Latest / Night Science Stories / Where Does the Solar System Actually End

Transcript

Where Does the Solar System Actually End · Night Science Stories

• 0:00
  Speaker 1: Good evening, and welcome to the Sleepy Scientist. I'm very
• 0:04
  Speaker 1: glad you've chosen to spend a little of your night here.
• 0:08
  Speaker 1: Before we travel too far from home, I'd love to
• 0:11
  Speaker 1: know where you're listening from tonight. Perhaps you're already tucked
• 0:14
  Speaker 1: into bed. Perhaps you're stretched out on the sofa. Or
• 0:18
  Speaker 1: maybe this is keeping you company during one of those
• 0:21
  Speaker 1: quiet hours when the rest of the house has gone still.
• 0:24
  Speaker 1: Wherever you happen to be, get comfortable. Let your shoulders relax.
• 0:29
  Speaker 1: and allow everything outside this moment to become a little
• 0:32
  Speaker 1: less important. And if you prefer these episodes without a
• 0:37
  Speaker 1: glowing screen nearby, The Sleepy Scientist is also on Spotify.
• 0:42
  Speaker 1: You'll find the link waiting for you in the description,
• 0:45
  Speaker 1: so whenever you're ready, you can put the phone down,
• 0:48
  Speaker 1: close your eyes, and simply listen. If these slow journeys
• 0:52
  Speaker 1: through science have become part of your evenings, leaving a
• 0:55
  Speaker 1: like or subscribing is a small thing that makes an
• 0:58
  Speaker 1: enormous difference to the channel. And to everyone who has
• 1:02
  Speaker 1: already done that, thank you. It's wonderful knowing that these
• 1:07
  Speaker 1: strange little trips through the universe are being shared with
• 1:10
  Speaker 1: so many of you. Tonight, we're going to travel outward,
• 1:15
  Speaker 1: past Saturn and its rings, beyond the methane lakes of
• 1:18
  Speaker 1: Titan and the icy geysers of Enceladus, past Uranus and Neptune.
• 1:24
  Speaker 1: and into regions where planets give way to frozen worlds,
• 1:28
  Speaker 1: the solar wind fades into interstellar space, and even the
• 1:32
  Speaker 1: sun's gravity begins to compete with the pull of the
• 1:35
  Speaker 1: wider galaxy. I'll try not to make any of that
• 1:39
  Speaker 1: too exciting, although discovering that the solar system may extend
• 1:44
  Speaker 1: more than a light year from the sun is perhaps
• 1:46
  Speaker 1: not ideal material for an early night. So, make yourself comfortable,
• 1:53
  Speaker 1: and let's begin. Where does the solar system actually end?
• 1:59
  Speaker 1: Most of us first meet the solar system in a
• 2:02
  Speaker 1: diagram that looks wonderfully tidy. The sun sits in the middle,
• 2:06
  Speaker 1: the planets are arranged around it in neat little orbits,
• 2:10
  Speaker 1: and somewhere beyond Neptune the picture simply seems to stop.
• 2:14
  Speaker 1: It is useful for learning the names of the planets,
• 2:17
  Speaker 1: but as a map of the solar system itself, it
• 2:20
  Speaker 1: leaves almost everything out. The real solar system is is
• 2:24
  Speaker 1: vastly larger, far emptier, and much harder to give a
• 2:29
  Speaker 1: clean-out of boundary. If you asked where the solar system ends,
• 2:34
  Speaker 1: one perfectly reasonable answer would be Neptune. Neptune is the
• 2:40
  Speaker 1: eighth and most distant major planet, orbiting the Sun at
• 2:43
  Speaker 1: an average distance of around 30 astronomical units. One astronomical
• 2:49
  Speaker 1: unit is roughly the average distance between Earth and the Sun,
• 2:53
  Speaker 1: about 150 million kilometres. By the time we reach Neptune,
• 3:00
  Speaker 1: we are already about 4.5 billion kilometres from the Sun.
• 3:04
  Speaker 1: But if Neptune is the end, then Pluto would somehow
• 3:08
  Speaker 1: be outside the solar system for much of its orbit,
• 3:11
  Speaker 1: along with countless other icy objects that unquestionably orbit the Sun.
• 3:17
  Speaker 1: So Neptune cannot really be the whole answer. Beyond Neptune
• 3:23
  Speaker 1: lies the Kuiper Belt, a broad region filled with frozen
• 3:27
  Speaker 1: remnants left over from the formation of the planets. Pluto
• 3:31
  Speaker 1: belongs to this population, along with dwarf planets and smaller
• 3:35
  Speaker 1: bodies made largely from rock and ices. The main part
• 3:40
  Speaker 1: of the Kuiper Belt extends for tens of astronomical units
• 3:43
  Speaker 1: beyond Neptune. If we define the solar system by the
• 3:48
  Speaker 1: family of substantial objects orbiting the Sun, in a broad,
• 3:52
  Speaker 1: flattened region, then perhaps this is where the edge begins
• 3:56
  Speaker 1: to make more sense. Except the Sun reaches much farther still.
• 4:02
  Speaker 1: The Sun is constantly releasing a stream of charged particles
• 4:07
  Speaker 1: known as the solar wind. These particles race outward in
• 4:11
  Speaker 1: every direction and carry part of the Sun's magnetic field
• 4:14
  Speaker 1: with them. Together they create an enormous bubble around the
• 4:19
  Speaker 1: planets called the heliosphere. Eventually, the solar wind becomes so
• 4:24
  Speaker 1: weak that it can no longer dominate the surrounding interstellar environment.
• 4:30
  Speaker 1: The boundary where this happens is called the heliopause. This
• 4:34
  Speaker 1: is a particularly interesting candidate for the edge of the
• 4:37
  Speaker 1: solar system, because it is something spacecraft have actually crossed.
• 4:42
  Speaker 1: Voyager 1 entered the region beyond the heliopause in 2012,
• 4:46
  Speaker 1: and Voyager 2 crossed it in 2018. Both spacecraft had
• 4:52
  Speaker 1: travelled more than 100 astronomical units from the Sun before
• 4:57
  Speaker 1: entering what scientists describe as interstellar space. That sounds fairly conclusive.
• 5:04
  Speaker 1: If a spacecraft has entered interstellar space, surely it has
• 5:08
  Speaker 1: left the solar system. Not quite. It has left the
• 5:13
  Speaker 1: Sun's solar wind behind. It has not left the Sun's
• 5:18
  Speaker 1: gravity behind. Gravity does not simply switch off at a
• 5:21
  Speaker 1: particular distance. The gravitational attraction of the Sun becomes weaker
• 5:26
  Speaker 1: and weaker as you move farther away, but there is
• 5:29
  Speaker 1: no invisible wall where it suddenly disappears. Even beyond the heliopause,
• 5:36
  Speaker 1: extremely distant objects can remain gravitationally bound to the Sun,
• 5:40
  Speaker 1: and that takes us to a much more distant possible edge.
• 5:44
  Speaker 1: Far beyond the Kuiper Belt, astronomers think there is an
• 5:47
  Speaker 1: immense population of frozen bodies known as the Oort Cloud.
• 5:52
  Speaker 1: Unlike the relatively flat arrangement of the planets in Kuiper Belt,
• 5:56
  Speaker 1: the Oort Cloud is thought to form something closer to
• 5:59
  Speaker 1: a vast spherical shell surrounding the solar system. We have
• 6:04
  Speaker 1: never directly seen the Oort Cloud as a complete structure.
• 6:08
  Speaker 1: Its existence is inferred largely from the behaviour of long-period comets,
• 6:13
  Speaker 1: which can arrive from enormous distances and from many different directions.
• 6:18
  Speaker 1: Estimates vary, but the outer Oort cloud may extend tens
• 6:22
  Speaker 1: of thousands of astronomical units from the Sun, perhaps reaching
• 6:26
  Speaker 1: something on the order of 100,000 astronomical units in its
• 6:30
  Speaker 1: most distant regions. At those scales, we are no longer
• 6:34
  Speaker 1: talking about a little extension beyond Neptune. We are talking
• 6:38
  Speaker 1: about distances measured in a significant fraction of a light year.
• 6:43
  Speaker 1: And even there, drawing a sharp boundary becomes difficult. At
• 6:48
  Speaker 1: such enormous distances, the Sun's gravitational hold becomes increasingly vulnerable
• 6:54
  Speaker 1: to disturbances from passing stars and from the gravitational field
• 6:58
  Speaker 1: of the Milky Way itself. An object may remain loosely
• 7:03
  Speaker 1: bound to the Sun for millions or billions of years,
• 7:06
  Speaker 1: only for a passing star to alter its orbit enough
• 7:09
  Speaker 1: to send it inward, push it farther outward. or remove
• 7:14
  Speaker 1: it from the solar system altogether. So the outer edge
• 7:19
  Speaker 1: is not really a wall. It is more like a
• 7:22
  Speaker 1: gradually fading border between one gravitational neighbourhood and another. This
• 7:28
  Speaker 1: is why several different statements can all be correct at
• 7:31
  Speaker 1: the same time. Neptune is the end of the major planets.
• 7:36
  Speaker 1: The Kuiper belt marks the distant realm of many of
• 7:39
  Speaker 1: the solar system's known icy worlds. The heliopause marks the
• 7:44
  Speaker 1: outer edge of the Sun's solar wind bubble and the
• 7:47
  Speaker 1: beginning of interstellar space. And the Oort cloud may represent
• 7:51
  Speaker 1: the farthest meaningful reach of the population still gravitationally associated
• 7:56
  Speaker 1: with the Sun. To understand how quickly the familiar solar
• 8:01
  Speaker 1: system begins to change, we can start much closer in.
• 8:05
  Speaker 1: Saturn is already almost ten times farther from the Sun
• 8:08
  Speaker 1: than Earth is. Sunlight is much weaker there. Journeys take
• 8:13
  Speaker 1: years rather than months. Temperatures are low enough for substances
• 8:17
  Speaker 1: that would behave very differently near Earth to remain frozen
• 8:21
  Speaker 1: across moons and rings. And surrounding Saturn are worlds that
• 8:25
  Speaker 1: immediately show us just how strange the outer solar system becomes.
• 8:30
  Speaker 1: There are enormous rings made from countless pieces of ice.
• 8:34
  Speaker 1: There is Titan, where methane forms clouds, falls as rain,
• 8:39
  Speaker 1: carves channels and gathers in lakes and seas. there is Enceladus,
• 8:45
  Speaker 1: a small frozen moon-spraying material from a hidden ocean directly
• 8:49
  Speaker 1: into space. From there, Uranus and Neptune wait farther out,
• 8:55
  Speaker 1: followed not by emptiness, but by an enormous territory of
• 8:59
  Speaker 1: frozen objects. Invisible boundaries, ancient comets and distances so large
• 9:05
  Speaker 1: that the eight planets begin to look less like the
• 9:08
  Speaker 1: whole solar system and more like a tightly packed little
• 9:11
  Speaker 1: group gathered very close to its centre. The solar system
• 9:16
  Speaker 1: does have edges. The difficulty is deciding which edge we mean.
• 9:23
  Speaker 1: Once we move beyond the familiar planets, distance begins to
• 9:26
  Speaker 1: behave almost like a different subject altogether. On Earth, we
• 9:31
  Speaker 1: are comfortable with kilometres. A journey of 100 kilometres feels substantial.
• 9:38
  Speaker 1: A journey of 1,000 kilometres can take us across countries.
• 9:42
  Speaker 1: even the circumference of Earth at roughly 40,000 kilometres, is
• 9:47
  Speaker 1: large enough to feel difficult to picture clearly. But those
• 9:51
  Speaker 1: numbers become almost useless once we begin moving through the
• 9:55
  Speaker 1: solar system. Astronomers therefore use a much larger unit called
• 10:00
  Speaker 1: the astronomical unit. One astronomical unit is approximately the average
• 10:06
  Speaker 1: distance between Earth and the Sun, around 150 million kilometres.
• 10:12
  Speaker 1: That single distance is already large enough that sunlight– travelling
• 10:17
  Speaker 1: at almost 300,000 kilometres every second, takes a little over
• 10:22
  Speaker 1: eight minutes to cross it. So when you look at
• 10:25
  Speaker 1: the Sun, you are not seeing it exactly as it
• 10:28
  Speaker 1: is now. You are seeing sunlight that began its journey
• 10:33
  Speaker 1: several minutes earlier. At first the astronomical unit makes the
• 10:37
  Speaker 1: solar system easier to describe. Earth sits roughly one astronomical
• 10:43
  Speaker 1: unit from the Sun. Mars averages about 1.5 astronomical units away.
• 10:49
  Speaker 1: Jupiter is a little more than 5 astronomical units from
• 10:53
  Speaker 1: the Sun. Then we reach Saturn. Saturn orbits at an
• 10:58
  Speaker 1: average distance of about 9.5 astronomical units. In kilometres, that
• 11:04
  Speaker 1: is roughly 1.4 billion kilometres from the Sun. Even light
• 11:09
  Speaker 1: needs around 80 minutes to travel from the Sun to Saturn.
• 11:14
  Speaker 1: that immediately changes the feeling of the journey. A signal
• 11:18
  Speaker 1: from Earth cannot simply reach a spacecraft near Saturn instantly.
• 11:24
  Speaker 1: Depending on the positions of Earth, Saturn and the spacecraft,
• 11:28
  Speaker 1: radio communication takes well over an hour in one direction.
• 11:32
  Speaker 1: There is no possibility of steering a spacecraft there with
• 11:36
  Speaker 1: the kind of immediate response we might expect from an
• 11:39
  Speaker 1: aircraft or a vehicle on Earth. Commands have to be planned, transmitted,
• 11:45
  Speaker 1: and then patiently awaited. Farther out, Uranus orbits at roughly
• 11:52
  Speaker 1: 19 astronomical units. Sunlight reaching Uranus has travelled for more
• 11:57
  Speaker 1: than two and a half hours. By the time you
• 11:59
  Speaker 1: saw something happen there through a perfect telescope, that event
• 12:03
  Speaker 1: would already be hours old. Neptune lies farther still, at
• 12:09
  Speaker 1: an average distance close to 30 astronomical units. or around 4.5
• 12:14
  Speaker 1: billion kilometres from the Sun. Light takes a little over
• 12:19
  Speaker 1: four hours to travel from the Sun to Neptune. A
• 12:23
  Speaker 1: message sent from Earth toward a spacecraft near Neptune therefore
• 12:27
  Speaker 1: takes hours to arrive, and any reply takes hours more
• 12:31
  Speaker 1: to return. Even at the speed of light, a simple
• 12:35
  Speaker 1: exchange becomes an all-day conversation. And Neptune is still nowhere
• 12:40
  Speaker 1: near the end. The main Kuiper belt begins around the
• 12:44
  Speaker 1: region of Neptune's orbit and extends broadly outward to around
• 12:49
  Speaker 1: 50 or 55 astronomical units. This region contains Pluto and
• 12:55
  Speaker 1: many other icy bodies, along with objects occupying resonant, inclined
• 12:59
  Speaker 1: or eccentric orbits. Some members remain relatively close to this
• 13:04
  Speaker 1: broad zone. Others travel much farther from the Sun. The
• 13:09
  Speaker 1: scattered disk contains bodies with highly elongated orbits that can
• 13:13
  Speaker 1: carry them far beyond the main Kuiper belt. Detached objects
• 13:18
  Speaker 1: can reach still greater distances. By this stage, even the
• 13:22
  Speaker 1: astronomical unit begins to lose some of its intuitive usefulness.
• 13:26
  Speaker 1: 50 astronomical units is already about 7.5 billion kilometres. 100
• 13:33
  Speaker 1: astronomical units is roughly 15 billion kilometres. the numbers remain accurate,
• 13:40
  Speaker 1: but they stop behaving like distances we can naturally imagine.
• 13:45
  Speaker 1: The Voyager spacecraft helped make this scale slightly more tangible.
• 13:49
  Speaker 1: Voyager 1 and Voyager 2 both travelled far beyond the
• 13:53
  Speaker 1: planets before reaching the helipause, the boundary where the outward-flowing
• 13:57
  Speaker 1: solar wind gives way to the surrounding interstellar environment. Their
• 14:02
  Speaker 1: crossings took place at distances of roughly 120 astronomical units
• 14:05
  Speaker 1: from the Sun, although the exact distance differed between the
• 14:11
  Speaker 1: two spacecraft. That means the bubble produced by the Sun's
• 14:15
  Speaker 1: charged particles extends several times farther than Neptune's orbit. If
• 14:22
  Speaker 1: Neptune marks the end of the major planets at around
• 14:24
  Speaker 1: 30 astronomical units, then the heliopause lies roughly four times
• 14:29
  Speaker 1: farther from the Sun. And still, the solar system may
• 14:34
  Speaker 1: continue enormously farther. The Oort cloud is thought to begin
• 14:40
  Speaker 1: thousands of astronomical units from the Sun, and perhaps extend
• 14:44
  Speaker 1: outward to something on the order of 100,000 astronomical units.
• 14:50
  Speaker 1: At that point, astronomical units themselves begin to sound awkward. 100,000
• 14:56
  Speaker 1: astronomical units is around 15 trillion kilometres. So another unit
• 15:02
  Speaker 1: becomes useful. The light year is the distance light travels
• 15:06
  Speaker 1: in one year. roughly 9.5 trillion kilometres. An outer Oort
• 15:12
  Speaker 1: cloud extending toward 100,000 astronomical units would therefore reach roughly 1.5
• 15:18
  Speaker 1: light-years from the Sun. Suddenly, we are using a unit
• 15:23
  Speaker 1: normally associated with the distances between stars. The nearest star
• 15:28
  Speaker 1: system beyond our own, Alpha Centauri, lies a little more
• 15:33
  Speaker 1: than 4 light-years away. So if the most distant estimates
• 15:38
  Speaker 1: for the Oort cloud are approximately correct, the Sun's gravitational
• 15:42
  Speaker 1: family may extend a surprisingly large fraction of the way
• 15:46
  Speaker 1: toward neighbouring stars. This is one reason ordinary diagrams of
• 15:50
  Speaker 1: the solar system can be so misleading. The planets are
• 15:55
  Speaker 1: usually drawn much too large and far too close together.
• 15:59
  Speaker 1: If they were shown at their correct sizes and distances
• 16:02
  Speaker 1: on the same page, most of them would be little
• 16:04
  Speaker 1: more than invisible specks separated by enormous stretches of blank space.
• 16:10
  Speaker 1: And if we attempted to include the Oort cloud on
• 16:12
  Speaker 1: the same scale, the planetary region would collapse into a
• 16:16
  Speaker 1: tiny feature near the centre. A useful way to imagine
• 16:20
  Speaker 1: this is to compress the solar system. Suppose the distance
• 16:23
  Speaker 1: from the Sun to Earth were reduced to just one metre.
• 16:27
  Speaker 1: Saturn would then orbit roughly nine and a half metres away,
• 16:31
  Speaker 1: Uranus would be about 19 metres away. Neptune would sit
• 16:36
  Speaker 1: around 30 metres from the Sun. The main Kuiper belt
• 16:40
  Speaker 1: would stretch outward to roughly 50 metres. The heliopause would
• 16:44
  Speaker 1: lie around 120 metres away. But an outer Oort cloud
• 16:49
  Speaker 1: reaching 100,000 astronomical units would extend for around 100 kilometres
• 16:55
  Speaker 1: on that same scale. Earth, Saturn, Uranus, Neptune, Pluto, and
• 17:02
  Speaker 1: the entire familiar planetary system would all be clustered within
• 17:07
  Speaker 1: the first few dozen metres. The outer gravitational structure of
• 17:11
  Speaker 1: the solar system would continue across the distance of a
• 17:15
  Speaker 1: substantial road journey. This gradual change in the units we
• 17:19
  Speaker 1: used tells us something important. Close to the Sun, we
• 17:24
  Speaker 1: talk comfortably in millions of kilometres. Across the planets, astronomical
• 17:30
  Speaker 1: units become more useful. Farther out, tens of thousands of
• 17:35
  Speaker 1: astronomical units begin to blur into fractions of a light-year.
• 17:40
  Speaker 1: And with every change in scale, the meaning of the
• 17:43
  Speaker 1: solar system's edge changes with it. The planets occupy only
• 17:48
  Speaker 1: a small central part. The Kuiper belt extends farther. The
• 17:53
  Speaker 1: heliosphere reaches farther still in another sense. and the Sun's
• 17:58
  Speaker 1: gravitational influence may continue so far outward that the language
• 18:02
  Speaker 1: of planetary distance finally gives way to the language of
• 18:05
  Speaker 1: the stars. By the time we reach Saturn, the solar
• 18:11
  Speaker 1: system has begun to feel noticeably different. Saturn is the
• 18:15
  Speaker 1: sixth planet from the Sun and the second largest planet
• 18:19
  Speaker 1: in the solar system, smaller only than Jupiter. It orbits
• 18:24
  Speaker 1: at an average distance of roughly 9.5 astronomical units, meaning
• 18:29
  Speaker 1: it is almost 10 times farther from the Sun than
• 18:32
  Speaker 1: Earth is. At that distance, sunlight is far weaker. Temperatures
• 18:39
  Speaker 1: are much lower. Substances that behave as gases or liquids
• 18:43
  Speaker 1: in warmer parts of the solar system can remain frozen
• 18:47
  Speaker 1: across moons, rings and smaller bodies. Water becomes hard ice.
