18:28
  Speaker 1: the cloud as a functioning, self-replenishing system will be gone.
• 2:18:33
  Speaker 1: There will be no mechanism to replace what has been lost.
• 2:18:37
  Speaker 1: No new objects forming in a protoplanetary disk. No ongoing
• 2:18:44
  Speaker 1: process of ejection from a living planetary system. What is
• 2:18:49
  Speaker 1: left in the very far future is a cold stellar
• 2:18:54
  Speaker 1: remnant surrounded by dispersed debris. Not a cloud, not a reservoir.
• 2:19:01
  Speaker 1: Just the last scattered objects on their slow, solitary orbits.
• 2:19:07
  Speaker 1: Long after the last comet has fallen inward, and the
• 2:19:10
  Speaker 1: last scattered object has drifted away, the outer edge of
• 2:19:15
  Speaker 1: the solar system will be gone, and there will be
• 2:19:18
  Speaker 1: nothing left to mark where it was. There is a
• 2:19:21
  Speaker 1: version of the solar system that most people carry around
• 2:19:25
  Speaker 1: in their heads, and it is almost entirely wrong in
• 2:19:28
  Speaker 1: one specific way. Not wrong about the names of the planets,
• 2:19:33
  Speaker 1: not wrong about the order they appear in, or the
• 2:19:36
  Speaker 1: rough fact that they orbit a star. Wrong about scale.
• 2:19:41
  Speaker 1: Wrong in a way that is so fundamental it changes
• 2:19:44
  Speaker 1: what the solar system actually is. Every diagram ever printed
• 2:19:49
  Speaker 1: in a school textbook, every animation that shows the planet
• 2:19:54
  Speaker 1: spinning around the sun, every poster hanging in a classroom,
• 2:19:58
  Speaker 1: they all commit the same distortion. They have to. If
• 2:20:02
  Speaker 1: you drew the solar system to true scale, the planets
• 2:20:06
  Speaker 1: would be invisible. You would need a sheet of paper
• 2:20:09
  Speaker 1: the size of a city block just to fit Neptune
• 2:20:12
  Speaker 1: at a readable size, and even then Earth would be
• 2:20:15
  Speaker 1: a speck too small to see without magnification. Here is
• 2:20:20
  Speaker 1: a version of that scale you can hold in your mind.
• 2:20:24
  Speaker 1: Imagine the sum reduced to a single printed period, the
• 2:20:29
  Speaker 1: kind at the end of a sentence on this page.
• 2:20:32
  Speaker 1: At that scale, Earth's orbit around the Sun is a
• 2:20:36
  Speaker 1: circle with a diameter of roughly one millimeter. Neptune sits
• 2:20:41
  Speaker 1: about three centimeters out. The Kuiper belt, the region that
• 2:20:45
  Speaker 1: includes Pluto and thousands of other icy bodies, ends at
• 2:20:50
  Speaker 1: maybe six or seven centimeters from that period. Everything, every planet,
• 2:20:57
  Speaker 1: every moon, every asteroid belt, The entire region of the
• 2:21:01
  Speaker 1: solar system that humans have explored with spacecraft. All of
• 2:21:05
  Speaker 1: it fits inside a circle smaller than a grain of
• 2:21:09
  Speaker 1: sand in that model. And the outer Oort cloud? At
• 2:21:13
  Speaker 1: that same scale, the outer Oort cloud forms a sphere
• 2:21:17
  Speaker 1: roughly 40 feet across. The period sits at the center
• 2:21:21
  Speaker 1: of a room. And the edge of the solar system
• 2:21:26
  Speaker 1: is somewhere outside the building. That is not a poetic exaggeration.
• 2:21:32
  Speaker 1: That is the actual geometry. The planets are not the
• 2:21:36
  Speaker 1: solar system. They are a tiny warm core at the
• 2:21:40
  Speaker 1: center of something almost incomprehensibly larger. A gravitational domain that
• 2:21:47
  Speaker 1: stretches outward for 100,000 AU in every direction. Everything inside
• 2:21:53
  Speaker 1: that domain, every object still orbiting the Sun rather than
• 2:21:58
  Speaker 1: the galaxy, is part of this solar system. Most of
• 2:22:02
  Speaker 1: it has never been mapped. Most of it has never
• 2:22:05
  Speaker 1: been seen. What does this mean for the search for
• 2:22:09
  Speaker 1: life elsewhere? It changes the framing entirely. When people talk
• 2:22:15
  Speaker 1: about searching for life in other star systems, the assumption
• 2:22:19
  Speaker 1: is usually that we are well inside our own and
• 2:22:22
  Speaker 1: they are somewhere out there across a vast and empty gulf.
