Latest / Talking Science / Exoplanets — Finding Worlds Around Other Stars

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Exoplanets — Finding Worlds Around Other Stars · Talking Science

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If you were to walk outside tonight and just
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look up at a clear dark sky, right, and you just
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pick out any star, you would actually be looking
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at this profound mystery that stumped humanity
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for, well, almost all of our history. Oh, absolutely.
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Like the absolute smartest minds on earth could
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not tell you. if that specific star had worlds
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orbiting around it. I mean, we had our own little
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family of planets in our solar system, ones you
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can actually see wandering across our own sky.
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But beyond that. Just a giant cosmic question
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mark. Exactly. Just a blank space. Yeah. For
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centuries, we were effectively blind to what
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was really up there. I mean, the idea that other
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stars had planets was a reasonable guess, you
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know, a logical assumption. Sure. It makes sense
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mathematically. Right. But a guess is all it
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was. So the mission for our deep dive today is
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to really unpack this. incredible paradigm shift.
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We're looking at a collection of astronomical
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data and some fascinating historical research
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that details how humanity went from knowing just
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a handful of planets to discovering over 5 ,000
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of them. In a single lifetime. That's the crazy
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part. Yeah, in one lifetime. And maybe more importantly
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for you listening, we want to understand the
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incredibly clever, honestly, kind of everyday
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physics we use to actually find them. The timeline
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of this. is what absolutely blows my mind. Because
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the very first confirmed planet around an ordinary
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sun -like star wasn't found until 1995. Just
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the blink of an eye ago in historical terms.
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Right. Think about that. If you are listening
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to this right now, there is a very good chance
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that when you were in school... The only known
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planets in the entire universe were the ones
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right here in our own backyard. And now we're
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finding new ones faster than we can even name
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them. We really are. But to appreciate how we
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got here, we kind of have to understand why we
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were stuck at zero confirmed exoplanets for so
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long. Exoplanets, that's just the scientific
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term for planets orbiting other stars. Exactly,
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exoplanets. And the core problem of finding them
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is just a massive matter of scale and contrast.
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I mean, a star is trillions of kilometers away.
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planet, well, doesn't shine with its own light.
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Right. It only reflects the light of its star.
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Exactly. Which makes a planet millions, sometimes
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literally billions of times fainter than the
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star it orbits. One of the researchers we're
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profiling today uses this beautiful analogy to
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ground the science. And I love this. He said,
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looking for a planet is like trying to spot a
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single firefly sitting right next to a lighthouse.
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Yes, that's the perfect way to picture it. But
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not just that. A firefly next to a lighthouse,
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from the other side of a city. Right. You just
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can't see it. The glare of the lighthouse completely
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washes out the tiny flicker of the firefly. So
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if we can't see the firefly, how do we know it's
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there? Well, astronomers realize that if we can't
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see the firefly, we have to stop looking for
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the firefly. Okay. Instead, we have to look for
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what the firefly does to the lighthouse. We look
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at the star itself. Oh, that's clever. And this
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brings up the first major breakthrough we need
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to talk about, right? Something called the transit
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method. Yeah, the transit method. It's brilliant
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in its simplicity. Let's walk through the mechanics
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of it. Imagine a distant solar system where a
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planet's orbit is lined up perfectly edge -on
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to our view here on Earth. Like completely flat
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to our line of sight. Exactly flat. Because of
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that specific geometric alignment. Once every
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orbit, that planet is going to pass directly
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in front of its star from our perspective. Right.
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And when it does, it blocks a tiny, tiny sliver
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of the star's light. The star dims just slightly
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and then it brightens again as the planet moves
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on. So it's essentially a mini eclipse. That
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is exactly what it is. If you are watching a
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star patiently and you see its light dip by the
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exact same small amount at regular predictable
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intervals, you very likely caught a planet crossing
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its face. And NASA. built a whole telescope just
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for this, didn't they? Yeah, the Kepler Space
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Telescope. It was designed specifically for this
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one trick. It launched into space and basically
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just stared at a single patch of sky containing
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150 ,000 stars. 150 ,000 stars all at once. Right.