• 18:53
  Speaker 1: Ammonia can freeze. Methane can survive in environments that would
• 18:58
  Speaker 1: be impossible much closer to the Sun. This colder chemistry
• 19:02
  Speaker 1: becomes one of the defining features of the outer solar system.
• 19:06
  Speaker 1: Saturn itself, however, is not a giant frozen world. It
• 19:11
  Speaker 1: is a gas giant composed predominantly of hydrogen and helium,
• 19:16
  Speaker 1: the same two elements that dominate the Sun. There is
• 19:19
  Speaker 1: no solid surface where a spacecraft could simply land. As
• 19:24
  Speaker 1: we move downward through Saturn's atmosphere, the gas becomes progressively
• 19:29
  Speaker 1: denser under increasing pressure. Hydrogen is compressed into deeper fluid layers,
• 19:36
  Speaker 1: eventually reaching conditions where it can behave as metallic hydrogen,
• 19:40
  Speaker 1: an unusual state in which electrons move more freely through
• 19:43
  Speaker 1: the material. Farther toward the centre lies a dense region
• 19:48
  Speaker 1: containing heavier elements. probably including rock, ice and other materials
• 19:53
  Speaker 1: mixed under extraordinary pressure. The exact structure of Saturn's interior
• 19:59
  Speaker 1: is still being studied. It is far more complicated than
• 20:02
  Speaker 1: a neat rocky core surrounded by simple layers of gas.
• 20:08
  Speaker 1: Saturn is enormous. Its diameter is roughly 120,000 kilometres, making
• 20:15
  Speaker 1: it more than nine times wider than Earth. Yet despite
• 20:19
  Speaker 1: its size, Saturn is the least dense planet in the
• 20:23
  Speaker 1: solar system. Its average density is lower than that of
• 20:26
  Speaker 1: liquid water. This occasionally leads to the cheerful observation that
• 20:31
  Speaker 1: Saturn would float if someone could find an ocean large
• 20:35
  Speaker 1: enough to put it in. Finding the ocean would probably
• 20:39
  Speaker 1: be the more difficult part. Saturn also spins remarkably quickly.
• 20:45
  Speaker 1: One rotation takes only about ten and three quarter hours.
• 20:49
  Speaker 1: A planet more than nine times wider than Earth completes
• 20:53
  Speaker 1: a day in less than half the time Earth does.
• 20:57
  Speaker 1: That rapid rotation causes Saturn to bulge noticeably around its
• 21:01
  Speaker 1: equator and flatten near its poles. Its year is a
• 21:06
  Speaker 1: very different matter. Saturn takes almost 30 Earth years to
• 21:10
  Speaker 1: complete a single orbit around the Sun. A person somehow
• 21:14
  Speaker 1: born on Saturn at the beginning of one Saturnian year
• 21:18
  Speaker 1: would be approaching 30 years old before celebrating their first birthday,
• 21:23
  Speaker 1: according to the planet's orbit. But Saturn is more than
• 21:27
  Speaker 1: one planet travelling around the Sun. It sits at the
• 21:31
  Speaker 1: centre of an enormous miniature system of its own, There
• 21:34
  Speaker 1: are rings extending hundreds of thousands of kilometres across. There
• 21:39
  Speaker 1: are numerous moons ranging from tiny, irregular fragments to Titan,
• 21:44
  Speaker 1: a moon larger than the planet Mercury. There is Saturn's
• 21:48
  Speaker 1: magnetic field and the vast magnetosphere shaped by it. There
• 21:53
  Speaker 1: are streams of charged particles. There are gravitational resonances connecting
• 21:58
  Speaker 1: moons and rings across enormous distances, seen this way. Saturn
• 22:04
  Speaker 1: begins to resemble a small planetary system within the larger
• 22:07
  Speaker 1: solar system. Human beings knew of Saturn long before telescopes existed.
• 22:14
  Speaker 1: It is bright enough to be seen with the unaided
• 22:16
  Speaker 1: eye and was observed by ancient civilizations thousands of years ago.
• 22:21
  Speaker 1: But for almost all of human history, Saturn was simply
• 22:25
  Speaker 1: a wandering point of light. Even after the invention of
• 22:29
  Speaker 1: the telescope, it took generations to understand what surrounded it.
• 22:34
  Speaker 1: The transformation in our knowledge accelerated dramatically once spacecraft began
• 22:39
  Speaker 1: travelling there. Voyager 1 passed Saturn in 1980. Voyager 2
• 22:45
  Speaker 1: followed in 1981. Their cameras and instruments revealed details of
• 22:51
  Speaker 1: the rings, atmosphere, magnetic environment and moons that could never
• 22:56
  Speaker 1: have been seen properly from Earth. Then came a much
• 23:00
  Speaker 1: more ambitious visitor. The Cassini-Huygens mission was a collaboration involving
• 23:07
  Speaker 1: the National Aeronautics and Space Administration, the European Space Agency
• 23:13
  Speaker 1: and the Italian Space Agency. Cassini arrived at Saturn in
• 23:18
  Speaker 1: 2004 and entered orbit, beginning one of the most productive
• 23:23
  Speaker 1: planetary exploration missions ever attempted. For 13 years, the spacecraft
• 23:29
  Speaker 1: repeatedly travelled through the Saturnian system. It watched storms move
• 23:35
  Speaker 1: across Saturn's atmosphere. It studied the structure of the rings.
• 23:39
  Speaker 1: It flew past moons again and again. It measured gravity,
• 23:44
  Speaker 1: magnetic fields, particles, ice, dust and gases. The hydrogen's probe
• 23:50
  Speaker 1: separated from Cassini and descended through the atmosphere of Titan,
• 23:54
  Speaker 1: eventually landing on its surface in 2005. Cassini also transformed
• 23:59
  Speaker 1: our understanding of the small moon Enceladus, after observing enormous
• 24:04
  Speaker 1: plumes of water-rich material erupting from fractures near its south pole.
• 24:10
  Speaker 1: These discoveries turned Saturn from a beautiful distant planet into
• 24:14
  Speaker 1: a place filled with active environments. Its moons revealed internal oceans,
• 24:20
  Speaker 1: its rings revealed the delicate effects of orbital resonance and
• 24:24
  Speaker 1: tidal gravity. Its atmosphere revealed storms and circulation operating under
• 24:30
  Speaker 1: conditions very different from those on Earth. Titan showed that
• 24:34
  Speaker 1: complex chemistry, weather, rivers, lakes and seas can exist using
• 24:39
  Speaker 1: substances other than liquid water at the surface. Enceladus showed
• 24:44
  Speaker 1: that a small frozen moon can contain a liquid ocean
• 24:47
  Speaker 1: hidden beneath its crust and release material from that ocean
• 24:51
  Speaker 1: directly into space. Saturn therefore marks an important change in
• 24:56
  Speaker 1: the journey outward. The outer solar system is colder, darker
• 25:01
  Speaker 1: and more distant. but it is not necessarily quieter. In
• 25:06
  Speaker 1: some ways, it becomes more varied. Cold temperatures preserve volatile
• 25:11
  Speaker 1: substances that disappear from warmer worlds. Powerful tidal forces can
• 25:16
  Speaker 1: maintain geological activity inside moons far from the Sun. Gravity
• 25:21
  Speaker 1: organises countless bodies into rings, resonances and orbital families. Cassini's
• 25:28
  Speaker 1: mission ended in 2017. By then, engineers faced a problem
• 25:34
  Speaker 1: created by the spacecraft's own success. Cassini had discovered that
• 25:39
  Speaker 1: moons such as Enceladus possessed environments of considerable astrobiological interest.
• 25:45
  Speaker 1: If the spacecraft were eventually left uncontrolled, there was a
• 25:49
  Speaker 1: small possibility that it could collide with one of these
• 25:52
  Speaker 1: moons and contaminate it with material carried from Earth. So
• 25:57
  Speaker 1: Cassini was deliberately directed into Saturn's atmosphere. During its final orbits,
• 26:04
  Speaker 1: it repeatedly passed through the narrow region between Saturn and
• 26:08
  Speaker 1: its innermost rings, collecting measurements that had never previously been possible.
• 26:14
  Speaker 1: Then it entered the atmosphere and was destroyed. It was
• 26:18
  Speaker 1: an unusually thoughtful end for a spacecraft. After travelling billions
• 26:24
  Speaker 1: of kilometres and spending more than a decade revealing Saturn's secrets,
• 26:29
  Speaker 1: Cassini disappeared into the planet it had spent so long studying,
• 26:33
  Speaker 1: and many of the questions it left behind begin with
• 26:36
  Speaker 1: the feature that makes Saturn instantly recognisable. From Earth, the
• 26:41
  Speaker 1: rings can appear almost smooth. They look like a single
• 26:45
  Speaker 1: elegant structure wrapped around the planet. Up close, nothing could
• 26:50
  Speaker 1: be further from the truth. The rings are not solid.
• 26:54
  Speaker 1: They are made from an astonishing population of separate particles,
• 26:58
  Speaker 1: each moving around Saturn under the same fundamental laws of
• 27:02
  Speaker 1: gravity that govern planets around the Sun. From tiny grains
• 27:06
  Speaker 1: to large blocks of ice, billions upon billions of individual
• 27:11
  Speaker 1: objects combine to create something that, from a distance, looks
• 27:15
  Speaker 1: almost impossibly perfect. When Galileo Galilei first turned a telescope
• 27:22
  Speaker 1: towards Saturn in 1610, he saw something that made almost
• 27:26
  Speaker 1: no sense. His instrument was far too crude to reveal
• 27:30
  Speaker 1: the rings clearly. Instead, Saturn appeared to have strange companions
• 27:35
  Speaker 1: attached to either side, almost as though the planet had
• 27:39
  Speaker 1: grown a pair of ears. Galileo returned to Saturn repeatedly,
• 27:45
  Speaker 1: but the view only became more confusing. As Saturn moved
• 27:49
  Speaker 1: through its orbit, the angle of the rings changed from
• 27:52
  Speaker 1: Earth's perspective. At times they appeared broad and obvious. At
• 27:57
  Speaker 1: other times they seemed to vanish almost completely. It was
• 28:01
  Speaker 1: not until several decades later that Christian Huygens offered the
• 28:04
  Speaker 1: correct interpretation. Saturn was surrounded by a thin, flat ring
• 28:10
  Speaker 1: that did not touch the planet. Then Giovanni Domenico Cassini
• 28:15
  Speaker 1: observed that the ring system itself was divided. The most
• 28:19
  Speaker 1: prominent dark gap between the bright A ring and B
• 28:22
  Speaker 1: ring became known as the Cassini Division. What looked simple
• 28:27
  Speaker 1: from Earth was already beginning to reveal structure. Modern observations
• 28:32
  Speaker 1: have taken that idea much further. Saturn's rings are not
• 28:37
  Speaker 1: solid disks. They are enormous populations of individual particles, each
• 28:42
  Speaker 1: travelling around Saturn in its own orbit. Most of the
• 28:46
  Speaker 1: material is water ice, mixed with smaller amounts of rocky
• 28:50
  Speaker 1: and dusty material. Some particles are tiny grains, others are
• 28:56
  Speaker 1: chunks the size of houses. There are probably countless objects
• 29:00
  Speaker 1: spanning a wide range of sizes, all sharing the same
• 29:04
  Speaker 1: broad region of space. From a distance, this collection blends
• 29:08
  Speaker 1: together into smooth bands of light. Up close, it is
• 29:12
  Speaker 1: more like an immense orbital traffic system. Every particle is
• 29:16
  Speaker 1: being guided primarily by Saturn's gravity, and importantly, Not every
• 29:21
  Speaker 1: part of the rings moves at the same speed. Material
• 29:24
  Speaker 1: closer to Saturn travels around the planet faster than material
• 29:28
  Speaker 1: farther away. This is exactly what we expect from orbital mechanics.
• 29:34
  Speaker 1: A particle in a lower orbit must move more quickly
• 29:37
  Speaker 1: to remain in that orbit, so the rings do not
• 29:40
  Speaker 1: rotate as one rigid object. If you painted a line
• 29:44
  Speaker 1: across the entire ring system, that line would quickly become
• 29:48
  Speaker 1: stretched and distorted because its inner parts would move ahead
• 29:52
  Speaker 1: of its outer parts. That alone tells us that the
• 29:55
  Speaker 1: rings are fundamentally dynamic. They are also extraordinarily thin compared
• 30:02
  Speaker 1: with their width. The main rings stretch across hundreds of
• 30:07
  Speaker 1: thousands of kilometres from one side of Saturn to the other,
• 30:11
  Speaker 1: yet many parts may be only tens of metres thick.
• 30:15
  Speaker 1: That makes them proportionally far thinner than a sheet of paper.
• 30:21
  Speaker 1: The main system is divided into several named regions. Beginning
• 30:25
  Speaker 1: close to Saturn is the faint D-ring. followed by the
• 30:29
  Speaker 1: brighter C ring and then the massive B ring. Farther
• 30:33
  Speaker 1: out lies the Cassini division, followed by the A ring.
• 30:37
  Speaker 1: Beyond that sits the narrow and complicated F ring, while
• 30:41
  Speaker 1: more distant structures include the G ring and the broad
• 30:45
  Speaker 1: E ring. These divisions are not arbitrary. Many of them
• 30:50
  Speaker 1: are shaped by gravity. Saturn's moons constantly disturb the ring particles.
• 30:56
  Speaker 1: and some of the most beautiful patterns in the system
• 30:58
  Speaker 1: are created by these repeated gravitational interactions. Mimmers, for example,
• 31:04
  Speaker 1: produces orbital resonances that help shape gaps within the rings.
• 31:09
  Speaker 1: A resonance occurs when the orbital periods of two objects
• 31:12
  Speaker 1: form a simple repeating relationship. Small gravitational tugs then occur
• 31:18
  Speaker 1: again and again in similar positions, allowing tiny effects to
• 31:22
  Speaker 1: build over time. Other moons interact with the rings much
• 31:26
  Speaker 1: more directly. Pan travels within the Encke gap in the
• 31:31
  Speaker 1: A ring. Daphnis orbits inside the Keeler gap and can
• 31:35
  Speaker 1: pull nearby ring particles vertically, creating rippling structures along the
• 31:40
  Speaker 1: edges of the gap. Prometheus and Pandora interact with the
• 31:44
  Speaker 1: narrow F ring, producing knots, strands, clumps and shifting features.
• 31:51
  Speaker 1: The ring system is therefore not a static arrangement frozen
• 31:54
  Speaker 1: in place. It is constantly being sculpted. Another important idea
• 32:00
  Speaker 1: is the Roche Limit. This is a region around a
• 32:03
  Speaker 1: planet where tidal forces can become strong enough to prevent
• 32:07
  Speaker 1: loosely held material from gathering into a large moon, or
• 32:12
  Speaker 1: in some circumstances can tear apart a body that lacks
• 32:15
  Speaker 1: enough internal strength to resist the difference in gravitational pull
• 32:20
  Speaker 1: across it. Saturn's rings lie largely within this kind of environment.
• 32:26
  Speaker 1: The side of an object facing Saturn feels a slightly
• 32:29
  Speaker 1: stronger gravitational pull than the side facing away. Across a
• 32:34
  Speaker 1: sufficiently large or weak object, that difference can become important.
• 32:40
  Speaker 1: This helps explain why so much material near Saturn remains
• 32:43
  Speaker 1: spread into rings rather than simply assembling into one large moon.
• 32:49
  Speaker 1: but the exact origin of the rings remains uncertain. One
• 32:54
  Speaker 1: possibility is that they formed from the remains of an
• 32:57
  Speaker 1: icy moon that was shattered by a collision or torn
• 33:01
  Speaker 1: apart by tidal forces. Another is that several smaller icy
• 33:06
  Speaker 1: bodies contributed material over time. There is also continuing debate
• 33:12
  Speaker 1: about the age of the bright main rings. Their unusually clean,
• 33:16
  Speaker 1: icy appearance has sometimes been interpreted as evidence that they
• 33:21
  Speaker 1: may be relatively young compared with Saturn itself, because a
• 33:25
  Speaker 1: very ancient ring system should gradually accumulate darker dust and debris. However,
• 33:32
  Speaker 1: ring material can be recycled, redistributed, buried, exposed, and altered
• 33:39
  Speaker 1: by collisions and interactions with Saturn's atmosphere and moons. That
• 33:44
  Speaker 1: makes the problem much less straightforward than simply measuring how
• 33:48
  Speaker 1: dirty the ice appears. There may not be one moment
• 33:52
  Speaker 1: when the entire ring system suddenly formed. Different parts could
• 33:57
  Speaker 1: have different histories. Some material may have been lost and replaced.
• 34:02
  Speaker 1: Some may have been broken apart and reassembled repeatedly. And
• 34:07
  Speaker 1: one of Saturn's rings is being supplied with fresh material
• 34:10
  Speaker 1: even now. The broad E ring is very different from
• 34:15
  Speaker 1: the dense, bright main rings. Its particles are extremely small
• 34:20
  Speaker 1: and widely spread. Much of that material comes from Enceladus.
• 34:25
  Speaker 1: This little icy moon sprays water-rich particles from fractures near
• 34:29
  Speaker 1: its south pole. Some of those particles escape Enceladus and
• 34:34
  Speaker 1: enter orbit around Saturn, gradually spreading out to help form
• 34:38
  Speaker 1: the E ring. So one of Saturn's moons, is literally
• 34:43
  Speaker 1: feeding part of the ring system. That connection captures what
• 34:47
  Speaker 1: makes Saturn so scientifically rich. The planet affects the moons.
• 34:53
  Speaker 1: The moons reshape the rings. The rings exchange material with
• 34:57
  Speaker 1: the moons. Tidal forces heat interiors. Orbital resonances rearrange particles. Ice, dust,
• 35:08
  Speaker 1: gravity and motion are tied together across enormous distances. From Earth,
• 35:15
  Speaker 1: Saturn's rings look almost impossibly calm and ordered. In reality,
• 35:20
  Speaker 1: they are the visible result of countless objects continually responding
• 35:24
  Speaker 1: to gravity. And hidden within that elegant structure is a
• 35:29
  Speaker 1: clue that the outer solar system is not simply a
• 35:32
  Speaker 1: collection of isolated worlds. It is a network of systems
• 35:36
  Speaker 1: within systems, each one influencing the others. Titan is one
• 35:43
  Speaker 1: of those worlds that becomes stranger the longer you look
• 35:45
  Speaker 1: at it. It is Saturn's largest moon and the second
• 35:49
  Speaker 1: largest moon in the solar system. With a diameter of
• 35:53
  Speaker 1: more than 5,000 kilometres, Titan is actually wider than the
• 35:57
  Speaker 1: planet Mercury. Christian Huygens discovered it in 1655, although he
• 36:04
  Speaker 1: could have had little idea of just how unusual the
• 36:07
  Speaker 1: distant point of light would eventually prove to be. Titan
• 36:11
  Speaker 1: is the only moon in the solar system known to
• 36:14
  Speaker 1: possess a dense, substantial atmosphere. At its surface, the atmospheric
• 36:20
  Speaker 1: pressure is roughly one and a half times that experienced
• 36:24
  Speaker 1: at sea level on Earth. The atmosphere is composed predominantly
• 36:28
  Speaker 1: of molecular nitrogen, making Titan surprisingly similar to Earth in
• 36:33
  Speaker 1: one respect. but mixed into that nitrogen is methane. High
• 36:39
  Speaker 1: in the atmosphere, sunlight and energetic particles break apart molecules
• 36:44
  Speaker 1: and drive complicated chemical reactions. The products combine into larger
• 36:50
  Speaker 1: organic molecules, helping create the thick orange haze that hides
• 36:55
  Speaker 1: Titan's surface from ordinary visible light observations. For many years,
• 37:01
  Speaker 1: this haze made Titan extraordinarily frustrating, Astronomers knew there was
• 37:06
  Speaker 1: a large world underneath it, but they could barely see
• 37:10
  Speaker 1: what that world looked like. The Cassini spacecraft changed that.