• 2:22:26
  Speaker 1: But the nearest star system, Alpha Centauri, is about 4.2
• 2:22:32
  Speaker 1: light years away. The outer Oort cloud extends to somewhere
• 2:22:36
  Speaker 1: around 1.5 light years from the Sun. That means, at
• 2:22:41
  Speaker 1: its true outer edge, our solar system reaches roughly a
• 2:22:46
  Speaker 1: third of the way to the nearest star. Alpha Centauri
• 2:22:50
  Speaker 1: almost certainly has its own equivalent of an Oort cloud,
• 2:22:54
  Speaker 1: its own shell of distant icy debris, extending outward from
• 2:22:58
  Speaker 1: its gravitational center. It is not impossible that the outermost
• 2:23:03
  Speaker 1: edges of these two systems are closer to each other
• 2:23:06
  Speaker 1: than the outer Oort cloud is to the Sun's own planets.
• 2:23:12
  Speaker 1: The boundary between our solar system and interstellar space is
• 2:23:16
  Speaker 1: not a clean wall. It is a zone where the
• 2:23:19
  Speaker 1: Sun's gravitational grip fades slowly into the competing pulls of
• 2:23:24
  Speaker 1: the galaxy. and its neighboring stars. This has consequences for
• 2:23:30
  Speaker 1: how we think about interstellar travel. The popular image is
• 2:23:34
  Speaker 1: a spacecraft leaving Earth, passing the outer planets, and then
• 2:23:39
  Speaker 1: crossing a kind of threshold into the void between stars.
• 2:23:44
  Speaker 1: But there is no threshold. There is the heliosphere, where
• 2:23:48
  Speaker 1: the solar wind stops, which Voyager 1 crossed around 120 AU,
• 2:23:54
  Speaker 1: And then there is the gravitational edge of the solar system,
• 2:23:58
  Speaker 1: which is not at 120 AU, but somewhere around 100,000 AU.
• 2:24:04
  Speaker 1: A spacecraft that crossed the heliopause and entered interstellar space
• 2:24:09
  Speaker 1: in the physical sense is still, gravitationally, inside the solar system.
• 2:24:15
  Speaker 1: It is still moving through territory that belongs to the sun.
• 2:24:19
  Speaker 1: spacecraft would have to travel a thousand times further than
• 2:24:23
  Speaker 1: Voyager has gone to reach the gravitational boundary of our
• 2:24:27
  Speaker 1: own solar system. Interstellar travel does not begin at the
• 2:24:32
  Speaker 1: edge of the planets. It begins at the edge of
• 2:24:35
  Speaker 1: the Oort cloud, and that edge is a light year
• 2:24:39
  Speaker 1: and a half away. The scale also reshapes what it
• 2:24:43
  Speaker 1: means to be a small rocky planet orbiting a medium-sized star.
• 2:24:49
  Speaker 1: We tend to think of Earth as sitting inside the
• 2:24:52
  Speaker 1: solar system. In one sense, that is true. In another sense,
• 2:24:57
  Speaker 1: Earth is pressed right up against the Sun, huddled in
• 2:25:00
  Speaker 1: the innermost fraction of a percent of the Sun's domain,
• 2:25:05
  Speaker 1: so close to the central star that the vast majority
• 2:25:08
  Speaker 1: of the solar system lies beyond us in all directions.
• 2:25:12
  Speaker 1: The cold, dark outer reaches of the cloud are far
• 2:25:16
  Speaker 1: more representative of what the solar system actually is than
• 2:25:20
  Speaker 1: the bright inner zone where the planets orbit. The planets
• 2:25:25
  Speaker 1: are the exception. The cold and dark are the rule.
• 2:25:29
  Speaker 1: This is the thing that no textbook diagram has ever conveyed.