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And it didn't blink. It used highly sensitive
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digital sensors. Imagine a massive ultra -precise
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version of the camera chip in your smartphone
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just patiently watching for these faint repeating
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winks of light. And from just a wink, we learn
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what? A massive amount, actually. We extract
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so much information from that simple shadow.
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First, the size of the dip tells you how big
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the planet is. So bigger shadow means bigger
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planet. Right. A bigger planet, say something
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the size of Jupiter, is going to block way more
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light, creating a much deeper dip than a small
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rocky planet like Earth. Makes perfect sense.
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And second, the spacing between those dips tells
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you the length of the planet's year. Because
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that's how long it takes to orbit and come back
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around. Exactly. If the star dims every 10 days,
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you know the planet takes 10 days to complete
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a full orbit. Okay, wait, let's pause here. Let's
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unpack this. Because this brings up a huge logical
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problem for me. Okay, hit me. Doesn't this trick
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rely on incredibly, impossibly lucky geometry?
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I mean, space is three -dimensional. It is. So
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if a distant solar system is tilted even slightly
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from our point of view, that planet is going
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to orbit. Like above or below the star from our
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perspective. Yeah. It will never cross the star's
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face. It will never cast a shadow toward Earth.
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And Kepler would just be completely blind to
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it. Right. You're entirely right. That is a massive
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limitation. The transit method only works for
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the small fraction of planetary systems that
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just happen purely by chance to be tilted edge
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on to Earth. So what are the odds of that actually
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happening? Pretty low. The probability of an
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Earth -like planet perfectly aligning to transit
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a sun -like star is roughly one half of 1%. Half
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of 1%. Yeah. So for every single planet we actually
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catch transiting, there are literally hundreds
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more out there that are totally hidden from us
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just because of our viewing angle. Wow. Which,
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honestly... Makes the numbers we're seeing even
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more staggering. Exactly. Because even with that
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massive limitation, this edge -on method is still
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responsible for the vast majority of all the
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exoplanets we've ever discovered. It is. It just
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goes to show how crowded the galaxy must actually
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be. I mean, if you're finding thousands of planets
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using a method that only catches a fraction of
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a percent of them, the total number of worlds
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out there must be astronomical. Pun intended.
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Huh. Yes. Pun intended. It is a statistical goldmine.
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But... You know, knowing a planet's physical
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size, its volume, that only gives us part of
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the picture. weighs that? Well, a giant puffy
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cloud of gas and a dense solid cannonball of
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rock could theoretically cast the exact same
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size shadow, assuming they have the same diameter.
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Oh right, because a shadow just tells you how
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wide it is, not what's inside it. Exactly. To
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know what a world is actually made of, we need
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to know how much stuff is inside it. We need
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its mass. Okay, so if the shadow can't tell us
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how heavy a planet is, how do we weigh a world
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we can't even see? We use gravity, and this requires
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correcting a very common misconception about
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how space actually works. Let's do it. Yeah.
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We usually say that a planet orbits a star, right?
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And we picture the star sitting perfectly still
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in the center while the planet does all the moving
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around it. Yeah, that's what we're taught in
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elementary school. Right, but physics tells us
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that's not quite right. Gravity pulls both ways.
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In reality, both the planet and the star orbit
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their shared center of mass. Astronomers call
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it the Berry Center. Okay, I was trying to visualize
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this when prepping for this deep dive, and it
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reminds me of this, like, really lopsided dance.
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Oh, tell me. So... Picture a heavy adult holding
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hands with a small child, and the adult is spinning
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the child around in circles. Okay, yeah. The
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child is swinging out in a really wide, fast
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circle. But the adult isn't perfectly still either,
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right? No, they can't be. Exactly. To balance
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the weight of the child swinging around, the
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adult has to shift their footing. They end up
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leaning back and swaying in this tiny, tight,
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slow circle themselves. That's a great analogy.