• 37:16
  Speaker 1: Because visible light could not easily penetrate the haze, Cassini
• 37:20
  Speaker 1: relied heavily on radar and infrared instruments to study the
• 37:24
  Speaker 1: landscape below. Slowly an entire world emerged. There were enormous
• 37:30
  Speaker 1: dune fields stretching across equatorial regions. There were channels that
• 37:35
  Speaker 1: appeared to have been carved by flowing liquid. There were plains, mountains,
• 37:41
  Speaker 1: possible cryovolcanic features and vast dark regions near the poles
• 37:45
  Speaker 1: that turned out to be lakes and seas. Titan has
• 37:49
  Speaker 1: something remarkably close to a hydrological cycle. Except there is
• 37:54
  Speaker 1: very little liquid water flowing across its surface. Instead, the
• 37:59
  Speaker 1: working fluids are mainly methane and ethane. Methane can evaporate
• 38:04
  Speaker 1: from lakes and seas. It can enter the atmosphere, form clouds, condense,
• 38:10
  Speaker 1: fall as rain, run through channels, collect in depressions, and
• 38:15
  Speaker 1: eventually return to larger bodies of liquid. The basic pattern
• 38:19
  Speaker 1: sounds strangely familiar. Evaporation, clouds, rainfall, rivers, lakes, seas. But
• 38:26
  Speaker 1: the chemistry is entirely different. Titan's surface temperature is around
• 38:31
  Speaker 1: 179 degrees below zero Celsius. At temperatures like these, water
• 38:37
  Speaker 1: ice is so hard that it behaves more like rock
• 38:40
  Speaker 1: does on Earth. Water forms much of the landscape itself.
• 38:45
  Speaker 1: Methane takes on the role normally played by water. Near
• 38:49
  Speaker 1: Titan's North Pole are some of the largest known bodies
• 38:51
  Speaker 1: of stable surface liquid anywhere beyond Earth. Kraken Mare is
• 38:56
  Speaker 1: the largest. Ligeia Mare is another enormous sea, filled predominantly
• 39:02
  Speaker 1: with liquid methane along with other hydrocarbons. Smaller lakes and
• 39:07
  Speaker 1: connecting channels surround them. The existence of these seas means
• 39:11
  Speaker 1: Titan possesses coastlines, islands, shorelines, drainage systems and landscapes modified
• 39:18
  Speaker 1: by weather. If you could somehow stand safely on the surface,
• 39:23
  Speaker 1: the scenery might feel oddly recognisable at first glance. There
• 39:27
  Speaker 1: could be clouds overhead. There could be rain. there could
• 39:31
  Speaker 1: be a shoreline in the distance. Then you would remember
• 39:34
  Speaker 1: that the rain is methane, the sea is filled with hydrocarbons,
• 39:38
  Speaker 1: the rocks beneath your feet may be water ice, and
• 39:42
  Speaker 1: Saturn is hanging somewhere beyond the orange haze. The Huygens
• 39:47
  Speaker 1: probe gave humanity its first direct experience of this landscape.
• 39:52
  Speaker 1: After separating from Cassini, Huygens entered Titan's atmosphere and descended
• 39:57
  Speaker 1: by parachute in January 2005. For more than two hours
• 40:02
  Speaker 1: it measured the atmosphere while slowly falling toward the surface.
• 40:07
  Speaker 1: Its cameras revealed branching channels and terrain that looked as
• 40:11
  Speaker 1: though flowing liquid had shaped it. Hygiens eventually landed on
• 40:16
  Speaker 1: a relatively flat region, scattered with rounded objects composed largely
• 40:21
  Speaker 1: of water ice. Their smooth shapes were consistent with material
• 40:25
  Speaker 1: that had been transported or modified by liquid, much as
• 40:29
  Speaker 1: stones in an earthly river can become rounded over time.
• 40:33
  Speaker 1: Titan is not simply a frozen copy of Earth. However,
• 40:37
  Speaker 1: its methane climate raises an important problem. Sunlight gradually destroys
• 40:44
  Speaker 1: methane in the upper atmosphere. Over geological time, the methane
• 40:49
  Speaker 1: should disappear unless some process replaces it. Scientists have therefore
• 40:54
  Speaker 1: investigated possible reservoirs inside Titan and mechanisms capable of releasing
• 41:00
  Speaker 1: methane into the atmosphere. Seasonal changes also affect clouds, rainfall,
• 41:07
  Speaker 1: winds and the distribution of methane around the Moon. One
• 41:12
  Speaker 1: Titan year is the same as one Saturn year, almost
• 41:15
  Speaker 1: 30 Earth years, so its seasons unfold very slowly by
• 41:20
  Speaker 1: human standards. Beneath all of this may lie another environment entirely.
• 41:28
  Speaker 1: Evidence from Titan's gravity, rotation, and interior modelling suggests that
• 41:33
  Speaker 1: a deep ocean of liquid water probably exists beneath its
• 41:37
  Speaker 1: frozen outer shell. So Titan may contain two profoundly different
• 41:43
  Speaker 1: liquid environments. At the surface are hydrocarbon lakes and seas
• 41:48
  Speaker 1: dominated by methane and ethane. Far below lies a probable
• 41:53
  Speaker 1: water-rich ocean concealed beneath kilometres of ice. This makes Titan
• 41:58
  Speaker 1: particularly interesting when scientists think about habitability and the chemistry
• 42:03
  Speaker 1: that can precede life. Complex organic molecules are abundant there.
• 42:10
  Speaker 1: carbon chemistry is active. There are liquids, energy sources and
• 42:15
  Speaker 1: environments unlike anything found naturally on Earth's surface. But these
• 42:20
  Speaker 1: ideas have to be handled carefully. Complex chemistry is not
• 42:25
  Speaker 1: the same thing as life. Prebiotic chemistry, meaning chemistry capable
• 42:31
  Speaker 1: of contributing to the processes that may precede biology, is
• 42:35
  Speaker 1: not evidence that biology has actually developed. and there is
• 42:40
  Speaker 1: currently no confirmed evidence of life on Titan. That distinction
• 42:46
  Speaker 1: is important precisely because Titan is so tempting. It offers
• 42:50
  Speaker 1: natural laboratories for studying how organic chemistry behaves under conditions
• 42:55
  Speaker 1: radically different from those on Earth. The National Aeronautics and
• 43:00
  Speaker 1: Space Administration plans to investigate those environments with Dragonfly. an
• 43:06
  Speaker 1: unusual robotic mission designed to travel between multiple locations on Titan.
• 43:12
  Speaker 1: Dragonfly is a rotorcraft rather than a conventional rover. Titan's
• 43:18
  Speaker 1: dense atmosphere and relatively weak gravity make powered flight unusually practical,
• 43:24
  Speaker 1: allowing the spacecraft to move across much greater distances than
• 43:28
  Speaker 1: a wheeled vehicle could easily cover. Its instruments are intended
• 43:31
  Speaker 1: to examine surface composition, organic chemistry, geological processes and the
• 43:38
  Speaker 1: conditions that may help scientists understand how far complex chemistry
• 43:42
  Speaker 1: can progress before biology is involved. Titan therefore changes an
• 43:48
  Speaker 1: assumption that is easy to carry into the outer solar system.
• 43:53
  Speaker 1: Travelling farther from the sun does not mean travelling into
• 43:57
  Speaker 1: chemical inactivity. Sunlight becomes weaker. Temperatures fall dramatically. Water freezes
• 44:04
  Speaker 1: hard enough to become landscape, yet chemistry continues. Weather continues.
• 44:11
  Speaker 1: Landscapes continue to evolve. On Titan, a cold world more
• 44:17
  Speaker 1: than a billion kilometres from the sun has clouds moving
• 44:20
  Speaker 1: through a nitrogen sky, methane falling as rain, rivers carving channels,
• 44:26
  Speaker 1: and dark seas waiting beneath an orange haze. The outer
• 44:30
  Speaker 1: solar system is not simply where activity fades away. It
• 44:34
  Speaker 1: is where familiar processes begin using unfamiliar ingredients. Enceladus does
• 44:41
  Speaker 1: not look like the sort of world that should be
• 44:43
  Speaker 1: particularly interesting. It is small, only about 500 kilometres across,
• 44:49
  Speaker 1: and its surface is covered almost entirely in bright water ice.
• 44:54
  Speaker 1: From a distance it appears pale, frozen and quiet. For
• 44:59
  Speaker 1: a long time, that was more or less what scientists
• 45:02
  Speaker 1: expected it to be. Then the Cassini spacecraft began looking closely.
• 45:08
  Speaker 1: What Cassini found transformed Enceladus from a minor icy moon
• 45:13
  Speaker 1: into one of the most scientifically important places in the
• 45:16
  Speaker 1: outer solar system. Near the moon's south pole are long,
• 45:21
  Speaker 1: roughly parallel fractures in the ice. These features became known
• 45:26
  Speaker 1: as tiger stripes. because of the way they appear across
• 45:29
  Speaker 1: the surface. Cassini discovered that these fractures were warmer than
• 45:34
  Speaker 1: the surrounding terrain. More surprisingly, enormous plumes of material were
• 45:39
  Speaker 1: escaping from them. Water vapour, tiny ice grains and other
• 45:44
  Speaker 1: particles were being fired directly into space. Enceladus, it turned out,
• 45:51
  Speaker 1: was not simply frozen. Something active was happening underneath the ice.
• 45:57
  Speaker 1: Cassini flew through the plumes repeatedly, allowing its instruments to
• 46:02
  Speaker 1: sample some of the material directly. The spacecraft detected water vapor, salts,
• 46:08
  Speaker 1: carbon-bearing compounds and particles of silica, along with other substances.
• 46:14
  Speaker 1: This immediately raised an extraordinary possibility. The material escaping from
• 46:20
  Speaker 1: Enceladus appeared to be coming from liquid water beneath the surface.
• 46:25
  Speaker 1: Further measurements strengthened that interpretation. Observations of Enceladus' gravity showed
• 46:32
  Speaker 1: that its interior did not behave like a completely solid
• 46:35
  Speaker 1: frozen body. Small variations in the way the moon rotates
• 46:41
  Speaker 1: also provided important evidence. Enceladus wobbles slightly as it travels
• 46:47
  Speaker 1: around Saturn. and the size of that motion is difficult
• 46:50
  Speaker 1: to explain if the outer ice shell is attached directly
• 46:54
  Speaker 1: to a solid interior. Instead, the evidence points toward a global,
• 47:00
  Speaker 1: or nearly global, liquid water ocean lying beneath the ice.
• 47:05
  Speaker 1: So beneath a moon that looks almost perfectly frozen from
• 47:09
  Speaker 1: the outside, there may be an entire hidden ocean. The
• 47:13
  Speaker 1: obvious question is how that ocean can remain liquid so
• 47:17
  Speaker 1: far from the sun. Sunlight at Saturn is much weaker
• 47:21
  Speaker 1: than it is at Earth. Solar heating alone cannot explain
• 47:25
  Speaker 1: the activity seen at Enceladus. The main source of energy
• 47:30
  Speaker 1: appears to be tidal heating. Enceladus does not travel around
• 47:35
  Speaker 1: Saturn in a perfectly circular orbit. Its orbit is slightly eccentric,
• 47:41
  Speaker 1: meaning its distance from Saturn changes as it moves. Because
• 47:46
  Speaker 1: Saturn's gravitational pull becomes stronger, when Enceladus is closer and
• 47:51
  Speaker 1: weaker when it is farther away. The moon is repeatedly
• 47:55
  Speaker 1: stretched and relaxed. The effect is subtle, but it happens constantly.
• 48:01
  Speaker 1: That repeated flexing produces friction inside Enceladus and converts mechanical
• 48:07
  Speaker 1: energy into heat. Orbital interactions with other moons help maintain
• 48:12
  Speaker 1: the eccentricity of Enceladus' orbit. preventing it from simply settling
• 48:18
  Speaker 1: into a perfectly circular path. As a result, the Moon
• 48:22
  Speaker 1: continues to be gently squeezed by Saturn again and again.
• 48:26
  Speaker 1: That internal heating helps keep water liquid beneath the outer shell,
• 48:31
  Speaker 1: and the evidence suggests that this water may be interacting
• 48:34
  Speaker 1: directly with rock. Cassini detected tiny silica particles in the
• 48:40
  Speaker 1: plume material. The best explanations for those particles are involve
• 48:44
  Speaker 1: warm water interacting with silicate rock beneath the ocean. On Earth,
• 48:50
  Speaker 1: similar kinds of water-rock interactions occur around hydrothermal environments on
• 48:55
  Speaker 1: the ocean floor. This does not mean Enceladus definitely has
• 48:59
  Speaker 1: hydrothermal vents identical to those found on Earth, but the
• 49:03
  Speaker 1: evidence is consistent with some form of warm hydrothermal activity
• 49:08
  Speaker 1: occurring where the ocean meets the rocky interior. That matters.
• 49:13
  Speaker 1: because environments involving liquid water, rock, heat and chemical reactions
• 49:18
  Speaker 1: can produce useful sources of chemical energy. And this is
• 49:22
  Speaker 1: where Enceladus becomes especially interesting when scientists think about habitability.
• 49:28
  Speaker 1: Life as we know it requires more than simply the
• 49:31
  Speaker 1: presence of water. A potentially habitable environment generally needs persistent
• 49:37
  Speaker 1: liquid water. biologically important elements such as carbon, hydrogen, nitrogen, oxygen,
• 49:45
  Speaker 1: phosphorus and sulphur, sources of usable energy, and enough environmental
• 49:51
  Speaker 1: stability for complex chemistry to continue over meaningful periods of time.
• 49:58
  Speaker 1: Enceladus appears to possess several of these ingredients. There is
• 50:03
  Speaker 1: liquid water, there are carbon-bearing compounds, There are salts and
• 50:09
  Speaker 1: evidence of chemical interactions between water and rock. There are
• 50:14
  Speaker 1: possible sources of chemical energy. And the ocean may have
• 50:18
  Speaker 1: existed for a considerable length of time. But one distinction
• 50:23
  Speaker 1: is absolutely essential. Habitability is not the same as inhabited.
• 50:31
  Speaker 1: Finding conditions in which life might be able to exist
• 50:34
  Speaker 1: does not mean that life actually exists there. No direct
• 50:38
  Speaker 1: evidence of organisms has been discovered on Enceladus. No spacecraft
• 50:43
  Speaker 1: has detected cells, metabolism, reproduction or any unambiguous biological process.
• 50:51
  Speaker 1: What scientists have found is an environment that appears to
• 50:55
  Speaker 1: contain several ingredients associated with habitability. That makes Enceladus exciting
• 51:02
  Speaker 1: precisely because the evidence is strong enough to justify deeper
• 51:06
  Speaker 1: investigations without requiring us to leap beyond what has actually
• 51:11
  Speaker 1: been observed. And unusually, this hidden ocean may be relatively accessible.
• 51:17
  Speaker 1: On many icy moons, studying the subsurface ocean would require
• 51:22
  Speaker 1: somehow drilling through kilometres of ice. Enceladus does something rather helpful.
• 51:29
  Speaker 1: It throws some of its ocean into space. The plumes
• 51:32
  Speaker 1: rising above the South Pole naturally carry material from beneath
• 51:37
  Speaker 1: the surface into an environment where spacecraft can fly directly
• 51:41
  Speaker 1: through it. In effect, the Moon provides samples without requiring
• 51:45
  Speaker 1: us to land, drill or melt through the crust. Some
• 51:50
  Speaker 1: of the particles escaping Enceladus do not leave the Saturnian system. Instead,
• 51:56
  Speaker 1: they spread around Saturn and help maintain the broad E-ring.
• 52:01
  Speaker 1: So the same hidden ocean that makes Enceladus fascinating as
• 52:05
  Speaker 1: a potentially habitable environment also contributes material to one of
• 52:10
  Speaker 1: Saturn's outer rings. The connection between moon and ring is
• 52:14
  Speaker 1: happening continuously. More recently, the James Webb Space Telescope observed
• 52:20
  Speaker 1: an enormous plume of water vapour extending thousands of kilometres
• 52:24
  Speaker 1: from Enceladus. The observations showed water spreading far beyond the
• 52:29
  Speaker 1: moon itself, and feeding material into the surrounding Saturnian environment.
• 52:36
  Speaker 1: For such a small world, Enceladus has an extraordinary reach.
• 52:40
  Speaker 1: Its surface may look frozen and still. Its interior is
• 52:45
  Speaker 1: anything but. Beneath the ice is liquid water. Saturn's gravity
• 52:50
  Speaker 1: supplies heat. Water interacts with rock. Material escapes through fractures
• 52:57
  Speaker 1: and travels into space. And as we continue farther from
• 53:01
  Speaker 1: the sun, Enceladus leaves us with an important lesson. Cold
• 53:06
  Speaker 1: does not necessarily mean dead. A world can sit more
• 53:10
  Speaker 1: than a billion kilometres from the sun, wrapped in ice
• 53:15
  Speaker 1: and receiving only a small fraction of the sunlight reaching
• 53:18
  Speaker 1: Earth while still hiding an active ocean beneath its surface.
• 53:23
  Speaker 1: The outer solar system may look quiet, but sometimes, All
• 53:28
  Speaker 1: that activity is simply happening underneath. Uranus is where the
• 53:34
  Speaker 1: outer solar system begins to feel genuinely remote. For most
• 53:38
  Speaker 1: of human history, nobody knew it was there. Mercury, Venus, Mars,
• 53:44
  Speaker 1: Jupiter and Saturn are all bright enough to be seen
• 53:47
  Speaker 1: without a telescope, and they had been watched since ancient times.
• 53:52
  Speaker 1: Uranus was different. It remained hidden in plain sight. until
• 53:57
  Speaker 1: William Herschel observed it through a telescope in 1781. At first,
• 54:03
  Speaker 1: Herschel thought he had discovered a comet. Continued observations showed
• 54:08
  Speaker 1: that the object was following the kind of orbit expected
• 54:11
  Speaker 1: from a planet. Uranus, therefore, became the first planet in
• 54:15
  Speaker 1: recorded history to be recognised through telescopic observation rather than
• 54:21
  Speaker 1: inherited from ancient astronomy. Its discovery immediately expanded the known
• 54:27
  Speaker 1: solar system. Saturn had seemed unimaginably distant. Uranus was roughly
• 54:34
  Speaker 1: twice as far from the Sun. Its average orbital distance
• 54:38
  Speaker 1: is about 19 astronomical units, placing it nearly 3 billion
• 54:43
  Speaker 1: kilometres from the Sun. Sunlight takes roughly 2 hours and
• 54:48
  Speaker 1: 40 minutes to reach it. By the time sunlight arrives
• 54:53
  Speaker 1: at Uranus, its intensity has fallen dramatically compared with what
• 54:57
  Speaker 1: Earth receives. The planet inhabits a colder, dimmer environment and
• 55:03
  Speaker 1: its composition reflects a different balance of materials from the
• 55:06
  Speaker 1: gas giants Jupiter and Saturn. Uranus is usually described as
• 55:11
  Speaker 1: an ice giant. The word ice can be slightly misleading.
• 55:16
  Speaker 1: It does not mean that Uranus is simply a giant
• 55:19
  Speaker 1: frozen ball covered in ordinary ice. Instead, The classification refers
• 55:24
  Speaker 1: largely to the greater importance of substances such as water, ammonia,
• 55:29
  Speaker 1: methane and related heavier compounds within its interior. These materials
• 55:35
  Speaker 1: are often called ices in planetary science because they condensed
• 55:39
  Speaker 1: as solid material under cold conditions in the early solar system.
• 55:44
  Speaker 1: Inside Uranus today, however, extreme pressure and temperature mean they
• 55:49
  Speaker 1: may exist as hot, dense fluids rather than familiar frozen substances.
• 55:55
  Speaker 1: Above these deeper layers lies an atmosphere dominated by hydrogen
• 55:59
  Speaker 1: and helium, with methane playing an important role in the
• 56:03
  Speaker 1: planet's appearance. Methane absorbs red wavelengths of sunlight more effectively
• 56:09
  Speaker 1: than blue and green wavelengths. As a result, Some of
• 56:13
  Speaker 1: the light reflected back into space gives Uranus its characteristic
• 56:17
  Speaker 1: pale blue-green colour. From a distance, the planet can look
• 56:22
  Speaker 1: remarkably calm. For a long time, this contributed to the
• 56:26
  Speaker 1: idea that Uranus was almost featureless compared with Jupiter, with
• 56:30
  Speaker 1: its enormous belts and storms, or Saturn, with its dramatic
• 56:34
  Speaker 1: ring system. But the apparent stillness is deceptive. Uranus has
• 56:40
  Speaker 1: atmospheric circulation, clouds, haze layers, changing storms and weather systems
• 56:47
  Speaker 1: that become more visible as observing techniques improve. Its most
• 56:51
  Speaker 1: famous feature, however, is not its colour, it is its orientation.
• 56:58
  Speaker 1: Uranus has an axial tilt of nearly 98 degrees. Earth
• 57:03
  Speaker 1: is tilted by about 23.5 degrees relative to the plane
• 57:07
  Speaker 1: of its orbit. Uranus is tilted so dramatically that that
• 57:11
  Speaker 1: it effectively rotates on its side. As the planet travels
• 57:16
  Speaker 1: around the Sun, its poles can point almost directly toward
• 57:20
  Speaker 1: or away from the Sun at certain parts of its orbit.