• 2:25:34
  Speaker 1: The solar system is not a warm, lit region with
• 2:25:37
  Speaker 1: a dark void beginning somewhere past Neptune. It is an
• 2:25:42
  Speaker 1: enormous sphere of gravitationally bound objects, most of them frozen,
• 2:25:48
  Speaker 1: most of them dark, most of them silent, extending outward
• 2:25:52
  Speaker 1: from the Sun for a distance that approaches the space
• 2:25:56
  Speaker 1: between stars. The sunlit planetary zone is a small interior room.
• 2:26:02
  Speaker 1: The Oort cloud is everything else, and everything else is
• 2:26:06
  Speaker 1: most of it. The full extent of what the Sun
• 2:26:10
  Speaker 1: holds the complete size of our local domain is larger
• 2:26:14
  Speaker 1: than any illustration has ever managed to show. We started
• 2:26:19
  Speaker 1: with the assumption that the solar system ends somewhere near Pluto.
• 2:26:24
  Speaker 1: That assumption is built into every diagram, every classroom poster,
• 2:26:30
  Speaker 1: every animation that shows eight planets circling a central star
• 2:26:34
  Speaker 1: and then just stops. The image is clean and confident
• 2:26:39
  Speaker 1: and almost entirely misleading. What the Oort cloud reveals is
• 2:26:43
  Speaker 1: not just a larger solar system, it reveals how narrow
• 2:26:47
  Speaker 1: our picture of it has always been. Think about what
• 2:26:51
  Speaker 1: humanity has actually managed to do in space. We have
• 2:26:55
  Speaker 1: telescopes positioned beyond Earth's atmosphere that can resolve the light
• 2:27:01
  Speaker 1: of galaxies 12 billion light years away. We have landed
• 2:27:06
  Speaker 1: instruments on the surface of a comet. We have mapped
• 2:27:10
  Speaker 1: the faint temperature variations in the cosmic microwave background, the
• 2:27:15
  Speaker 1: oldest light in the universe, radiation from roughly 380,000 years
• 2:27:21
  Speaker 1: after the Big Bang. We have sent spacecraft out of
• 2:27:25
  Speaker 1: the planetary system entirely. Voyager 1 is crossing interstellar space
• 2:27:31
  Speaker 1: right now, past the heliopause, in the region between stars.
• 2:27:37
  Speaker 1: By almost any measure, human astronomy has reached things so
• 2:27:41
  Speaker 1: far away they are nearly incomprehensible. And yet, the outer
• 2:27:48
  Speaker 1: Oort cloud sits roughly one to one and a half
• 2:27:51
  Speaker 1: light years away, not billions, not millions, one to one
• 2:27:56
  Speaker 1: and a half. a fraction of the distance to the
• 2:27:59
  Speaker 1: nearest star, the closest truly deep astronomical unknown in existence,
• 2:28:06
  Speaker 1: and we have never directly seen a single object in it,
• 2:28:10
  Speaker 1: not one. Every estimate of how many objects are out there,
• 2:28:15
  Speaker 1: every map of its two regions, every calculation of its
• 2:28:19
  Speaker 1: total mass is built on inference. We are reading the
• 2:28:24
  Speaker 1: cloud from its debris, the comets it sends inward, the
• 2:28:29
  Speaker 1: orbital signatures they carry. We have never looked at the source.
• 2:28:34
  Speaker 1: The Oort cloud is right here surrounding the solar system
• 2:28:38
  Speaker 1: on all sides and it remains completely invisible to us.
• 2:28:43
  Speaker 1: This is the strange situation astronomy finds itself in. The
• 2:28:48
  Speaker 1: universe at large is in many ways better observed than
• 2:28:52
  Speaker 1: our own backyard. That is not a figure of speech.
• 2:28:56
  Speaker 1: The Hubble Space Telescope spent a million seconds staring at
• 2:29:00
  Speaker 1: a patch of sky the apparent size of a grain
• 2:29:04
  Speaker 1: of sand held at arm's length and found 10,000 galaxies
• 2:29:09
  Speaker 1: in it. We have chemical profiles of stars hundreds of
• 2:29:13
  Speaker 1: light years away. We have detected planets orbiting other suns
• 2:29:18
  Speaker 1: using the faint dimming of starlight. All of that And
• 2:29:22
  Speaker 1: the reservoir enclosing our own solar system remains a theoretical object,
• 2:29:28
  Speaker 1: named after a man who deduced it from 19 comet orbits.