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Thanks. So if you're watching from far away in
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the dark, you might not be able to see the small
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child at all. But if you see the heavy adult
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swaying in that rhythmic circular motion, you
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know for a fact that they are swinging someone
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around. You nailed it. The star is the adult
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and the planet is the child. The star's movement
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is incredibly small compared to the planet's
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orbit. I mean, the star barely moves while the
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planet swings wide, but the star does physically
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move. It is tugged around by the planet's gravity.
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So it wobbles. Yes. In fact, this technique is
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officially called the radial velocity method,
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but more casually, everyone just calls it the
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wobble method. But wait, how on earth do we see
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a star wobbling? Because if the star is just
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a speck of light trillions of kilometers away,
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it's not like we can see it swaying back and
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forth through a telescope. Right. We can't see
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the motion directly. We don't look at the position
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of the star. We look at the light waves leaving
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the star. Oh, interesting. Yeah. So as the star
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wobbles in that tiny circle, half of the time
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it is moving slightly away from us and half of
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the time it is wobbling slightly toward us. Okay.
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Think of light as a spring or a slinky traveling
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through space. When the star moves toward us,
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it compresses those light waves, squishing them
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together. Like the Doppler effect. Exactly like
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the Doppler effect. The same thing that makes
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an ambulance siren drop and titch as it speeds
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past you. But with light, compressed waves shift
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toward the blue end of the color spectrum. And
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when the star wobbles away from us, those light
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waves are stretched out, shifting the color toward
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the red end. So the star is actually rhythmically
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changing color. Like... Blue, red, blue, red.
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Very, very slightly, yes. And astronomers use
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incredibly sensitive instruments called spectrographs
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to measure this tiny teller shift. When they
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see that rhythmic stretching and compressing
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of light, they know they've found an unseen planet
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yanking its star around. That is wild. It is.
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And crucially, just like the adult and the child
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in your dance analogy, a heavier child is going
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to make the adult sway much harder. So by measuring
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exactly how fast the star is wobbling, we can
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calculate the mass, the weight of the unseen
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planet. Ah, so this is the grand synthesis of
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modern astronomy. You use the transit method.
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watching the shadow to get the planet's volume.
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Yes. And then you use the wobble method, watching
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the color shift to get the planet's mass. You've
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got it. And once you have the volume and the
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mass, it's just basic division to get the density.
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And density is the key to everything. Because
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density tells us what it's made of. Exactly.
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It allows us to deduce if a distant world is
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a dense, rocky ball like Earth or Venus, or if
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it's a puffy, lightweight ball of gas like Jupiter
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or Saturn. From just a couple of very clever,
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very patient measurements of starlight, we can
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completely reconstruct the physical nature of
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a world we will never, ever visit. Which is just
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incredible. So armed with these two methods,
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the shadow and the wobble astronomers turned
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these tools to the sky in the late 90s and early
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2000s. Yeah. That's when things really took off.
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And the very first major discovery completely
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broke our scientific models, didn't it? Like
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it threw the textbooks out the window. It really
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did. Astronomers had naturally assumed that other
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solar systems would look somewhat like ours.
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Right. Why wouldn't they? Exactly. You'd have
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small rocky planets on the inside near the star
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and big cold gas giants on the outside, all moving
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in nice, orderly, slow orbits. But the earliest
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discoveries were a class of planets we now call
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hot Jupiters. Hot Jupiters. What exactly? These
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are gas giants, often even bigger than our Jupiter,
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but they are whipping around their stars in a
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matter of days, not years, days. Wow. They are
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orbiting closer to their star than Mercury is
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to our sun. They are absolutely roasting. But
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nobody predicted them. I mean, a gas giant that
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close to a star. didn't make any sense based
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on how we thought planetary systems formed. Shouldn't
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the heat have just blown all the gas away? That
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was the big mystery. Why were... our models so
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wrong yeah why were they well the models weren't
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entirely wrong our perspective was just heavily
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skewed by selection bias we found hot jupiters
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first because they were simply the easiest things
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to spot oh i see think about the two methods
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we just discussed a massive giant planet is going
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to cause a massive easy to see wobble in its
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star right and a planet that is huge and orbits
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very fast very close to its star is going to
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cause deep frequent dips in light that are very
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easy to catch with the transit method. So they
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weren't necessarily the most common, they were
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just the most obvious. Exactly. They were the
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loudest signals in the data. They showed up first
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simply because they were the low -hanging fruit.