• 57:25
  Speaker 1: This creates extraordinary seasons. Uranus takes about 84 Earth years
• 57:31
  Speaker 1: to complete one journey around the Sun, so each season
• 57:34
  Speaker 1: lasts roughly 21 Earth years. For long periods, One hemisphere
• 57:41
  Speaker 1: can experience something close to continuous sunlight, while the other
• 57:45
  Speaker 1: remains in prolonged darkness. Then, as Uranus continues around its orbit,
• 57:51
  Speaker 1: those conditions slowly reverse. Exactly how the planet ended up
• 57:56
  Speaker 1: tilted so severely remains uncertain. One leading idea is that
• 58:01
  Speaker 1: Uranus suffered an enormous collision early in the history of
• 58:05
  Speaker 1: the solar system. a sufficiently massive impact could have altered
• 58:09
  Speaker 1: its rotation dramatically. Other models suggest the final tilt may
• 58:14
  Speaker 1: have resulted from several large impacts rather than one single
• 58:18
  Speaker 1: catastrophic event. There were also more complicated possibilities involving gravitational
• 58:24
  Speaker 1: interactions during the early rearrangement of the planets. The important
• 58:30
  Speaker 1: point is that Uranus probably did not simply form in
• 58:33
  Speaker 1: its present orientation without disturbance. Something significant happened to it.
• 58:39
  Speaker 1: The planet also possesses rings. They are much darker and
• 58:43
  Speaker 1: fainter than Saturn's, which is one reason they remained undiscovered
• 58:47
  Speaker 1: for so long. In 1977, astronomers studying Uranus watched the
• 58:55
  Speaker 1: planet pass in front of a distant star. This technique
• 58:59
  Speaker 1: is called stellar occultation. The intention was to study Uranus
• 59:05
  Speaker 1: itself by observing how the starlight changed as it passed behind.
• 59:10
  Speaker 1: the planet. But before Uranus covered the star, the light
• 59:14
  Speaker 1: briefly dimmed several times. It dimmed again after the planet
• 59:18
  Speaker 1: had passed. The pattern revealed narrow rings surrounding Uranus. The
• 59:24
  Speaker 1: discovery was a reminder that ring systems are not unique
• 59:27
  Speaker 1: to Saturn. Uranus also has an unusual magnetic environment. Its
• 59:33
  Speaker 1: magnetic field is tilted strongly relative to its axis of rotation,
• 59:37
  Speaker 1: and the magnetic field is significantly offset from the planet's centre.
• 59:42
  Speaker 1: This produces a magnetosphere that is highly asymmetric. As Uranus rotates,
• 59:48
  Speaker 1: the magnetic environment around it changes in a complicated way,
• 59:53
  Speaker 1: creating a very different system from the more centrally organised
• 59:56
  Speaker 1: magnetic fields of planets such as Earth. Much of what
• 1:00:00
  Speaker 1: we know directly about Uranus comes from one brief encounter.
• 1:00:05
  Speaker 1: Voyager 2 flew past the planet in January 1986. It
• 1:00:09
  Speaker 1: remains the only spacecraft ever to visit Uranus at close range.
• 1:00:15
  Speaker 1: During that encounter, Voyager 2 measured the atmosphere and magnetic field,
• 1:00:20
  Speaker 1: photographed the rings and observed several major moons. Among them
• 1:00:26
  Speaker 1: are Miranda, Ariel, Umbriel, Titania and Oberon. These moons are
• 1:00:33
  Speaker 1: themselves complex worlds. Miranda contains extraordinary cliffs, ridges, fault systems
• 1:00:40
  Speaker 1: and patches of terrain that look as though the surface
• 1:00:43
  Speaker 1: was broken apart and rearranged. Aerial shows valleys, fractures and
• 1:00:49
  Speaker 1: evidence of substantial geological modification. Titania and Oberon also preserve
• 1:00:55
  Speaker 1: signs of large-scale tectonic activity. Some of these moons may
• 1:01:01
  Speaker 1: even have experienced internal oceans at different stages of their histories,
• 1:01:05
  Speaker 1: and the possibility that some could retain liquid layers today
• 1:01:09
  Speaker 1: remains an area of scientific interest. Since Voyager 2, astronomers
• 1:01:16
  Speaker 1: have continued studying Uranus from afar. The Hubble Space Telescope
• 1:01:21
  Speaker 1: has tracked storms, clouds and seasonal changes. The James Webb
• 1:01:26
  Speaker 1: Space Telescope has revealed atmospheric details, and the planet's faint
• 1:01:31
  Speaker 1: rings in infrared light. Ground-based observatories have added further measurements
• 1:01:36
  Speaker 1: of winds, storms, chemistry and temperature. But there is a
• 1:01:41
  Speaker 1: clear difference between watching Uranus from Earth and placing a
• 1:01:45
  Speaker 1: spacecraft into orbit around it. Saturn has been studied for
• 1:01:50
  Speaker 1: years by an orbiting spacecraft. Uranus has been visited once,
• 1:01:54
  Speaker 1: during a rapid flyby several decades ago. That contrast tells
• 1:01:59
  Speaker 1: us something important about the journey outward. Beyond Saturn, direct
• 1:02:05
  Speaker 1: exploration becomes sparse very quickly. The distances grow. Travel times lengthen.
• 1:02:13
  Speaker 1: Communication slows. Solar power becomes less practical. And enormous worlds
• 1:02:19
  Speaker 1: remain known from only a few hours of close observation.
• 1:02:24
  Speaker 1: Uranus is not merely another planet on the way to
• 1:02:26
  Speaker 1: the edge. It is the beginning of a region where
• 1:02:29
  Speaker 1: our maps remain surprisingly incomplete. And farther out still waits Neptune.
• 1:02:37
  Speaker 1: The last major planet and the final giant world before
• 1:02:40
  Speaker 1: the planetary system gives way to the frozen populations beyond.
• 1:02:47
  Speaker 1: Neptune has a particularly unusual place in the history of
• 1:02:50
  Speaker 1: astronomy because it was not simply found by looking into
• 1:02:53
  Speaker 1: the sky. It was predicted. By the early 19th century,
• 1:03:00
  Speaker 1: astronomers had become increasingly aware that the orbit of Uranus
• 1:03:04
  Speaker 1: did not behave exactly as expected. Its position showed small
• 1:03:09
  Speaker 1: deviations that could not be explained perfectly by the gravitational
• 1:03:13
  Speaker 1: influence of the planets already known. One possibility was that
• 1:03:17
  Speaker 1: another planet existed farther out, gently disturbing Uranus through gravity.
• 1:03:23
  Speaker 1: Two mathematicians, Urban Le Verrier in France, and John Couch
• 1:03:28
  Speaker 1: Adams in Britain, independently worked on the problem and calculated
• 1:03:32
  Speaker 1: where such a planet might be found. In 1846, Johann
• 1:03:38
  Speaker 1: Gottfried Galley and Heinrich Loewy de Rest observed the sky
• 1:03:42
  Speaker 1: near the position predicted by Urban Le Verrier. There was Neptune.
• 1:03:48
  Speaker 1: It was one of the most remarkable demonstrations of gravitational
• 1:03:52
  Speaker 1: theory in the history of astronomy. A planet billions of
• 1:03:56
  Speaker 1: kilometres away had effectively announced its presence through the tiny
• 1:04:00
  Speaker 1: influence it exerted on another world. Neptune orbits the Sun
• 1:04:05
  Speaker 1: at an average distance of roughly 30 astronomical units, about 4.5
• 1:04:10
  Speaker 1: billion kilometres away. One journey around the Sun takes almost
• 1:04:15
  Speaker 1: 165 Earth years. In fact, Neptune completed its first full
• 1:04:22
  Speaker 1: orbit since its discovery only in the early 21st century.
• 1:04:27
  Speaker 1: Like Uranus, Neptune is classified as an ice giant. Its
• 1:04:33
  Speaker 1: atmosphere is dominated by hydrogen and helium, with methane contributing
• 1:04:38
  Speaker 1: to its blue appearance by absorbing red wavelengths of sunlight.
• 1:04:42
  Speaker 1: But beneath the atmosphere lies a much denser interior, containing
• 1:04:47
  Speaker 1: larger proportions of substances such as water, ammonia, methane and
• 1:04:53
  Speaker 1: heavier material under enormous pressures and temperatures. Calling these substances
• 1:04:58
  Speaker 1: ices refers mainly to their chemical role in the formation
• 1:05:02
  Speaker 1: of the outer planets. Deep inside Neptune, they would not
• 1:05:07
  Speaker 1: resemble the neat frozen cubes we normally imagine when we
• 1:05:10
  Speaker 1: hear the word ice. From a distance, Neptune can appear
• 1:05:15
  Speaker 1: calm and beautifully blue. Up close, it is anything but calm.
• 1:05:21
  Speaker 1: When Voyager 2 passed the planet in 1989, it observed
• 1:05:25
  Speaker 1: an enormous storm system known as the Great Dark Spot.
• 1:05:29
  Speaker 1: The feature was comparable in scale to Earth. Voyager also
• 1:05:33
  Speaker 1: recorded bright clouds, rapidly changing atmospheric structures and winds travelling
• 1:05:39
  Speaker 1: at extraordinary speeds. Some winds on Neptune can exceed 2,000
• 1:05:45
  Speaker 1: km per hour. That makes them among the fastest measured
• 1:05:49
  Speaker 1: anywhere in the solar system. The strange part is how
• 1:05:53
  Speaker 1: little sunlight Neptune receives. At 30 astronomical units from the Sun,
• 1:05:58
  Speaker 1: the planet receives only a tiny fraction of the solar
• 1:06:02
  Speaker 1: energy reaching Earth. It might seem reasonable to expect such
• 1:06:06
  Speaker 1: a distant atmosphere to be sluggish. Instead, Neptune has powerful
• 1:06:12
  Speaker 1: storms and extremely fast winds. Part of the explanation appears
• 1:06:18
  Speaker 1: to come from inside the planet. Neptune emits significantly more
• 1:06:23
  Speaker 1: energy than it receives from the Sun. Heat escaping from
• 1:06:28
  Speaker 1: its interior contributes to atmospheric motion and helps drive circulation.
• 1:06:33
  Speaker 1: This is an important reminder that planetary weather does not
• 1:06:36
  Speaker 1: always depend mainly on sunlight. A planet can carry its
• 1:06:41
  Speaker 1: own reservoir of internal energy, and that energy can continue
• 1:06:45
  Speaker 1: shaping the atmosphere billions of kilometres from the Sun. Neptune
• 1:06:51
  Speaker 1: also has rings. They are faint, narrow and dusty, compared
• 1:06:56
  Speaker 1: with the spectacular rings of Saturn. Some regions appear brighter
• 1:07:00
  Speaker 1: than others, producing unusual concentrations called ring arcs. Instead of
• 1:07:06
  Speaker 1: material being distributed evenly around an entire ring, particles can
• 1:07:11
  Speaker 1: become concentrated into particular regions through gravitational interactions. Neptune's moons
• 1:07:17
  Speaker 1: also influence this environment. The most important of them is Triton,
• 1:07:22
  Speaker 1: a large moon whose unusual orbit suggests that it was
• 1:07:26
  Speaker 1: probably captured rather than formed alongside Neptune in the ordinary way.
• 1:07:32
  Speaker 1: Voyager 2 remains the only spacecraft ever to visit Neptune
• 1:07:36
  Speaker 1: at close range. Its flyby in August 1989 completed the
• 1:07:41
  Speaker 1: spacecraft's grand tour of the giant planets. After visiting Jupiter, Saturn,
• 1:07:47
  Speaker 1: Uranus and finally Neptune, Voyager 2 continued outward on a
• 1:07:52
  Speaker 1: trajectory that would eventually carry it beyond the heliosphere. That
• 1:07:57
  Speaker 1: brief encounter remains our only close view of the most
• 1:08:00
  Speaker 1: distant major planet. Everything since then has been studied from
• 1:08:05
  Speaker 1: much farther away. And yet Neptune influences a region far
• 1:08:10
  Speaker 1: beyond its own atmosphere and moons. Its gravity plays an
• 1:08:15
  Speaker 1: important role in organising the Kuiper Belt. Many icy bodies
• 1:08:20
  Speaker 1: beyond Neptune occupy orbital resonances with the planet. An orbital
• 1:08:26
  Speaker 1: resonance occurs when two objects complete their orbits in a
• 1:08:29
  Speaker 1: repeating numerical relationship. Pluto provides the most famous example. For
• 1:08:36
  Speaker 1: every three orbits Neptune completes around the Sun, Pluto completes
• 1:08:40
  Speaker 1: roughly two. This three-to-two resonance keeps their movements organised in
• 1:08:46
  Speaker 1: such a way that, despite Pluto's orbit crossing inside the
• 1:08:50
  Speaker 1: distance of Neptune's at certain points, the two worlds do
• 1:08:54
  Speaker 1: not come dangerously close. Neptune has shaped many other distant
• 1:08:59
  Speaker 1: objects in similar ways. Its gravity helped scatter bodies outward
• 1:09:03
  Speaker 1: during the early history of the solar system, captured others
• 1:09:07
  Speaker 1: into resonances, and contributed to the complex architecture that exists
• 1:09:12
  Speaker 1: beyond the major planets today. So Neptune can reasonably be
• 1:09:17
  Speaker 1: called the end of the planetary solar system. There are
• 1:09:22
  Speaker 1: no recognised major planets beyond it. But it is not
• 1:09:26
  Speaker 1: the end of the solar system itself. Beyond Neptune are
• 1:09:30
  Speaker 1: dwarf planets, icy worlds, scattered objects, comets, and populations extending
• 1:09:37
  Speaker 1: across billions upon billions of kilometres. The character of exploration
• 1:09:43
  Speaker 1: changes here. Closer to the Sun, We can visit planets
• 1:09:48
  Speaker 1: and photograph entire worlds. Beyond Neptune, much of the structure
• 1:09:53
  Speaker 1: has to be reconstructed from faint points of light, slow
• 1:09:58
  Speaker 1: movements against background stars, gravitational relationships, and the occasional spacecraft
• 1:10:04
  Speaker 1: travelling through an almost unimaginable emptiness. Neptune is therefore less
• 1:10:10
  Speaker 1: like a wall at the edge of the solar system,
• 1:10:12
  Speaker 1: and more like the final lighthouse before a much wider ocean.
• 1:10:17
  Speaker 1: The major planets end here. The solar system does not.
• 1:10:23
  Speaker 1: Triton is one of the clearest signs that the outer
• 1:10:26
  Speaker 1: solar system has not always been arranged the way we
• 1:10:29
  Speaker 1: see it today. It is Neptune's largest moon, discovered by
• 1:10:34
  Speaker 1: William LaSalle in 1846, only a few weeks after Neptune
• 1:10:38
  Speaker 1: itself was identified. At first glance, Triton might seem like
• 1:10:43
  Speaker 1: an ordinary large moon orbiting an outer planet. but its
• 1:10:47
  Speaker 1: motion immediately tells us otherwise. Triton travels around Neptune in
• 1:10:53
  Speaker 1: a retrograde orbit. That means it moves in the opposite
• 1:10:56
  Speaker 1: direction to Neptune's rotation. Most large moons orbit their planets
• 1:11:01
  Speaker 1: in the same general direction that the planets rotate, because
• 1:11:05
  Speaker 1: they formed from material moving together in a disk around
• 1:11:08
  Speaker 1: the young planet. Triton does not fit that pattern. Its
• 1:11:13
  Speaker 1: backward orbit is strong evidence that it did not form
• 1:11:16
  Speaker 1: around Neptune in the usual way. Instead, Triton was probably captured.
• 1:11:23
  Speaker 1: Before that happened, it may have been a trans-Neptunian object,
• 1:11:27
  Speaker 1: part of the broader population of icy worlds beyond Neptune.
• 1:11:32
  Speaker 1: One leading idea is that Triton originally belonged to a
• 1:11:35
  Speaker 1: binary system, meaning two objects orbiting one another. If that
• 1:11:40
  Speaker 1: pair passed close to Neptune, the planet's gravity could have
• 1:11:44
  Speaker 1: disrupted the system. One object may have been thrown away
• 1:11:49
  Speaker 1: while Triton lost enough energy to remain bound to Neptune.
• 1:11:53
  Speaker 1: That capture would have been violent in gravitational terms. Triton's
• 1:11:58
  Speaker 1: original orbit was probably far more elongated than it is today,
• 1:12:03
  Speaker 1: and as it gradually settled into a more circular path,
• 1:12:07
  Speaker 1: enormous amounts of orbital energy would have been converted into heat.
• 1:12:12
  Speaker 1: The event may also have disrupted Neptune's original system of moons.
• 1:12:16
  Speaker 1: Existing satellites could have been scattered, collided with one another,
• 1:12:20
  Speaker 1: or been destroyed, with some of the present smaller moons
• 1:12:24
  Speaker 1: forming later from the debris. Triton therefore may be less
• 1:12:28
  Speaker 1: like a moon born around Neptune, and more like a
• 1:12:31
  Speaker 1: captured world that reshaped the entire system around it. Its
• 1:12:36
  Speaker 1: surface strengthens that impression. Triton is one of the coldest
• 1:12:41
  Speaker 1: known large bodies in the solar system. Temperatures near the
• 1:12:45
  Speaker 1: surface can fall to around 235 degrees below zero Celsius.
• 1:12:51
  Speaker 1: Frozen nitrogen covers large areas along with water ice, carbon
• 1:12:55
  Speaker 1: dioxide ice and other volatile materials. Yet despite the cold,
• 1:13:00
  Speaker 1: Voyager 2 found a surface that looked unexpectedly active. When
• 1:13:06
  Speaker 1: the spacecraft passed Triton in 1989, It revealed smooth plains,
• 1:13:12
  Speaker 1: long faults, relatively few impact craters and strange textured regions
• 1:13:17
  Speaker 1: sometimes described as cantaloupe terrain. The lack of heavy cratering
• 1:13:22
  Speaker 1: across much of the surface suggests that parts of Triton
• 1:13:25
  Speaker 1: have been resurfaced relatively recently in geological terms. Even more
• 1:13:31
  Speaker 1: surprising were dark streaks associated with active geysers. Voyager 2
• 1:13:36
  Speaker 1: observed plumes of nitrogen gas and dark particles rising several
• 1:13:41
  Speaker 1: kilometres above the surface before being carried sideways by the
• 1:13:45
  Speaker 1: thin atmosphere. These eruptions are thought to be powered largely
• 1:13:49
  Speaker 1: by weak sunlight passing through translucent nitrogen ice and warming
• 1:13:54
  Speaker 1: darker material beneath it. Gas pressure then builds until material
• 1:13:59
  Speaker 1: escapes through the surface. Even at Neptune's distance from the Sun,
• 1:14:05
  Speaker 1: Faint sunlight can still drive active processes. Triton may also
• 1:14:10
  Speaker 1: hide something deeper. Its internal composition, past tidal heating and
• 1:14:16
  Speaker 1: evidence of geological activity have led scientists to consider the
• 1:14:20
  Speaker 1: possibility of a buried liquid water ocean beneath the frozen crust.
• 1:14:26
  Speaker 1: That remains uncertain. No spacecraft has directly detected such an ocean.
• 1:14:33
  Speaker 1: and Triton has not been revisited since Voyager 2. But
• 1:14:37
  Speaker 1: the possibility is scientifically plausible enough to make the Moon
• 1:14:41
  Speaker 1: an important target for future exploration. Triton's story is also unfinished.
• 1:14:48
  Speaker 1: Because it orbits Neptune in the opposite direction to the
• 1:14:51
  Speaker 1: planet's rotation, tidal interactions are gradually stealing orbital energy from
• 1:14:57
  Speaker 1: the Moon. Very slowly, Triton is spiralling inward, Over billions
• 1:15:03
  Speaker 1: of years, it may eventually approach Neptune's Roche limit, where
• 1:15:07
  Speaker 1: tidal forces could become strong enough to break it apart.
• 1:15:12
  Speaker 1: If that happens, Triton could one day contribute material to
• 1:15:15
  Speaker 1: a spectacular new ring system around Neptune. For now, though,
• 1:15:21
  Speaker 1: it remains intact, and its real importance lies in what
• 1:15:25
  Speaker 1: it tells us about the territory beyond Neptune, Triton. may
• 1:15:29
  Speaker 1: be a captured member of the same broad family of
• 1:15:32
  Speaker 1: icy worlds that populates the outer solar system. In that sense,
• 1:15:37
  Speaker 1: it is a preview. Before we even leave the final
• 1:15:42
  Speaker 1: major planet behind, Neptune is already carrying a piece of
• 1:15:45
  Speaker 1: the distant frontier around with it. For much of the
• 1:15:48
  Speaker 1: 20th century, Pluto seemed to offer a wonderfully simple ending
• 1:15:53
  Speaker 1: to the planetary solar system. It was discovered in 1930.
• 1:15:58
  Speaker 1: by Clyde Tombaugh, a young astronomer working at Lowell Observatory
• 1:16:03
  Speaker 1: in Arizona. Tombaugh had been comparing photographs of the night sky,
• 1:16:09
  Speaker 1: searching for a faint object that moved against the background stars.