• 2:29:33
  Speaker 1: Never photographed, never resolved, just inferred. What makes this genuinely
• 2:29:39
  Speaker 1: unsettling is the scale problem. The Oort cloud is not
• 2:29:44
  Speaker 1: difficult to observe because it is far away in the
• 2:29:47
  Speaker 1: way that distant galaxies are far away. It is difficult
• 2:29:51
  Speaker 1: to observe because its objects are small, dark, cold and
• 2:29:55
  Speaker 1: spread across an almost inconceivably large volume of space. The
• 2:30:00
  Speaker 1: average separation between neighbouring Oort cloud objects is tens of
• 2:30:05
  Speaker 1: millions of kilometres. Even if you aimed every telescope on
• 2:30:09
  Speaker 1: Earth at the right patch of sky, you would likely
• 2:30:13
  Speaker 1: see nothing. The cloud does not glow. It does not emit.
• 2:30:18
  Speaker 1: It sits in frozen silence at temperatures only a few
• 2:30:22
  Speaker 1: degrees above absolute zero, waiting. There is no signal. There
• 2:30:28
  Speaker 1: is no edge that lights up. There is just the dark.
• 2:30:32
  Speaker 1: Every comet that has ever crossed the night sky was
• 2:30:36
  Speaker 1: a piece of that dark falling inward. Ancient observers in China,
• 2:30:42
  Speaker 1: in Babylon, in Egypt watched bright comets appear without warning.
• 2:30:47
  Speaker 1: and recorded them as omens, as signs of disaster or
• 2:30:52
  Speaker 1: of divine attention. They were not wrong that something significant
• 2:30:56
  Speaker 1: was happening. They just did not know what. What they
• 2:31:00
  Speaker 1: were watching was a fragment of original solar system material.
• 2:31:05
  Speaker 1: Ice and dust preserved unchanged for four and a half
• 2:31:08
  Speaker 1: billion years, making its first journey toward the sun since
• 2:31:13
  Speaker 1: the planets formed. Some of those comets had never been
• 2:31:17
  Speaker 1: close to the Sun before. Their surfaces had never been heated.
• 2:31:22
  Speaker 1: Their ice had never sublimated. And then, a passing star
• 2:31:26
  Speaker 1: or a ripple in the galactic tide nudged their orbit
• 2:31:30
  Speaker 1: just slightly, and they began the long fall inward, taking
• 2:31:36
  Speaker 1: millions of years to arrive, and finally appearing in the
• 2:31:40
  Speaker 1: inner solar system as a bright smear of light, that
• 2:31:45
  Speaker 1: lasted a few weeks before disappearing back into darkness. The
• 2:31:50
  Speaker 1: cloud is still out there. Right now, in every direction.
• 2:31:56
  Speaker 1: Trillions of objects in slow, wide orbits that take hundreds
• 2:32:01
  Speaker 1: of thousands of years to complete. Most of them will
• 2:32:05
  Speaker 1: never fall inward. Most will drift in their distant loops
• 2:32:09
  Speaker 1: until the sun swells and dies and the cloud disperses.
• 2:32:14
  Speaker 1: But some of them are already falling. Somewhere in the
• 2:32:18
  Speaker 1: outer cloud right now, an orbit is being disturbed. An
• 2:32:22
  Speaker 1: object that has been in the same slow path for
• 2:32:25
  Speaker 1: four billion years is beginning a new trajectory, one that
• 2:32:30
  Speaker 1: will eventually carry it past Neptune, past Jupiter, past the
• 2:32:35
  Speaker 1: asteroid belt. into the inner solar system where the sun's
• 2:32:39
  Speaker 1: heat will boil its surface and build a tail stretching
• 2:32:44
  Speaker 1: millions of kilometers behind it. That process has probably already begun,
• 2:32:50
  Speaker 1: for comets we will see centuries from now. The solar
• 2:32:54
  Speaker 1: system ends somewhere out in the dark, more than a
• 2:32:57
  Speaker 1: light year from the sun, and every time a comet
• 2:33:01
  Speaker 1: falls inward out of that dark and crosses the sky
• 2:33:05
  Speaker 1: above us, It is sending a fragment of that answer
• 2:33:10
  Speaker 1: our way.