• 1:16:14
  Speaker 1: When he found Pluto, it was quickly accepted as the
• 1:16:16
  Speaker 1: ninth planet. For decades, schoolchildren learned the solar system as
• 1:16:21
  Speaker 1: a sequence ending neatly with Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune,
• 1:16:31
  Speaker 1: and finally Pluto. But Pluto was always an unusual member
• 1:16:36
  Speaker 1: of that list. It is tiny compared with the eight
• 1:16:39
  Speaker 1: major planets. Its diameter is only about 2,370 kilometres, making
• 1:16:46
  Speaker 1: it considerably smaller than Earth's moon. Its orbit is also
• 1:16:51
  Speaker 1: much more eccentric than those of the major planets. Instead
• 1:16:56
  Speaker 1: of remaining at roughly the same distance from the Sun
• 1:16:59
  Speaker 1: throughout its journey, Pluto moves between about 30 and nearly
• 1:17:04
  Speaker 1: 50 astronomical units away. Its orbital plane is also strongly
• 1:17:09
  Speaker 1: inclined compared with the relatively flat arrangement followed by the
• 1:17:13
  Speaker 1: major planets. For part of each orbit, Pluto actually travels
• 1:17:17
  Speaker 1: closer to the Sun than Neptune does. That sounds as
• 1:17:21
  Speaker 1: though the two worlds should occasionally risk a collision. They
• 1:17:25
  Speaker 1: do not. Pluto and Neptune are locked into a gravitational
• 1:17:29
  Speaker 1: relationship known as a 3-to-2 orbital resonance. For every three
• 1:17:35
  Speaker 1: orbits completed by Neptune, Pluto completes roughly two. Their motions
• 1:17:40
  Speaker 1: are synchronised in such a way that when Pluto passes
• 1:17:43
  Speaker 1: through the part of its orbit that lies inside Neptune's
• 1:17:46
  Speaker 1: average distance from the Sun, Neptune is safely positioned elsewhere.
• 1:17:52
  Speaker 1: This resonance has remained stable for immense periods of time.
• 1:17:58
  Speaker 1: Pluto was strange, but for many years it still seemed unique.
• 1:18:02
  Speaker 1: Then astronomers began finding more objects beyond Neptune. Some were
• 1:18:07
  Speaker 1: large enough to complicate the old picture considerably. Hormea and
• 1:18:11
  Speaker 1: Makemake were discovered in the outer solar system, along with
• 1:18:15
  Speaker 1: many other trans-Neptunian objects. Then came Eris. Eris is similar
• 1:18:23
  Speaker 1: to Pluto in overall scale, and was initially thought to
• 1:18:26
  Speaker 1: be slightly larger. Its discovery created an awkward question. If
• 1:18:31
  Speaker 1: Pluto was a planet, should Eris be a planet as well?
• 1:18:36
  Speaker 1: And if Eris qualified, how many more worlds might eventually
• 1:18:40
  Speaker 1: need to be added? The issue forced astronomers to reconsider
• 1:18:44
  Speaker 1: what the word planet was actually supposed to mean. In 2007,
• 1:18:50
  Speaker 1: International Astronomical Union adopted a formal definition for planets in
• 1:18:55
  Speaker 1: the solar system. Under that definition, a planet must orbit
• 1:19:00
  Speaker 1: the sun, be massive enough for its own gravity to
• 1:19:03
  Speaker 1: pull it into a nearly round shape, and have cleared
• 1:19:06
  Speaker 1: the neighbourhood around its orbit. Pluto meets the first two conditions.
• 1:19:11
  Speaker 1: It does not meet the third. Its orbital region contains
• 1:19:15
  Speaker 1: many other objects, and Pluto is part of a much
• 1:19:18
  Speaker 1: broader population, rather than gravitationally dominating its surroundings in the
• 1:19:23
  Speaker 1: way Earth or Neptune does. Pluto was therefore classified as
• 1:19:28
  Speaker 1: a dwarf planet. That decision became surprisingly emotional for something
• 1:19:33
  Speaker 1: involving a frozen world billions of kilometres away. But scientifically,
• 1:19:39
  Speaker 1: the change reflected an important improvement in our understanding. Pluto
• 1:19:44
  Speaker 1: had not physically changed. The solar system had. Or rather,
• 1:19:49
  Speaker 1: our knowledge of it had. Instead of one isolated ninth
• 1:19:54
  Speaker 1: planet beyond Neptune, astronomers had discovered an entire population of
• 1:19:58
  Speaker 1: icy worlds. Pluto was no longer an odd object sitting
• 1:20:03
  Speaker 1: alone near the edge. It was one important member of
• 1:20:06
  Speaker 1: the Kuiper Belt. And Pluto itself turned out to be
• 1:20:10
  Speaker 1: far more interesting than the old planetary diagrams had suggested.
• 1:20:15
  Speaker 1: The National Aeronautics and Space Administration spacecraft New Horizons flew
• 1:20:21
  Speaker 1: past Pluto in July 2015. Before that encounter, even the
• 1:20:27
  Speaker 1: best images showed Pluto as a small, blurred world. New
• 1:20:32
  Speaker 1: Horizons transformed it into a landscape. The spacecraft revealed towering
• 1:20:37
  Speaker 1: mountains made largely from water ice, broad plains of frozen nitrogen,
• 1:20:43
  Speaker 1: layered atmospheric haze, possible cryovolcanic structures and terrain that appeared
• 1:20:49
  Speaker 1: surprisingly young. One of its most striking features is Sputnik Planitia,
• 1:20:57
  Speaker 1: the bright heart-shaped region visible in many images of Pluto.
• 1:21:03
  Speaker 1: Much of Sputnik Planitia is filled with nitrogen ice. The
• 1:21:06
  Speaker 1: surface is divided into broad cells that appear to be
• 1:21:10
  Speaker 1: created by slow convection. Relatively warmer nitrogen ice rises from below, spreads,
• 1:21:18
  Speaker 1: cools and eventually sinks again. This process continually renews parts
• 1:21:24
  Speaker 1: of the surface and helps explain why some areas contain
• 1:21:27
  Speaker 1: very few impact craters. Pluto is not simply an ancient
• 1:21:33
  Speaker 1: frozen object that stopped changing billions of years ago. It
• 1:21:37
  Speaker 1: has active geology. It has a thin atmosphere, mainly composed
• 1:21:42
  Speaker 1: of nitrogen with smaller amounts of methane and carbon monoxide.
• 1:21:47
  Speaker 1: As Pluto moves closer to and farther from the Sun,
• 1:21:51
  Speaker 1: atmospheric gases can freeze onto the surface and later return
• 1:21:55
  Speaker 1: to the atmosphere. Pluto also has five known moons. The largest, Charon,
• 1:22:02
  Speaker 1: is unusually large compared with Pluto itself. Charon is more
• 1:22:06
  Speaker 1: than half Pluto's diameter. The two objects orbit a common
• 1:22:10
  Speaker 1: centre of mass that lies outside Pluto's surface. For that reason,
• 1:22:15
  Speaker 1: Pluto and Charon are sometimes described as behaving rather like
• 1:22:19
  Speaker 1: a binary system. They continually face the same sides toward
• 1:22:24
  Speaker 1: one another, locked together by tidal forces while the smaller
• 1:22:28
  Speaker 1: moons orbit farther out. Everything about Pluto therefore weakens the
• 1:22:33
  Speaker 1: idea that the solar system should have a neat planetary ending.
• 1:22:37
  Speaker 1: Its orbit does not define a fixed boundary. At some
• 1:22:41
  Speaker 1: points Pluto is near 30 astronomical units from the Sun,
• 1:22:45
  Speaker 1: while at others it approaches 50. And even at its
• 1:22:49
  Speaker 1: most distant point, many known objects continue far beyond it.
• 1:22:54
  Speaker 1: The old map ended with Pluto, because Pluto was once
• 1:22:58
  Speaker 1: the last large world we knew. The modern map does
• 1:23:02
  Speaker 1: not end there at all. Beyond Neptune, named planets begin
• 1:23:07
  Speaker 1: to give way to populations. Instead of travelling from one
• 1:23:11
  Speaker 1: major world to another, we enter a broad region filled
• 1:23:15
  Speaker 1: with dwarf planets, icy bodies, resonant objects, binaries, fragments and
• 1:23:21
  Speaker 1: ancient remnants of planetary formation. Pluto did not become less
• 1:23:26
  Speaker 1: important when it stopped being called the 9th planet. In
• 1:23:30
  Speaker 1: many ways it became more useful. It showed us that
• 1:23:33
  Speaker 1: the apparent edge of the planetary system was not an
• 1:23:36
  Speaker 1: edge at all. It was the entrance to the Kuiper belt.
• 1:23:43
  Speaker 1: For a long time, Pluto seemed almost alone beyond Neptune.
• 1:23:47
  Speaker 1: That picture changed dramatically in the late 20th century. Astronomers
• 1:23:52
  Speaker 1: had already suspected that more objects might be waiting in
• 1:23:56
  Speaker 1: the distant outer solar system. Kenneth Edgeworth had discussed the
• 1:24:01
  Speaker 1: possibility of a population of small bodies beyond Neptune, and
• 1:24:05
  Speaker 1: Gerard Kuiper later considered how material in this region might
• 1:24:09
  Speaker 1: have behaved during the early history of the solar system.
• 1:24:13
  Speaker 1: But the modern Kuiper belt is more complicated than either
• 1:24:17
  Speaker 1: astronomer originally imagined. The breakthrough came in 1992, when David
• 1:24:23
  Speaker 1: Jewett and Jane Liu discovered a distant object beyond Pluto
• 1:24:27
  Speaker 1: and Charon. That object, later named Albion, was the first
• 1:24:32
  Speaker 1: recognised member of a much larger population of trans-Neptunian worlds. Suddenly,
• 1:24:39
  Speaker 1: Pluto no longer looked like an isolated oddity. It looked
• 1:24:43
  Speaker 1: like part of a system. The Kuiper Belt is a broad,
• 1:24:47
  Speaker 1: donut-shaped region beginning near the orbit of Neptune and extending
• 1:24:52
  Speaker 1: outward through much of the space between roughly 30 and
• 1:24:55
  Speaker 1: 50 or 55 years. astronomical units from the Sun. It
• 1:25:01
  Speaker 1: is sometimes compared with the asteroid belt between Mars and Jupiter,
• 1:25:05
  Speaker 1: but the comparison can be misleading. The Kuiper belt is
• 1:25:10
  Speaker 1: vastly larger. It is also much colder. And while the
• 1:25:15
  Speaker 1: asteroid belt contains many rocky and metallic bodies, Kuiper belt
• 1:25:20
  Speaker 1: objects often contain substantial amounts of frozen water, methane, ammonia,
• 1:25:27
  Speaker 1: and other volatile substances. These are remnants from the formation
• 1:25:32
  Speaker 1: of the solar system, preserved in deep cold for billions
• 1:25:36
  Speaker 1: of years. The region is not one uniform ring. Different
• 1:25:42
  Speaker 1: groups of objects occupy different kinds of orbits. Some belong
• 1:25:46
  Speaker 1: to what astronomers call the classical Kuiper belt. Within this
• 1:25:50
  Speaker 1: population are dynamically cold and dynamically hot objects. Those terms
• 1:25:55
  Speaker 1: have nothing to do with their actual temperatures. Almost everything
• 1:26:00
  Speaker 1: out there is extremely cold. Instead, cold and hot describe
• 1:26:05
  Speaker 1: the shapes and orientations of the orbits. Dynamically cold objects
• 1:26:09
  Speaker 1: tend to follow more circular paths with relatively small inclinations
• 1:26:13
  Speaker 1: to the main plane of the solar system. Dynamically hot
• 1:26:17
  Speaker 1: objects generally have more tilted and eccentric orbits, suggesting a
• 1:26:22
  Speaker 1: more disturbed history. Neptune is responsible for much of this structure.
• 1:26:28
  Speaker 1: Its gravity does not simply affect nearby objects randomly. Many
• 1:26:33
  Speaker 1: Kuiper belt bodies occupy orbital resonances with Neptune. Pluto belongs
• 1:26:39
  Speaker 1: to the best known group. Objects in the same 3
• 1:26:43
  Speaker 1: to 2 resonance as Pluto are often called Plutinos. For
• 1:26:49
  Speaker 1: every 3 orbits Neptune completes, a Plutino completes approximately 2.
• 1:26:55
  Speaker 1: This repeated relationship prevents dangerous close encounters while creating long-term
• 1:27:01
  Speaker 1: orbital stability. Other groups occupy different resonances, forming patterns that
• 1:27:07
  Speaker 1: preserve evidence of how Neptune moved through the early solar system.
• 1:27:12
  Speaker 1: Farther out and on more disturbed paths lies the scattered disk.
• 1:27:18
  Speaker 1: Objects there often have highly eccentric orbits, carrying them from
• 1:27:22
  Speaker 1: regions relatively close to Neptune to enormous distances farther away.
• 1:27:27
  Speaker 1: Many appear to have been scattered by gravitational encounters with Neptune,
• 1:27:32
  Speaker 1: especially during the chaotic early history of the outer planets.
• 1:27:36
  Speaker 1: The worlds found throughout this region are surprisingly diverse. Pluto
• 1:27:41
  Speaker 1: has glaciers, mountains, haze and a complex atmosphere. Haumea rotates
• 1:27:47
  Speaker 1: extraordinarily quickly and has an elongated shape. Makemake is a bright,
• 1:27:54
  Speaker 1: icy dwarf planet. Quao has a large moon and an
• 1:27:57
  Speaker 1: unusual ring lying surprisingly far from its surface. Orcus occupies
• 1:28:03
  Speaker 1: an orbit that has some similarities to Pluto's, although the
• 1:28:06
  Speaker 1: two remain positioned very differently. And these are only the larger,
• 1:28:11
  Speaker 1: easier objects to detect. Thousands of trans-Neptunian objects have been catalogued,
• 1:28:17
  Speaker 1: but astronomers believe the actual population is vastly greater. Smaller
• 1:28:23
  Speaker 1: bodies are extraordinarily difficult to see. They are far from
• 1:28:27
  Speaker 1: the sun, so they receive very little sunlight. They reflect
• 1:28:32
  Speaker 1: only a small amount of that light back toward Earth,
• 1:28:36
  Speaker 1: and against the immense background of distant stars, many move slowly.
• 1:28:40
  Speaker 1: slowly that identifying them requires repeated observations over long periods.
• 1:28:46
  Speaker 1: The Kuiper Belt has also been shaped by collisions. Over
• 1:28:51
  Speaker 1: billions of years, icy bodies have struck one another, producing fragments, moons, dust,
• 1:28:58
  Speaker 1: and families of objects. sharing similar compositions and orbital histories.
• 1:29:03
  Speaker 1: Some binary systems may preserve clues about gentler interactions from
• 1:29:08
  Speaker 1: much earlier times. Others record violent events. To understand why
• 1:29:15
  Speaker 1: the Kuiper Belt looks this way, scientists have had to
• 1:29:19
  Speaker 1: reconstruct the early movement of the giant planets. One influential
• 1:29:24
  Speaker 1: explanation is the Nice model, named after the city of
• 1:29:28
  Speaker 1: Nice in France. In this model, Jupiter, Saturn, Uranus and
• 1:29:33
  Speaker 1: Neptune did not remain exactly where they formed. Instead, their
• 1:29:39
  Speaker 1: orbits changed over time. The giant planets exchanged gravitational energy
• 1:29:44
  Speaker 1: with one another and with enormous populations of smaller bodies.
• 1:29:49
  Speaker 1: Neptune migrated outward. As it moved, its gravity swept through
• 1:29:54
  Speaker 1: the distant disk of icy objects. Some bodies were captured
• 1:29:59
  Speaker 1: into resonances. Others were scattered inward or outward. Many were
• 1:30:05
  Speaker 1: removed from the region entirely. This process could explain why
• 1:30:10
  Speaker 1: the Kuiper Belt contains such a complicated mixture of stable, resonant, inclined,
• 1:30:17
  Speaker 1: eccentric and scattered populations. It also suggests that the belt
• 1:30:22
  Speaker 1: we see today is only a surviving remnant of something
• 1:30:26
  Speaker 1: that was once much more massive. That makes the Kuiper
• 1:30:31
  Speaker 1: Belt valuable for more than simply mapping the outer solar system.
• 1:30:36
  Speaker 1: It is an archaeological record. Its objects preserve evidence of
• 1:30:40
  Speaker 1: planetary migration, collisions, orbital disruption, and the earliest stages of
• 1:30:46
  Speaker 1: solar system formation. But despite its enormous scale, the Kuiper
• 1:30:51
  Speaker 1: Belt still does not give us a clean outer edge.
• 1:30:55
  Speaker 1: Beyond its main populations, objects continue onto more distant and
• 1:31:00
  Speaker 1: more extreme orbits, and even after the visible family of
• 1:31:04
  Speaker 1: icy worlds begins to thin out, the influence of the
• 1:31:08
  Speaker 1: Sun continues much farther. The Kuiper Belt may feel like
• 1:31:12
  Speaker 1: the frontier, but the frontier itself keeps moving outward. Beyond
• 1:31:19
  Speaker 1: the main Kuiper Belt, The solar system does not suddenly
• 1:31:23
  Speaker 1: become empty. Instead, the neatness begins to disappear. The broad
• 1:31:28
  Speaker 1: population of icy bodies beyond Neptune gradually gives way to
• 1:31:33
  Speaker 1: objects travelling on increasingly stretched, tilted and distant orbits. There
• 1:31:38
  Speaker 1: is no clean outer wall where the Kuiper belt ends
• 1:31:42
  Speaker 1: and something completely different begins. The structure simply becomes more scattered.
• 1:31:49
  Speaker 1: One important population is known as the Scattered Disk. Its
• 1:31:54
  Speaker 1: members are thought to have been strongly influenced by Neptune,
• 1:31:58
  Speaker 1: particularly during the early history of the solar system when
• 1:32:01
  Speaker 1: the giant planets were still changing their orbits. Close gravitational
• 1:32:06
  Speaker 1: encounters with Neptune could give smaller bodies enormous kicks. Some
• 1:32:11
  Speaker 1: were pushed inward. Some were thrown out of the solar
• 1:32:15
  Speaker 1: system altogether. Others survived on highly eccentric and inclined paths
• 1:32:20
  Speaker 1: that carry them far beyond the main Kuiper belt. Eris
• 1:32:25
  Speaker 1: is one of the best known examples. Its discovery was
• 1:32:29
  Speaker 1: announced in 2005 and played a major role in the
• 1:32:33
  Speaker 1: debate over what should count as a planet. Eris is
• 1:32:37
  Speaker 1: similar in overall scale to Pluto, but its orbit is
• 1:32:41
  Speaker 1: much more distant and elongated. At its closest, it remains
• 1:32:46
  Speaker 1: well beyond Neptune. At its most distant, it travels to
• 1:32:50
  Speaker 1: roughly 100 astronomical units from the Sun. Its existence helped
• 1:32:55
  Speaker 1: reveal that Pluto was not a lone object sitting near
• 1:32:58
  Speaker 1: the edge of the known planetary system. There were other
• 1:33:02
  Speaker 1: substantial worlds scattered throughout this distant region. But even the
• 1:33:07
  Speaker 1: scattered disk does not contain all of the strange objects
• 1:33:10
  Speaker 1: found beyond Neptune. Some worlds appear almost detached from the
• 1:33:15
  Speaker 1: known planets. A detached object follows an orbit whose closest
• 1:33:20
  Speaker 1: approach to the Sun is so distant that Neptune cannot
• 1:33:24
  Speaker 1: easily explain its present motion through ordinary repeated encounters. That
• 1:33:29
  Speaker 1: creates a puzzle. If Neptune is too far away to
• 1:33:33
  Speaker 1: disturb these objects strongly today, what placed them on such
• 1:33:37
  Speaker 1: unusual orbits? Sedna is the most famous example. Sedna was
• 1:33:43
  Speaker 1: discovered in 2003 by a team including Michael Brown, Chad
• 1:33:47
  Speaker 1: Trujillo and David Rabinowitz. Its orbit is extraordinary. Even at
• 1:33:53
  Speaker 1: its closest approach to the Sun, Sedna remains around 76
• 1:33:58
  Speaker 1: astronomical units away. At its most distant, it travels many
• 1:34:03
  Speaker 1: hundreds of astronomical units from the Sun. One complete orbit
• 1:34:08
  Speaker 1: takes roughly 11,000 years. For almost all of that time,
• 1:34:13
  Speaker 1: Sedna is unimaginably remote. Its closest point is already far
• 1:34:20
  Speaker 1: beyond Neptune's reach, compared with most scattered objects. That has
• 1:34:25
  Speaker 1: made its orbit particularly difficult to explain. Several possibilities have
• 1:34:30
  Speaker 1: been proposed. One idea reaches back to the birth of
• 1:34:34
  Speaker 1: the Sun itself. The Sun probably formed alongside many other
• 1:34:38
  Speaker 1: stars inside a stellar nursery or birth cluster. If another
• 1:34:43
  Speaker 1: young star passed relatively close to the early solar system,
• 1:34:47
  Speaker 1: its gravity could have disturbed distant icy bodies and lifted
• 1:34:52
  Speaker 1: their orbits away from the region controlled by Neptune. Those
• 1:34:56
  Speaker 1: ancient encounters may have left signatures that survive even today.
• 1:35:01
  Speaker 1: Another possibility involves the early migration of the giant planets.
• 1:35:06
  Speaker 1: When Jupiter, Saturn, Uranus and Neptune were moving through the
• 1:35:09
  Speaker 1: young solar system, their gravitational interactions may have produced configurations
• 1:35:15
  Speaker 1: capable of sending objects into unusually distant orbits. At still
• 1:35:20
  Speaker 1: greater distances, the gravitational field of the Milky Way can
• 1:35:24
  Speaker 1: also become relevant. Galactic tides are extremely weak across ordinary
• 1:35:29
  Speaker 1: planetary distances, but over immense spans of space and very
• 1:35:34
  Speaker 1: long periods of time, they can gradually alter the orbits
• 1:35:37
  Speaker 1: of objects only loosely attached to the Sun. And then
• 1:35:42
  Speaker 1: there is the possibility that something large remains undiscovered. In 2016,
• 1:35:49
  Speaker 1: Konstantin Batygin and Michael Brown proposed what became known as
• 1:35:53
  Speaker 1: the Planet 9 hypothesis. They argued that certain extremely distant
• 1:35:58
  Speaker 1: trans-Neptunian objects appeared to have orbital orientations that were not
• 1:36:03
  Speaker 1: randomly distributed. One possible explanation was the gravitational influence of
• 1:36:09
  Speaker 1: a large planet orbiting far beyond Neptune. In many versions
• 1:36:13
  Speaker 1: of the hypothesis, this hypothetical world would have several times
• 1:36:17
  Speaker 1: the mass of Earth and follow a large, elongated orbit
• 1:36:21
  Speaker 1: hundreds of astronomical units from the Sun. Such a planet could,
• 1:36:26
  Speaker 1: in principle, gravitationally organise the paths of smaller distant bodies.
• 1:36:32
  Speaker 1: But Planet 9 has never been directly observed. That point matters.
• 1:36:38
  Speaker 1: It remains a hypothesis, not a discovered planet. Astronomers continue
• 1:36:45
  Speaker 1: debating whether the apparent clustering of distant orbits is genuinely
• 1:36:49
  Speaker 1: caused by an unseen massive object. The sample of known
• 1:36:54
  Speaker 1: distant bodies is still small. Observational surveys do not examine
• 1:36:59
  Speaker 1: every part of the sky equally. Some regions are easier
• 1:37:04
  Speaker 1: to search than others. and some types of orbit make
• 1:37:07
  Speaker 1: objects more likely to be discovered. These selection effects can
• 1:37:11
  Speaker 1: create patterns that look physically meaningful, even when part of
• 1:37:15
  Speaker 1: the pattern comes from the way observations were made. Alternative
• 1:37:20
  Speaker 1: dynamical explanations have also been proposed. More discoveries are needed
• 1:37:26
  Speaker 1: before the question can be settled. Finding those objects is
• 1:37:30
  Speaker 1: exceptionally difficult. A world hundreds of astronomical units away receives
• 1:37:36
  Speaker 1: very little sunlight. The sunlight that does reach it must
• 1:37:40
  Speaker 1: then reflect from its surface and travel all the way
• 1:37:43
  Speaker 1: back toward telescopes near Earth. The result is an extremely
• 1:37:48
  Speaker 1: faint point of light. Distance also makes apparent motion painfully slow.
• 1:37:54
  Speaker 1: A nearby asteroid can visibly shift against the background stars
• 1:37:59
  Speaker 1: over a relatively short observing period. A distant trans-Neptunian object
• 1:38:04
  Speaker 1: may creep across the sky so gradually that astronomers need
• 1:38:08
  Speaker 1: repeated images separated by substantial periods of time to recognise
• 1:38:13
  Speaker 1: the movement. Large astronomical surveys therefore collect enormous numbers of
• 1:38:19
  Speaker 1: images and search them for tiny changes. The National Aeronautics
• 1:38:23
  Speaker 1: and Space Administration-funded Backyard Worlds project has involved members of
• 1:38:28
  Speaker 1: the public in examining why observations for faint moving objects,
• 1:38:33
  Speaker 1: particularly nearby brown dwarfs and possible distant solar system bodies.
• 1:38:39
  Speaker 1: Professional surveys use increasingly powerful cameras, automated processing, and long-term
• 1:38:46
  Speaker 1: observations to perform similar searches across vast areas of sky.
• 1:38:51
  Speaker 1: Yet our map remains incomplete. This is partly why the
• 1:38:55
  Speaker 1: scattered disk and detached populations are so important. They occupy
• 1:39:01
  Speaker 1: an uncertain region between the comparatively familiar Kuiper belt and
• 1:39:06
  Speaker 1: something much more distant. Some detached objects may represent an
• 1:39:10
  Speaker 1: intermediate population, leading toward the inner portion of the Oort cloud.
• 1:39:16
  Speaker 1: Others may have completely different histories. The categories themselves can
• 1:39:20
  Speaker 1: become blurred because the outer solar system is not divided
• 1:39:24
  Speaker 1: into perfectly separated shells. The farther we travel from the planets,
• 1:39:29
  Speaker 1: the less useful the picture of tidy concentric rings becomes. Instead,
• 1:39:34
  Speaker 1: there are overlapping populations. Some were scattered by Neptune. Some
• 1:39:39
  Speaker 1: were trapped into resonances. Some may preserve evidence of stellar
• 1:39:43
  Speaker 1: encounters billions of years ago. Some travel so far outward
• 1:39:48
  Speaker 1: that the wider galaxy begins to influence them. And somewhere
• 1:39:52
  Speaker 1: among these faint, slowly moving worlds may still be objects
• 1:39:56
  Speaker 1: large enough to change our understanding of the solar system again.
• 1:40:00
  Speaker 1: The edge is becoming harder to draw because the solar
• 1:40:03
  Speaker 1: system itself is becoming less orderly. What began as a
• 1:40:08
  Speaker 1: row of planets has dissolved into an enormous gravitational landscape,
• 1:40:13
  Speaker 1: filled with remnants whose strange orbits preserve the history of
• 1:40:16
  Speaker 1: events that happened when the Sun was still young. For
• 1:40:21
  Speaker 1: most of human history, the Kuiper Belt could only be
• 1:40:24
  Speaker 1: studied from a distance. Its worlds were points of light
• 1:40:28
  Speaker 1: moving slowly against the stars, their surfaces hidden across billions
• 1:40:32
  Speaker 1: of kilometres of space. New Horizons changed that. The mission
• 1:40:38
  Speaker 1: was created by the National Aeronautics and Space Administration to
• 1:40:42
  Speaker 1: explore Pluto and then continue into the Wydek Hyperbelt, becoming
• 1:40:47
  Speaker 1: the first spacecraft designed specifically to examine this distant population
• 1:40:52
  Speaker 1: at close range. The spacecraft was developed and is operated
• 1:40:57
  Speaker 1: by the Johns Hopkins University Applied Physics Laboratory, working with
• 1:41:01
  Speaker 1: the Southwest Research Institute. Alan Stern serves as the mission's
• 1:41:07
  Speaker 1: principal investigator. New Horizons launched in January 2006. Reaching Pluto
• 1:41:15
  Speaker 1: directly would have required an extremely long journey, so the
• 1:41:19
  Speaker 1: spacecraft followed a carefully designed path past Jupiter. In 2007,
• 1:41:26
  Speaker 1: it flew through the Jovian system and used Jupiter's gravity
• 1:41:29
  Speaker 1: to increase its speed. This gravity assist shortened the journey
• 1:41:34
  Speaker 1: to Pluto by several years. Even with that help, New
• 1:41:39
  Speaker 1: Horizons still spent about nine and a half years travelling
• 1:41:42
  Speaker 1: through space before reaching its primary destination. During most of
• 1:41:47
  Speaker 1: that time, there was very little scenery. The spacecraft crossed
• 1:41:52
  Speaker 1: enormous stretches of the outer solar system, with the planets
• 1:41:56
  Speaker 1: growing increasingly distant behind it and sunlight steadily weakening. Then,
• 1:42:02
  Speaker 1: in July 2015, Pluto stopped being a distant blur. New
• 1:42:07
  Speaker 1: Horizons passed within roughly 12,500 kilometres of its surface. The
• 1:42:14
  Speaker 1: encounter happened quickly. Because the spacecraft was travelling at many
• 1:42:18
  Speaker 1: kilometres per second and was not slowing down to enter orbit,
• 1:42:22
  Speaker 1: the most important observations had to be collected during a
• 1:42:26
  Speaker 1: narrow window surrounding the flyby. New Horizons was effectively racing
• 1:42:31
  Speaker 1: past an unexplored world while turning its instruments toward carefully
• 1:42:35
  Speaker 1: selected targets. It photographed the surface. It measured the atmosphere.
• 1:42:42
  Speaker 1: It studied Pluto's moons. It examined temperatures, composition, geology and
• 1:42:48
  Speaker 1: escaping gases. And then it continued onward. Because Pluto was
• 1:42:54
  Speaker 1: billions of kilometres from Earth, New Horizons could not simply
• 1:42:58
  Speaker 1: send all of its information home immediately. Radio signals took
• 1:43:03
  Speaker 1: hours to cross the distance. The spacecraft stored its observations
• 1:43:08
  Speaker 1: and transmitted them gradually through the deep space network. The
• 1:43:12
  Speaker 1: complete return of the flyby data took more than a year.
• 1:43:16
  Speaker 1: What arrived completely changed the image of Pluto. Instead of
• 1:43:21
  Speaker 1: an ancient, uniformly cratered, frozen world, Scientists saw landscapes that
• 1:43:26
  Speaker 1: appeared surprisingly active. There were mountains several kilometres high made
• 1:43:31
  Speaker 1: largely from water ice. There were extensive deposits of frozen nitrogen.
• 1:43:36
  Speaker 1: There were layers of atmospheric haze rising above the surface.
• 1:43:41
  Speaker 1: There were signs of glacial movement, varied surface chemistry and
• 1:43:45
  Speaker 1: terrain that had been renewed relatively recently in geological terms.
• 1:43:50
  Speaker 1: Some large features have also been interpreted as possible cryovolcanoes,
• 1:43:55
  Speaker 1: structures formed when icy material or other volatile substances rise
• 1:44:00
  Speaker 1: from the interior rather than molten rock erupting as they
• 1:44:03
  Speaker 1: would in ordinary terrestrial volcanism. The most famous region, Sputnik Planitia,
• 1:44:10
  Speaker 1: revealed broad expanses of nitrogen ice divided into enormous cells.
• 1:44:15
  Speaker 1: These patterns are consistent with slow convection within the ice.
• 1:44:19
  Speaker 1: showing that even at Pluto's extraordinary distance from the Sun,
• 1:44:23
  Speaker 1: the surface can continue to evolve. But Pluto was not
• 1:44:27
  Speaker 1: the end of the mission. New Horizons was already travelling
• 1:44:31
  Speaker 1: through the Kuiper Belt, and astronomers began searching for another
• 1:44:35
  Speaker 1: object lying close enough to the spacecraft's trajectory to permit
• 1:44:39
  Speaker 1: a second encounter. They found one. On the first day
• 1:44:45
  Speaker 1: of 2019, New Horizons flew past Arrokoth. The encounter took
• 1:44:52
  Speaker 1: place more than 6 billion kilometres from the Sun. Arrokoth
• 1:44:56
  Speaker 1: is dramatically smaller than Pluto, only a few tens of
• 1:45:00
  Speaker 1: kilometres long, but in some respects that made it even
• 1:45:03
  Speaker 1: more valuable. It has two distinct lobes joined together, producing
• 1:45:08
  Speaker 1: what astronomers call a contact binary. Rather than appearing to
• 1:45:13
  Speaker 1: have formed through a violent high-speed collision, Its shape suggests
• 1:45:17
  Speaker 1: that two separate objects approached one another comparatively gently before
• 1:45:22
  Speaker 1: becoming permanently joined. That seemingly modest detail reaches back more
• 1:45:28
  Speaker 1: than 4.5 billion years. During the formation of the solar system,
• 1:45:34
  Speaker 1: tiny particles gradually assembled into larger bodies. Exactly how those
• 1:45:41
  Speaker 1: objects grew into planetesimals, and eventually into planets, remains an
• 1:45:46
  Speaker 1: important question. Arrokoth appears to preserve evidence of a relatively
• 1:45:52
  Speaker 1: gentle form of assembly in which nearby bodies formed within
• 1:45:55
  Speaker 1: the same region lost relative speed and gradually came together.
• 1:46:01
  Speaker 1: Unlike larger planets and moons, Arrokoth has not been strongly
• 1:46:06
  Speaker 1: transformed by internal heat. It has not developed a substantial atmosphere.
• 1:46:11
  Speaker 1: It has not experienced the extensive geological recycling found on
• 1:46:15
  Speaker 1: larger worlds, and its remote location means it has remained
• 1:46:19
  Speaker 1: comparatively isolated from many of the processes that dramatically altered
• 1:46:24
  Speaker 1: objects closer to the sun. It is therefore something close
• 1:46:29
  Speaker 1: to a preserved sample from the solar system's earliest construction.
• 1:46:34
  Speaker 1: New Horizons did not simply photograph it. Measurements of its shape,
• 1:46:38
  Speaker 1: surface colour, composition, cratering and geology provided clues about how
• 1:46:43
  Speaker 1: primitive bodies formed within the outer protoplanetary disk. The spacecraft
• 1:46:49
  Speaker 1: has also studied other Kuiper belt objects from a distance,
• 1:46:53
  Speaker 1: and measured the dust environment through which it travels. These
• 1:46:57
  Speaker 1: observations help scientists estimate how much small material exists in
• 1:47:02
  Speaker 1: regions that are almost impossible to examine directly from Earth.
• 1:47:06
  Speaker 1: but there is an important distinction between travelling through the
• 1:47:09
  Speaker 1: Kuiper Belt and crossing a precise edge. The Kuiper Belt
• 1:47:15
  Speaker 1: does not end like a coastline. Its populations gradually thin.
• 1:47:20
  Speaker 1: Different classes of objects occupy different distances and different kinds
• 1:47:25
  Speaker 1: of orbit. Some remain within the classical belt. Some are
• 1:47:30
  Speaker 1: controlled by resonances with Neptune. Others belong to the scattered disk.
• 1:47:35
  Speaker 1: and travel far beyond the main concentration of objects. So
• 1:47:40
  Speaker 1: there is no moment when New Horizons can pass a
• 1:47:42
  Speaker 1: sign announcing that the Kuiper Belt has officially ended. The
• 1:47:46
  Speaker 1: spacecraft simply travels into regions where the known population becomes
• 1:47:51
  Speaker 1: increasingly sparse. Operating anything at these distances is extraordinarily difficult.
• 1:47:58
  Speaker 1: Sunlight is too weak for conventional solar panels to provide
• 1:48:01
  Speaker 1: practical power, so New Horizons relies on a radioisotope power source.
• 1:48:08
  Speaker 1: Communication becomes slower with every kilometre travelled. Radio signals now
• 1:48:13
  Speaker 1: require many hours to cross the distance between the spacecraft
• 1:48:17
  Speaker 1: and Earth. Commands must therefore be prepared long before anyone
• 1:48:22
  Speaker 1: can know the result. Navigation is similarly unforgiving. At billions
• 1:48:27
  Speaker 1: of kilometres from Earth, a tiny error in direction can
• 1:48:31
  Speaker 1: become a very large miss by the time a spacecraft
• 1:48:34
  Speaker 1: reaches its target. There is no real time correction from
• 1:48:38
  Speaker 1: a person holding a joystick. Everything depends on careful planning,
• 1:48:43
  Speaker 1: precise navigation, autonomous spacecraft systems, and patience. New Horizons has
• 1:48:50
  Speaker 1: shown us that the Kuiper Belt is not merely a
• 1:48:52
  Speaker 1: distant collection of frozen debris. It contains complex worlds and
• 1:48:57
  Speaker 1: remarkably well-preserved remnants of planetary formation. Pluto revealed active geology.
• 1:49:05
  Speaker 1: Arrokoth revealed something closer to the raw material from which
• 1:49:08
  Speaker 1: worlds were assembled. And beyond these objects, the spacecraft continues
• 1:49:13
  Speaker 1: into a part of the solar system where solid worlds
• 1:49:16
  Speaker 1: become increasingly sparse and another kind of structure becomes more important.
• 1:49:23
  Speaker 1: Even where there are fewer planets, moons and icy bodies
• 1:49:26
  Speaker 1: to see, the Sun is still creating an enormous invisible
• 1:49:30
  Speaker 1: environment around them all. The Sun does much more than
• 1:49:35
  Speaker 1: illuminate the solar system. It also fills the space around
• 1:49:39
  Speaker 1: it with matter. Every second, enormous numbers of charged particles
• 1:49:44
  Speaker 1: stream away from the Sun in all directions. Most are
• 1:49:48
  Speaker 1: protons and electrons, moving outward at hundreds of kilometres per second.
• 1:49:54
  Speaker 1: This continuous flow is called the solar wind. Long before
• 1:49:58
  Speaker 1: spacecraft could measure it directly, Eugene Parker argued in the
• 1:50:03
  Speaker 1: 1950s that the Sun's outer atmosphere could not remain perfectly static.
• 1:50:08
  Speaker 1: The extremely hot corona should expand outward into space. Parker's
• 1:50:14
  Speaker 1: calculations predicted a persistent, supersonic stream of charged particles leaving
• 1:50:20
  Speaker 1: the Sun. At first, the idea was controversial. Space was
• 1:50:25
  Speaker 1: still often imagined as almost completely empty. But early spacecraft
• 1:50:30
  Speaker 1: measurements confirmed that the solar wind was real. The Sun
• 1:50:34
  Speaker 1: was not sitting quietly at the centre of a vacuum.
• 1:50:37
  Speaker 1: it was constantly breathing material into the solar system. As
• 1:50:42
  Speaker 1: the solar wind moves outward, it carries more than particles.
• 1:50:46
  Speaker 1: It also carries the Sun's magnetic field. Because the Sun rotates,
• 1:50:52
  Speaker 1: those magnetic field lines become stretched and twisted into an
• 1:50:55
  Speaker 1: enormous spiral pattern through space. The result is a vast
• 1:51:00
  Speaker 1: region around the Sun known as the heliosphere. The heliosphere
• 1:51:05
  Speaker 1: surrounds all the major planets, Earth is inside it. Saturn
• 1:51:10
  Speaker 1: is inside it. Neptune is inside it. The Kuiper belt
• 1:51:15
  Speaker 1: lies within much of it as well. But the heliosphere
• 1:51:18
  Speaker 1: should not be confused with the complete gravitational solar system.
• 1:51:23
  Speaker 1: They are defined by different things. An object belongs gravitationally
• 1:51:28
  Speaker 1: to the solar system if it remains bound to the
• 1:51:30
  Speaker 1: Sun through gravity. The heliosphere, by contrast, is defined by
• 1:51:37
  Speaker 1: the behaviour of plasma, magnetic fields and pressure. A spacecraft
• 1:51:42
  Speaker 1: can leave the heliosphere while still remaining strongly affected by
• 1:51:46
  Speaker 1: the Sun's gravity. That distinction becomes extremely important when we
• 1:51:50
  Speaker 1: ask where the solar system ends. The solar wind is
• 1:51:55
  Speaker 1: also far from uniform. Its speed changes, its density changes,
• 1:52:01
  Speaker 1: its magnetic structure changes, different regions of the Sun can
• 1:52:06
  Speaker 1: produce faster or slower streams. The Sun's roughly 11-year activity
• 1:52:11
  Speaker 1: cycle alters its magnetic field and changes the conditions flowing outward.
• 1:52:17
  Speaker 1: Solar eruptions can throw enormous clouds of plasma into space.
• 1:52:22
  Speaker 1: These events, known as coronal mass ejections, can disturb the
• 1:52:26
  Speaker 1: solar wind and create large changes in the magnetic environment
• 1:52:30
  Speaker 1: across the solar system. Near Earth, These disturbances can produce
• 1:52:35
  Speaker 1: geomagnetic storms and auroras. Farther away, their influence spreads into
• 1:52:40
  Speaker 1: an enormous bubble billions of kilometres across. Eventually, however, the
• 1:52:46
  Speaker 1: solar wind begins to lose the contest. The space between
• 1:52:50
  Speaker 1: stars is not truly empty. It contains the interstellar medium.
• 1:52:55
  Speaker 1: This is an extremely thin mixture of neutral gas, charged plasma,
• 1:53:01
  Speaker 1: dust grains. energetic particles and magnetic fields. Compared with Earth's atmosphere,
• 1:53:09
  Speaker 1: it is almost unimaginably sparse. But over enormous distances, even
• 1:53:14
  Speaker 1: this faint material exerts pressure. The heliosphere exists because the
• 1:53:20
  Speaker 1: outward pressure of the solar wind pushes against the surrounding
• 1:53:23
  Speaker 1: interstellar environment. Closer to the Sun, the solar wind dominates.
• 1:53:30
  Speaker 1: Farther away, the balance begins to change. One of the
• 1:53:34
  Speaker 1: first major boundaries is called the termination shock. Until this point,
• 1:53:39
  Speaker 1: much of the solar wind has been moving faster than
• 1:53:42
  Speaker 1: the speed at which pressure disturbances can travel through the plasma.
• 1:53:46
  Speaker 1: In that sense, the flow is supersonic. At the termination shock,
• 1:53:51
  Speaker 1: pressure from the surrounding interstellar environment forces the solar wind
• 1:53:56
  Speaker 1: to slow abruptly. It becomes subsonic. The particles do not stop.
• 1:54:01
  Speaker 1: They continue moving outward but now they are slower, hotter
• 1:54:05
  Speaker 1: and more compressed. Beyond the termination shock lies a region
• 1:54:10
  Speaker 1: called the heliosheath. This is the outer layer of the heliosphere.
• 1:54:15
  Speaker 1: The solar wind has been slowed and disturbed here, and
• 1:54:18
  Speaker 1: the plasma becomes increasingly turbulent as it approaches the boundary
• 1:54:22
  Speaker 1: with interstellar space. The heliosheath is not a calm shell.
• 1:54:28
  Speaker 1: It is a complicated region where charged particles, magnetic fields
• 1:54:32
  Speaker 1: and pressure interact across enormous distances. Eventually comes the heliopause.
• 1:54:39
  Speaker 1: This is the boundary where the outward influence of the
• 1:54:42
  Speaker 1: solar wind is balanced by the surrounding interstellar environment. Beyond it,
• 1:54:48
  Speaker 1: the plasma is no longer dominated by material flowing directly
• 1:54:53
  Speaker 1: outward from the Sun. The heliopause is therefore one of
• 1:54:58
  Speaker 1: the strongest candidates for what we might call an edge
• 1:55:02
  Speaker 1: of the solar system. but it is not shaped like
• 1:55:06
  Speaker 1: a perfect glass bubble. The heliosphere changes. Solar activity varies
• 1:55:13
  Speaker 1: with time. The surrounding interstellar medium is not perfectly uniform.
• 1:55:19
  Speaker 1: The Sun itself is moving through the Milky Way, carrying
• 1:55:23
  Speaker 1: the heliosphere with it. All of these factors affect the
• 1:55:26
  Speaker 1: pressure balance. For many years, the heliosphere was often pictured
• 1:55:31
  Speaker 1: rather like a comet, with a rounded region facing the
• 1:55:35
  Speaker 1: direction of the Sun's motion and a very long tail
• 1:55:38
  Speaker 1: extending behind it. Modern models and observations suggest that the
• 1:55:43
  Speaker 1: true geometry may be more complicated. Different models produce shapes
• 1:55:48
  Speaker 1: ranging from stretched structures to more compact forms with broad lobes.
• 1:55:54
  Speaker 1: The important point is that there is no perfectly smooth
• 1:55:57
  Speaker 1: spherical wall surrounding the Sun. The heliosphere is dynamic. It
• 1:56:03
  Speaker 1: responds to both the sun and the environment through which
• 1:56:06
  Speaker 1: the solar system is travelling. It also offers us some protection.
• 1:56:11
  Speaker 1: Galactic cosmic rays are extremely energetic particles arriving from outside
• 1:56:16
  Speaker 1: the solar system. Some originate from violent processes elsewhere in
• 1:56:21
  Speaker 1: the Milky Way, including supernova remnants and other high-energy environments.
• 1:56:27
  Speaker 1: The magnetic fields and charged particles of the heliosphere can
• 1:56:31
  Speaker 1: deflect or reduce some of these incoming particles. The protection
• 1:56:36
  Speaker 1: is not complete. Cosmic rays still reach Earth and the
• 1:56:39
  Speaker 1: other planets, but conditions inside the heliosphere are different from
• 1:56:44
  Speaker 1: those outside it. In that sense, the Sun creates an
• 1:56:47
  Speaker 1: enormous environmental bubble around its planetary family. Nothing about this
• 1:56:53
  Speaker 1: boundary can be seen in the way we see the
• 1:56:55
  Speaker 1: surface of a planet. There is no glowing wall in
• 1:56:58
  Speaker 1: the darkness. No obvious line appears in photographs. To detect
• 1:57:04
  Speaker 1: the heliosphere's outer regions, spacecraft must measure particles, plasma density,
• 1:57:10
  Speaker 1: magnetic fields and energetic radiation. The farther outward we travel,
• 1:57:16
  Speaker 1: the less the solar system resembles a collection of visible worlds.
• 1:57:21
  Speaker 1: Its boundaries become invisible. They are revealed not by what
• 1:57:26
  Speaker 1: we can see, but by changes in the behaviour of
• 1:57:29
  Speaker 1: matter itself, and eventually a spacecraft can cross one of
• 1:57:33
  Speaker 1: those invisible boundaries and find that the particles surrounding it
• 1:57:38
  Speaker 1: no longer belong mainly to the Sun. At that point,
• 1:57:43
  Speaker 1: in one very specific and scientifically meaningful sense, interstellar space
• 1:57:48
  Speaker 1: has begun. There is a point far beyond Neptune and
• 1:57:54
  Speaker 1: far beyond the main Kuiper belt. where the Sun's influence
• 1:57:58
  Speaker 1: changes in a very specific way. This boundary is called
• 1:58:03
  Speaker 1: the heliopause. It marks the outer edge of the heliosphere,
• 1:58:08
  Speaker 1: the enormous region filled and shaped by the solar wind.
• 1:58:13
  Speaker 1: Inside the heliopause, the space environment is still dominated by
• 1:58:17
  Speaker 1: particles and magnetic fields carried outward from the Sun. Beyond it,
• 1:58:23
  Speaker 1: the surrounding interstellar plasma becomes dominant. The boundary is invisible
• 1:58:29
  Speaker 1: and There is no wall, no surface and no sudden
• 1:58:33
  Speaker 1: change in scenery. A spacecraft crossing it would not see
• 1:58:38
  Speaker 1: a line pass beneath it. Instead, the evidence appears in measurements.
• 1:58:44
  Speaker 1: Particle populations change. Plasma density changes. Magnetic conditions change. And
• 1:58:52
  Speaker 1: the two spacecraft that have measured this transition directly are
• 1:58:56
  Speaker 1: Voyager 1 and Voyager 2. Both were launched by the
• 1:59:01
  Speaker 1: National Aeronautics and Space Administration in 1977. Their original purpose
• 1:59:08
  Speaker 1: was planetary exploration. Voyager 1 travelled past Jupiter and Saturn.
• 1:59:15
  Speaker 1: Voyager 2 continued farther, becoming the only spacecraft ever to
• 1:59:21
  Speaker 1: visit Uranus and Neptune at close range. After those encounters,
• 1:59:26
  Speaker 1: both spacecraft continued outward. their planetary missions gradually became interstellar missions.
• 1:59:34
  Speaker 1: For decades, Edward Stone served as Voyager project scientist, helping
• 1:59:39
  Speaker 1: guide the interpretation of measurements arriving from increasingly distant parts
• 1:59:43
  Speaker 1: of the solar system. The Jet Propulsion Laboratory has managed
• 1:59:48
  Speaker 1: the mission. While communication has depended on the Deep Space Network,
• 1:59:53
  Speaker 1: a global system of enormous radio antennas capable of detecting
• 1:59:57
  Speaker 1: extraordinarily weak signals from deep space. By the early 21st century,
• 2:00:03
  Speaker 1: Voyager 1 was approaching an environment that no functioning spacecraft
• 2:00:08
  Speaker 1: had ever sampled directly. In 2004, it crossed the termination shock.
• 2:00:14
  Speaker 1: This is the region where the solar wind, previously travelling
• 2:00:17
  Speaker 1: outward at supersonic speed, slows sharply after encountering pressure from
• 2:00:22
  Speaker 1: the surrounding interstellar medium. Voyager 1 then entered the heliosheath.
• 2:00:28
  Speaker 1: For years it travelled through this turbulent outer region of
• 2:00:31
  Speaker 1: the heliosphere. Scientists knew that the heliopause should lie farther ahead,
• 2:00:37
  Speaker 1: but there was no simple marker announcing exactly where it
• 2:00:40
  Speaker 1: would be. Then, in August 2012, Voyager 1 recorded a
• 2:00:46
  Speaker 1: major change. The number of energetic particles associated with the
• 2:00:51
  Speaker 1: heliosphere fell sharply. At almost the same time, the spacecraft
• 2:00:56
  Speaker 1: detected an increase in galactic cosmic rays arriving from outside
• 2:01:00
  Speaker 1: the heliosphere. Something fundamental about the surrounding environment had changed.
• 2:01:08
  Speaker 1: The evidence became stronger when Voyager 1's plasma wave instrument
• 2:01:12
  Speaker 1: detected oscillations associated with a denser plasma than that typically
• 2:01:17
  Speaker 1: found inside the heliosphere. The spacecraft's direct plasma instrument had
• 2:01:23
  Speaker 1: stopped functioning many years earlier, so scientists could not simply
• 2:01:27
  Speaker 1: measure the surrounding plasma in the most straightforward way. Instead,
• 2:01:32
  Speaker 1: they used the frequency of plasma waves to infer the density.
• 2:01:37
  Speaker 1: Those measurements showed that Voyager 1 had entered the denser
• 2:01:40
  Speaker 1: interstellar plasma beyond the heliopause. The conclusion became clear. Voyager
• 2:01:47
  Speaker 1: 1 had entered interstellar space. Its crossing occurred at a
• 2:01:51
  Speaker 1: distance of roughly 120 astronomical units from the Sun. That
• 2:01:56
  Speaker 1: is around four times Neptune's average distance. Six years later,
• 2:02:01
  Speaker 1: Voyager 2 followed. Voyager 2 had crossed the termination shock
• 2:02:06
  Speaker 1: in 2007. It then travelled through its own section of
• 2:02:10
  Speaker 1: the heliosheath before reaching the heliopause in November 2018. Its
• 2:02:16
  Speaker 1: crossing occurred at a slightly different distance. and along a
• 2:02:20
  Speaker 1: different direction from the Sun. This was scientifically valuable because
• 2:02:25
  Speaker 1: the heliopause is not expected to be a perfectly spherical boundary.
• 2:02:29
  Speaker 1: The Sun is moving through the surrounding interstellar medium. Solar
• 2:02:34
  Speaker 1: activity changes over time. The pressure of the solar wind varies.
• 2:02:40
  Speaker 1: The local interstellar environment is also not perfectly uniform. As
• 2:02:44
  Speaker 1: a result, the heliopause can sit at different distances in
• 2:02:48
  Speaker 1: different directions and can shift as conditions change. Voyager 2
• 2:02:54
  Speaker 1: provided another advantage. Unlike Voyager 1, its plasma instrument was
• 2:03:00
  Speaker 1: still operating when it crossed the boundary. This allowed the
• 2:03:03
  Speaker 1: spacecraft to measure the plasma directly. The data showed a
• 2:03:09
  Speaker 1: transition from the thinner, hotter plasma associated with the outer
• 2:03:13
  Speaker 1: heliosphere into the denser interstellar plasma beyond it. Together, the
• 2:03:18
  Speaker 1: two Voyager crossings gave scientists the first direct measurements of
• 2:03:22
  Speaker 1: the Sun's outer plasma boundary from two different locations, and
• 2:03:27
  Speaker 1: yet the phrase interstellar space can easily create the wrong impression.
• 2:03:32
  Speaker 1: Voyager 1 and Voyager 2 have entered interstellar space in
• 2:03:36
  Speaker 1: the sense that they are outside the heliosphere. They are
• 2:03:40
  Speaker 1: no longer travelling through plasma dominated by the solar wind.
• 2:03:44
  Speaker 1: but they have not escaped the Sun's gravitational influence in
• 2:03:47
  Speaker 1: any absolute sense. They are also nowhere near the probable
• 2:03:51
  Speaker 1: outer gravitational boundary of the solar system. The distinction matters.
• 2:03:57
  Speaker 1: The heliopause defines the edge of the Sun's solar wind environment.
• 2:04:02
  Speaker 1: It does not define the outermost distance at which objects
• 2:04:05
  Speaker 1: can remain gravitationally bound to the Sun. That more distant
• 2:04:10
  Speaker 1: frontier may lie thousands of times farther away. The Voyagers
• 2:04:15
  Speaker 1: themselves continue travelling outward at remarkable speeds, but even those
• 2:04:20
  Speaker 1: speeds become modest when compared with the distances between stars.
• 2:04:25
  Speaker 1: Neither spacecraft is heading toward a close encounter with another
• 2:04:28
  Speaker 1: star any time soon. Their journeys through interstellar space will
• 2:04:33
  Speaker 1: continue for tens of thousands of years before they pass
• 2:04:36
  Speaker 1: anywhere relatively near another stellar system. Communication with them is
• 2:04:42
  Speaker 1: already difficult. Radio signals take many hours to travel between
• 2:04:47
  Speaker 1: Earth and the spacecraft. By the time engineers transmit a
• 2:04:50
  Speaker 1: command and receive confirmation that it has been carried out,
• 2:04:55
  Speaker 1: much of a day can pass. Their electrical power also
• 2:04:59
  Speaker 1: continues to decline. Both spacecraft rely on radioisotope generators, which
• 2:05:05
  Speaker 1: convert heat from the natural radioactive decay of plutonium into electricity.
• 2:05:12
  Speaker 1: As that heat source slowly weakens and the spacecraft systems age,
• 2:05:17
  Speaker 1: engineers have had to switch off instruments and heaters to
• 2:05:20
  Speaker 1: conserve power. The mission has become an exercise in careful survival.
• 2:05:27
  Speaker 1: Every remaining watt matters. Despite that practical reality, both spacecraft
• 2:05:34
  Speaker 1: carry something designed for a timescale far beyond their electronics.
• 2:05:39
  Speaker 1: Attached to each Voyager is a golden record. The records
• 2:05:43
  Speaker 1: contain sounds, images, music, greetings and information intended to represent
• 2:05:49
  Speaker 1: something of Earth and humanity. They were never expected to
• 2:05:53
  Speaker 1: reach another civilisation in any foreseeable future. Their real significance
• 2:05:59
  Speaker 1: is symbolic. Long after the spacecraft fall silent, the records
• 2:06:04
  Speaker 1: may continue drifting through the Milky Way. The Voyager missions
• 2:06:08
  Speaker 1: have therefore crossed one genuine edge of the solar system.
• 2:06:12
  Speaker 1: They have left the Sun's solar wind bubble and entered
• 2:06:16
  Speaker 1: the interstellar environment beyond it. But the heliopause answers only
• 2:06:21
  Speaker 1: one version of the original question. The Sun's particles may
• 2:06:25
  Speaker 1: have largely given way to interstellar plasma. Its gravity has not.
• 2:06:31
  Speaker 1: And somewhere much farther out, enormous populations of objects may
• 2:06:35
  Speaker 1: still be orbiting the same star the Voyagers left behind.
• 2:06:41
  Speaker 1: There is something slightly confusing about the way we talk
• 2:06:44
  Speaker 1: about the Voyager spacecraft. Voyager 1 and Voyager 2 are
• 2:06:49
  Speaker 1: often described as interstellar travellers. That is true. Both spacecraft
• 2:06:55
  Speaker 1: have crossed the heliopause and entered interstellar space, but they
• 2:06:59
  Speaker 1: are still vastly closer to the Sun than the expected
• 2:07:02
  Speaker 1: outer reaches of the Oort cloud. So how can something
• 2:07:06
  Speaker 1: be an interstellar space while still being inside the broader
• 2:07:10
  Speaker 1: gravitational solar system? The answer comes down to the fact
• 2:07:14
  Speaker 1: that different boundaries are defined by different physical effects. The
• 2:07:19
  Speaker 1: heliopause is a plasma boundary. It marks the region where
• 2:07:23
  Speaker 1: the solar wind, the stream of charged particles flowing outward
• 2:07:27
  Speaker 1: from the Sun, can no longer dominate the surrounding interstellar plasma.
• 2:07:33
  Speaker 1: Crossing that boundary tells us something about the material surrounding
• 2:07:37
  Speaker 1: a spacecraft. It does not mean that the gravity of
• 2:07:41
  Speaker 1: the Sun has suddenly disappeared. Gravity behaves very differently. The
• 2:07:46
  Speaker 1: gravitational attraction between two objects weakens as they become farther apart.
• 2:07:52
  Speaker 1: More specifically, gravitational strength follows what is known as an
• 2:07:57
  Speaker 1: inverse square relationship. If the distance from the sun doubles,
• 2:08:02
  Speaker 1: its gravitational pull becomes four times weaker. If the distance
• 2:08:07
  Speaker 1: becomes three times greater, the pull becomes nine times weaker.
• 2:08:12
  Speaker 1: But there is no particular distance where gravity suddenly drops
• 2:08:15
  Speaker 1: to nothing. It simply becomes progressively weaker. This means Voyager
• 2:08:21
  Speaker 1: 1 and Voyager 2 continue to experience the gravitational influence
• 2:08:25
  Speaker 1: of the Sun even after leaving the heliosphere. The important
• 2:08:30
  Speaker 1: detail is that neither spacecraft is gravitationally bound to the
• 2:08:34
  Speaker 1: Sun anymore in the ordinary sense. Their trajectories carry enough
• 2:08:38
  Speaker 1: energy for them to continue escaping outward rather than eventually
• 2:08:43
  Speaker 1: turning around and returning. That gives us three related ideas
• 2:08:48
  Speaker 1: that are easy to mix together. An object can be
• 2:08:51
  Speaker 1: influenced by the Sun's gravity. An object can be gravitationally
• 2:08:56
  Speaker 1: bound to the Sun. And an object can travel through
• 2:09:00
  Speaker 1: a region where the Sun remains the strongest nearby gravitational influence.
• 2:09:06
  Speaker 1: Those are not necessarily the same thing. Earth is strongly
• 2:09:10
  Speaker 1: influenced by the Sun and gravitationally bound to it. A
• 2:09:14
  Speaker 1: distant comet in the Oort cloud may also remain bound
• 2:09:17
  Speaker 1: to the Sun. even though the attraction is incredibly weak. Voyager,
• 2:09:23
  Speaker 1: by contrast, still feels the sun's gravity, but has enough
• 2:09:27
  Speaker 1: energy to escape permanently. The farther away we travel, the
• 2:09:31
  Speaker 1: more complicated the situation becomes, because the sun is not
• 2:09:35
  Speaker 1: the only source of gravity. The solar system exists inside
• 2:09:40
  Speaker 1: the Milky Way. The galaxy contains hundreds of billions of stars,
• 2:09:46
  Speaker 1: along with gas, dust, dark matter, stellar remnants, and other
• 2:09:51
  Speaker 1: sources of mass. All of these contribute to a much
• 2:09:55
  Speaker 1: larger gravitational environment. At ordinary planetary distances, the Sun dominates
• 2:10:02
  Speaker 1: so strongly that the galactic contribution is almost irrelevant when
• 2:10:07
  Speaker 1: describing an orbit. But far enough away, that begins to change.
• 2:10:13
  Speaker 1: Astronomers sometimes describe the Sun's broad gravitational neighbourhood using the
• 2:10:18
  Speaker 1: idea of a hill sphere. A hill sphere is an
• 2:10:21
  Speaker 1: approximate region around one object where its gravity can dominate
• 2:10:25
  Speaker 1: the tidal influence of a much larger gravitational system. For
• 2:10:30
  Speaker 1: the Sun, that larger system is the Milky Way. The
• 2:10:34
  Speaker 1: idea is useful, but it should not be imagined as
• 2:10:37
  Speaker 1: a perfectly spherical wall. The true gravitational environment is much messier.
• 2:10:43
  Speaker 1: The Sun is moving through the galaxy. Nearby stars are
• 2:10:47
  Speaker 1: moving as well. The distribution of mass in the Milky
• 2:10:51
  Speaker 1: Way is not perfectly uniform, and extremely distant objects can
• 2:10:55
  Speaker 1: be disturbed by influences that would barely matter closer to
• 2:10:59
  Speaker 1: the Sun. Passing stars are particularly important. A star passing
• 2:11:05
  Speaker 1: within a few light years can exert a small gravitational
• 2:11:08
  Speaker 1: tug on distant solar system objects. For Earth or Jupiter,
• 2:11:13
  Speaker 1: that effect would be negligible. For an icy body orbiting
• 2:11:18
  Speaker 1: tens of thousands of astronomical units from the Sun, the
• 2:11:22
  Speaker 1: same encounter can matter much more. Its attachment to the
• 2:11:26
  Speaker 1: Sun is so weak that a passing star may alter
• 2:11:29
  Speaker 1: the shape of its orbit. It might push the object
• 2:11:32
  Speaker 1: deeper into the solar system. It might send it farther outward.
• 2:11:37
  Speaker 1: In some cases, it could even remove the object from
• 2:11:40
  Speaker 1: the Sun's control completely. Large molecular clouds can produce similar
• 2:11:45
  Speaker 1: disturbances over longer timescales. These vast regions of gas can
• 2:11:50
  Speaker 1: contain enormous amounts of mass. If the solar system passes
• 2:11:56
  Speaker 1: sufficiently close to one during its journey around the Milky Way,
• 2:12:00
  Speaker 1: the gravitational influence can perturb the most distant objects orbiting
• 2:12:04
  Speaker 1: the Sun. There is also the galactic tide. This is
• 2:12:09
  Speaker 1: the subtle difference in the gravitational pull of the Milky Way.
• 2:12:13
  Speaker 1: across the enormous width of the outer solar system. Again,
• 2:12:17
  Speaker 1: the effect is tiny on human scales, but over millions
• 2:12:21
  Speaker 1: or billions of years, tiny gravitational influences have plenty of
• 2:12:26
  Speaker 1: time to accumulate. The result is that the outer gravitational
• 2:12:30
  Speaker 1: boundary of the solar system is not really a boundary
• 2:12:34
  Speaker 1: at all. It is a transition. Closer in, motion is
• 2:12:39
  Speaker 1: overwhelmingly controlled by the sun. Farther out, the gravitational influence
• 2:12:45
  Speaker 1: of the wider galaxy becomes increasingly important. Eventually, objects become
• 2:12:50
  Speaker 1: so weakly attached to the Sun that passing stars, galactic
• 2:12:55
  Speaker 1: tides and other disturbances can compete with or overwhelm that attachment.
• 2:13:01
  Speaker 1: There is no gate to pass through. No alarm sounds.
• 2:13:06
  Speaker 1: No spacecraft instrument suddenly announces that solar gravity has ended.
• 2:13:11
  Speaker 1: the neighbourhood simply becomes less clearly dominated by the Sun.
• 2:13:16
  Speaker 1: The distances involved are enormous. The nearest star system to
• 2:13:21
  Speaker 1: our own is Alpha Centauri, more than four light-years away.
• 2:13:26
  Speaker 1: That sounds relatively close when compared with the immense size
• 2:13:30
  Speaker 1: of the Milky Way, but four light-years is an extraordinary distance.
• 2:13:36
  Speaker 1: Even the broadest estimates for the outer solar system leave
• 2:13:40
  Speaker 1: a great deal of space between the Sun's most distant
• 2:13:43
  Speaker 1: gravitationally associated objects and neighbouring stars. And these stellar systems
• 2:13:49
  Speaker 1: are not fixed in place. The Sun moves around the
• 2:13:52
  Speaker 1: centre of the Milky Way. Alpha Centauri moves along its
• 2:13:56
  Speaker 1: own galactic path. Other stars pass closer or farther away
• 2:14:01
  Speaker 1: over immense periods of time. So the geometry of the
• 2:14:05
  Speaker 1: Sun's gravitational neighbourhood is continually changing. the outer edge today
• 2:14:11
  Speaker 1: would not necessarily have exactly the same shape or extent
• 2:14:15
  Speaker 1: hundreds of millions of years from now. This is why
• 2:14:19
  Speaker 1: the description of Voyager as interstellar is perfectly accurate, while
• 2:14:24
  Speaker 1: also being incomplete if we interpret it as meaning that
• 2:14:28
  Speaker 1: the spacecraft have crossed every meaningful edge of the solar system.
• 2:14:34
  Speaker 1: They have crossed the heliopause, they have entered the interstellar
• 2:14:38
  Speaker 1: plasma environment, they are escaping the Sun permanently, but they
• 2:14:43
  Speaker 1: remain vastly closer to the Sun than the outermost objects
• 2:14:47
  Speaker 1: that may still be gravitationally bound to it. To find
• 2:14:51
  Speaker 1: the broadest plausible edge of the solar system, we have
• 2:14:55
  Speaker 1: to travel much farther. Beyond the planets. Beyond the Kuiper Belt.
• 2:15:03
  Speaker 1: beyond the heliopause, out into a region so distant that
• 2:15:06
  Speaker 1: its objects can take hundreds of thousands or even millions
• 2:15:10
  Speaker 1: of years to complete a single orbit. There, almost lost
• 2:15:15
  Speaker 1: within interstellar space, lies the enormous and still largely hypothetical
• 2:15:20
  Speaker 1: Oort Cloud. Far beyond the planets, beyond the Kuiper Belt,
• 2:15:26
  Speaker 1: and even beyond the heliopause, the Sun may still be
• 2:15:30
  Speaker 1: holding on to an enormous population of icy bodies. This
• 2:15:34
  Speaker 1: distant region is known as the Oort Cloud. It is
• 2:15:37
  Speaker 1: not something we have photographed directly. No spacecraft has reached it,
• 2:15:42
  Speaker 1: no telescope has ever produced an image showing a vast
• 2:15:46
  Speaker 1: shell of comets surrounding the solar system. Its existence is inferred,
• 2:15:52
  Speaker 1: and that makes it one of the strangest parts of
• 2:15:54
  Speaker 1: the solar system. because it may represent the broadest practical
• 2:15:58
  Speaker 1: edge of the Sun's gravitational family while remaining almost completely invisible.
• 2:16:04
  Speaker 1: The idea developed from the behaviour of comets. In the
• 2:16:08
  Speaker 1: early 20th century, Ernst Pick suggested that a distant reservoir
• 2:16:13
  Speaker 1: of comet-like bodies could exist far beyond the known planets. Then,
• 2:16:19
  Speaker 1: in 1950, Jan Oort examined the orbits of long-period comets
• 2:16:24
  Speaker 1: and noticed something important. Many seemed to arrive from extremely
• 2:16:29
  Speaker 1: large distances. Their orbital orientations also suggested that they were
• 2:16:34
  Speaker 1: not coming from one narrow, flat region. Instead, they appeared
• 2:16:39
  Speaker 1: from many different directions. Oort argued that the solar system
• 2:16:43
  Speaker 1: must contain an enormous distant population of icy bodies surrounding
• 2:16:48
  Speaker 1: the Sun. This became known as the Oort Cloud. Unlike
• 2:16:53
  Speaker 1: the Kuiper Belt, which is arranged roughly along the broad
• 2:16:56
  Speaker 1: plane of the planetary system, the Oort Cloud is thought
• 2:17:00
  Speaker 1: to be much more spherical. Its objects may approach the
• 2:17:03
  Speaker 1: inner solar system from above, below, or almost any other direction.
• 2:17:09
  Speaker 1: There may also be more than one broad component. The
• 2:17:13
  Speaker 1: inner region is sometimes called the Inner Oort Cloud, or
• 2:17:16
  Speaker 1: Hills Cloud. Farther away lies the Outer Oort Cloud, where
• 2:17:21
  Speaker 1: objects are bound to the sun only very weakly. Exactly
• 2:17:27
  Speaker 1: where either region begins or ends remains uncertain. Different models
• 2:17:31
  Speaker 1: produce different estimates. The inner Oort cloud may begin somewhere
• 2:17:36
  Speaker 1: around 1,000 to several thousand astronomical units from the sun.
• 2:17:42
  Speaker 1: The outer edge may extend toward roughly 100,000 astronomical units.
• 2:17:48
  Speaker 1: That distance is almost absurdly large by planetary standards. 100,000
• 2:17:54
  Speaker 1: astronomical units is around one and a half light-years. The
• 2:17:58
  Speaker 1: nearest star system, Alpha Centauri, lies a little more than
• 2:18:02
  Speaker 1: four light-years away. so the outer reaches of the Oort
• 2:18:06
  Speaker 1: cloud may extend a significant fraction of the distance toward
• 2:18:09
  Speaker 1: neighbouring stars. At that scale, the familiar planetary solar system
• 2:18:15
  Speaker 1: becomes tiny. Neptune orbits at about 30 astronomical units. Even
• 2:18:21
  Speaker 1: the heliopause crossed by the Voyager spacecraft lies only a
• 2:18:25
  Speaker 1: little beyond 100 astronomical units. If the Oort Cloud truly
• 2:18:31
  Speaker 1: extends toward 100,000 astronomical units, then the part of the
• 2:18:35
  Speaker 1: solar system containing all eight major planets occupies only a
• 2:18:40
  Speaker 1: minute central region. The Oort Cloud probably began forming when
• 2:18:45
  Speaker 1: the solar system itself was young. During the formation of
• 2:18:49
  Speaker 1: the giant planets, enormous numbers of smaller icy bodies moved
• 2:18:54
  Speaker 1: through the outer regions. Jupiter, Saturn, Uranus and Neptune repeatedly
• 2:18:59
  Speaker 1: disturbed them gravitationally. Some objects were thrown inward. Some were
• 2:19:05
  Speaker 1: ejected from the solar system entirely. Others were scattered outward
• 2:19:10
  Speaker 1: onto enormous orbits. If the Sun had formed completely alone,
• 2:19:15
  Speaker 1: many of those objects might simply have escaped. But the
• 2:19:18
  Speaker 1: Sun probably formed inside a cluster containing many other young stars.
• 2:19:23
  Speaker 1: the gravity of nearby stars could have slowed or redirected
• 2:19:27
  Speaker 1: some of the outward scattered bodies before they escaped completely.
• 2:19:32
  Speaker 1: Instead of leaving forever, they may have been placed onto
• 2:19:35
  Speaker 1: vast weakly bound orbits around the Sun. Over billions of years,
• 2:19:41
  Speaker 1: those orbits could have evolved into the broad population we
• 2:19:44
  Speaker 1: now call the Oort Cloud, and the same external forces
• 2:19:48
  Speaker 1: that may have helped create the cloud can continue disturbing
• 2:19:52
  Speaker 1: it today. At these distances, the Sun's gravitational hold is
• 2:19:57
  Speaker 1: extremely weak. A passing star can alter an object's orbit.
• 2:20:03
  Speaker 1: So can the gravitational tide of the Milky Way. Most
• 2:20:07
  Speaker 1: of the time, an Oort cloud body may remain far
• 2:20:09
  Speaker 1: from the Sun, completing an orbit so slowly that one
• 2:20:13
  Speaker 1: journey can take hundreds of thousands or even millions of years.
• 2:20:18
  Speaker 1: But occasionally, a small gravitational disturbance changes that path. An
• 2:20:24
  Speaker 1: icy body begins falling inward. As it approaches the sun,
• 2:20:28
  Speaker 1: it warms. Frozen gases begin escaping. Dust is released. A
• 2:20:35
  Speaker 1: coma and tail can form. What had spent most of
• 2:20:39
  Speaker 1: its existence as a dark, frozen object at the edge
• 2:20:42
  Speaker 1: of the solar system becomes a long-period comet visible from
• 2:20:46
  Speaker 1: the inner planets. This is one of the main reasons
• 2:20:50
  Speaker 1: scientists think the Oort cloud exists. Short-period comets, which return
• 2:20:56
  Speaker 1: relatively frequently, are commonly associated with the Kuiper belt and
• 2:21:00
  Speaker 1: scattered disk. Long-period comets can arrive on far more extended
• 2:21:05
  Speaker 1: orbits and from a much wider range of directions. Their
• 2:21:10
  Speaker 1: behaviour points toward a distant, more spherical reservoir. But because
• 2:21:15
  Speaker 1: the Oort cloud itself has never been directly observed, much
• 2:21:19
  Speaker 1: of its structure remains theoretical. Its estimated population is uncertain.
• 2:21:26
  Speaker 1: Its inner boundary is uncertain. Its outer boundary is uncertain.
• 2:21:33
  Speaker 1: Even its shape is likely more complicated than a perfectly
• 2:21:36
  Speaker 1: smooth sphere. This is not surprising. The farther we move
• 2:21:40
  Speaker 1: from the sun, the less strongly the sun controls everything.
• 2:21:45
  Speaker 1: The outermost objects may be so weakly bound that encounters
• 2:21:49
  Speaker 1: with passing stars can strip them away. In other circumstances,
• 2:21:54
  Speaker 1: the Sun could potentially capture distant material that originated elsewhere.
• 2:21:59
  Speaker 1: The solar system's edge is therefore not permanent. Its most
• 2:22:03
  Speaker 1: distant members can be lost. Their orbits can change. The
• 2:22:07
  Speaker 1: surrounding galactic environment can reshape the frontier over enormous spans
• 2:22:12
  Speaker 1: of time. The Voyager spacecraft give a useful sense of
• 2:22:17
  Speaker 1: just how extreme these distances are. Voyager 1 and Voyager
• 2:22:23
  Speaker 1: 2 have been traveling outward for decades and have already
• 2:22:27
  Speaker 1: crossed the heliopause. Yet at their present speeds, it would
• 2:22:31
  Speaker 1: take roughly 300 years for them even to approach some
• 2:22:34
  Speaker 1: estimates of the inner Oort cloud. Passing through the broader
• 2:22:39
  Speaker 1: Oort cloud could take on the order of 30,000 years.
• 2:22:43
  Speaker 1: Humanity has reached interstellar space. We are nowhere close to
• 2:22:48
  Speaker 1: reaching the full gravitational edge of our own solar system.
• 2:22:52
  Speaker 1: And that finally gives us the clearest answer to the
• 2:22:55
  Speaker 1: question of where the solar system ends. If we mean
• 2:22:59
  Speaker 1: the system of major planets, it ends at Neptune. If
• 2:23:03
  Speaker 1: we mean the main flattened population of distant icy worlds,
• 2:23:08
  Speaker 1: the Kuiper belt provides another edge. If we mean the
• 2:23:12
  Speaker 1: bubble created by the solar wind, the heliopause is the boundary.
• 2:23:16
  Speaker 1: But if we mean the farthest population of objects still
• 2:23:20
  Speaker 1: gravitationally associated with the sun, then the Oort cloud is
• 2:23:23
  Speaker 1: the best broad answer we currently have. Even then, there
• 2:23:27
  Speaker 1: is no precise line. The solar system fades gradually into
• 2:23:32
  Speaker 1: the wider galaxy. The sun's influence becomes weaker. Passing stars
• 2:23:38
  Speaker 1: matter more. Galactic tides become more important. And somewhere across
• 2:23:43
  Speaker 1: that enormous transition, objects that once clearly belonged to the
• 2:23:48
  Speaker 1: sun become part of interstellar space instead. The solar system
• 2:23:53
  Speaker 1: is therefore not simply eight planets arranged around a star.
• 2:23:58
  Speaker 1: It is a nested structure of worlds, belts, particles, magnetic
• 2:24:03
  Speaker 1: fields and distant frozen bodies. The planets occupy the bright,
• 2:24:08
  Speaker 1: crowded centre. Around them lies the Kuiper belt. Farther out,
• 2:24:14
  Speaker 1: the heliosphere gives way to interstellar plasma. And beyond even
• 2:24:19
  Speaker 1: that may stretch an almost invisible halo of ancient icy objects,
• 2:24:24
  Speaker 1: moving so slowly that a single orbit can outlast entire species.
• 2:24:29
  Speaker 1: At its broadest, the solar system does not finish with
• 2:24:32
  Speaker 1: a planet, or a wall, or even a visible boundary.
• 2:24:37
  Speaker 1: It simply becomes less and less the sun's, until eventually
• 2:24:41
  Speaker 1: the galaxy takes over. So where does the solar system
• 2:24:46
  Speaker 1: actually end? The most accurate answer is that there is
• 2:24:50
  Speaker 1: no single edge. It depends entirely on what we are measuring.
• 2:24:55
  Speaker 1: Saturn is not an ending, but it is where the
• 2:24:58
  Speaker 1: solar system begins to feel unmistakably different from the warmer
• 2:25:01
  Speaker 1: worlds closer to the Sun. Its rings reveal countless particles
• 2:25:06
  Speaker 1: moving under gravity. Titan carries methane clouds, rain, rivers, lakes
• 2:25:12
  Speaker 1: and seas beneath an orange atmosphere. Enceladus hides a liquid
• 2:25:18
  Speaker 1: water ocean beneath its ice and sprays some of that
• 2:25:22
  Speaker 1: ocean directly into space. The colder solar system is not
• 2:25:27
  Speaker 1: empty or inactive. It is simply active in different ways.
• 2:25:32
  Speaker 1: Farther out, Uranus and Neptune form the ice giant frontier.
• 2:25:37
  Speaker 1: Neptune is the final recognised major planet, orbiting roughly 30
• 2:25:42
  Speaker 1: astronomical units from the Sun. But crossing Neptune's orbit does
• 2:25:47
  Speaker 1: not mean leaving the solar system. Beyond it lies the
• 2:25:52
  Speaker 1: Keeper Belt, a vast population of frozen remnants left over
• 2:25:56
  Speaker 1: from the formation of the planets. Pluto is only one
• 2:26:00
  Speaker 1: member of that population. And even the Keeper Belt does
• 2:26:05
  Speaker 1: not finish neatly. The scattered disk contains objects moving on
• 2:26:09
  Speaker 1: far more elongated and tilted paths. Detached worlds such as
• 2:26:14
  Speaker 1: Sedna travel on orbits extending hundreds of astronomical units from
• 2:26:19
  Speaker 1: the Sun. The farther outward we go, the less the
• 2:26:23
  Speaker 1: solar system resembles a set of tidy rings. Its structure
• 2:26:27
  Speaker 1: gradually becomes more diffuse. Then comes another kind of boundary entirely.
• 2:26:34
  Speaker 1: The heliopause marks the outer edge of the heliosphere, where
• 2:26:38
  Speaker 1: the solar wind gives way to the surrounding interstellar plasma.
• 2:26:42
  Speaker 1: Voyager 1 and Voyager 2 have crossed this boundary. In
• 2:26:47
  Speaker 1: that specific sense, both spacecraft have entered interstellar space. But
• 2:26:52
  Speaker 1: interstellar space does not mean the Sun has stopped influencing
• 2:26:56
  Speaker 1: them gravitationally. Gravity has no sharp outer wall. It simply
• 2:27:02
  Speaker 1: becomes weaker with distance. And this is why the broadest
• 2:27:06
  Speaker 1: possible edge of the solar system lies much farther away.
• 2:27:12
  Speaker 1: The Oort cloud may extend tens of thousands of astronomical
• 2:27:15
  Speaker 1: units from the Sun, perhaps reaching roughly 100,000 astronomical units
• 2:27:21
  Speaker 1: in its outermost regions. That would place its distant members
• 2:27:26
  Speaker 1: around one and a half light-years away. We have never
• 2:27:30
  Speaker 1: directly observed the Oort cloud as a complete structure. Its
• 2:27:33
  Speaker 1: existence is inferred mainly from long-period comets, gravitational dynamics and
• 2:27:39
  Speaker 1: models of how the solar system formed. Its exact boundaries
• 2:27:43
  Speaker 1: therefore remain uncertain. At those distances, the Sun holds objects
• 2:27:49
  Speaker 1: only weakly. Passing stars can disturb them. The gravitational field
• 2:27:55
  Speaker 1: of the Milky Way can slowly reshape their orbits. Some
• 2:27:59
  Speaker 1: may eventually be pulled away entirely. So even the farthest
• 2:28:04
  Speaker 1: edge is not permanent. It changes as the Sun moves
• 2:28:08
  Speaker 1: through the galaxy. The simplest answer is therefore a layered one.
• 2:28:14
  Speaker 1: The major planets end at Neptune. The main flattened population
• 2:28:18
  Speaker 1: of icy worlds fades beyond the Kuiper belt. The Sun's
• 2:28:23
  Speaker 1: solar wind ends at the heliopause. and the Sun's distant
• 2:28:27
  Speaker 1: gravitational family probably reaches outward through the Oort cloud. Almost
• 2:28:33
  Speaker 1: everything normally shown in diagrams of the solar system occupies
• 2:28:38
  Speaker 1: only a tiny region near the middle of that much
• 2:28:41
  Speaker 1: larger structure. Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and
• 2:28:53
  Speaker 1: nearly every familiar moon and asteroid, are crowded astonishingly close
• 2:28:58
  Speaker 1: to the Sun when compared with the full possible scale
• 2:29:01
  Speaker 1: of the system. Earth is not simply one planet among
• 2:29:05
  Speaker 1: eight circling a nearby star. It sits deep inside an
• 2:29:10
  Speaker 1: immense collection of nested environments stretching outward for potentially more
• 2:29:14
  Speaker 1: than a light year, and if you could somehow drift
• 2:29:18
  Speaker 1: near the outer reaches of the Oort cloud and look
• 2:29:20
  Speaker 1: back toward home, the Sun would no longer dominate the
• 2:29:24
  Speaker 1: sky as it does from Earth. It would appear as
• 2:29:28
  Speaker 1: a bright star among other stars. Yet even there, faint
• 2:29:33
  Speaker 1: and distant, its gravity could still be guiding frozen bodies
• 2:29:38
  Speaker 1: through orbits lasting hundreds of thousands of years. Beyond them,
• 2:29:42
  Speaker 1: the wider Milky Way would be pulling too. And somewhere
• 2:29:46
  Speaker 1: in that quiet balance between one star and the galaxy
• 2:29:49
  Speaker 1: around it, the solar system would finally fade away.