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That makes total sense. So as our technology
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improved, as we launched better space telescopes
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and wrote better software to filter out the noise,
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what happened well smaller quieter worlds started
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to emerge from the data and this is where we
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learn a mind -bending fact about our own place
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in the universe hit me it turns out The most
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abundant type of planet in the galaxy, the actual
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quote unquote normal out there, is a type of
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planet size somewhere between Earth and Neptune.
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Astronomers call them super Earths or mini Neptunes.
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And what's truly wild about that discovery is
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that this is a type of planet that doesn't even
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exist in our own solar system. Yeah, it's completely
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missing. We have small rocky planets like Earth
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and Mars and we have huge gas giants like Jupiter
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and Saturn. But we completely lack this intermediate
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size. Which is strange, right? It is, yet it
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seems to be the standard model for the rest of
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the Milky Way. These could be massive rocky worlds,
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or they could be entirely covered in oceans hundreds
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of miles deep. We just, we simply don't have
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a local example to study. So we are the weird
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ones. Yeah, pretty much. When you zoom out and
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look at the big picture of these discoveries,
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the scale is just breathtaking. It really is.
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The current best estimates, based on the thousands
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of planets we've found, and extrapolating for
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the ones hiding from our transit view, suggest
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that planets actually outnumber stars in the
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Milky Way. Let that sink in. Planets outnumber
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stars. When you look up at the night sky, you
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are looking at a galaxy where rocky, roughly
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Earth -sized planets exist in the billions. Billions.
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Which naturally leads to the ultimate question
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for anyone listening. Yeah. Could any of them
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support life? Ah, the million -dollar question.
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Right. And you see it in the news headlines all
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the time. Astronomers find Earth -sized planet
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in the habitable zone. I see those all the time.
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It sounds like we should pop champagne and pack
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our bags. We found Earth 2 .0. But I really want
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to push back on that because the data we are
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reviewing shows that this term is incredibly
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misleading. Yeah, the habitable zone is very
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specific. And I would say heavily misunderstood
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astronomical term. People also called the Goldilocks
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zone. Right. The Goldilocks zone. It is defined
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simply as the band of orbits around a star where
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the temperature would allow liquid water to exist
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on a planet's surface. So not too hot, not too
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cold. Exactly. It's not so close to the star
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that all the water boils away into steam. And
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it's not so far away that all the water freezes
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solid into ice. It is theoretically just right.
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OK. But in the habitable. zone does not mean
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habitable no absolutely not just look at our
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closest planetary neighbors venus and mars if
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you map out our sun's habitable zone both venus
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and mars are actually inside it they skirt the
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inner and outer edges yep they technically qualify
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but if you visit them Neither one is a tropical
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paradise with flowing rivers. I mean, Venus is
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a crushing toxic furnace with surface temperatures
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hot enough to melt lead. And Mars is an irradiated
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frozen desert. Neither one has oceans of liquid
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water on its surface today. Right. So why is
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that? Why does distance not guarantee a habitable
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surface? Because whether a planet can actually
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hold on to liquid water depends on far, far more
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than just its distance from the star. It depends
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on whether the planet has a atmosphere to provide
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pressure. It depends on what specific greenhouse
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gases make up that atmosphere. Which traps the
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heat. Exactly. And it depends on whether the
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planet has a strong internal magnetic field to
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protect that atmosphere from being violently
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blown away by solar winds. Like what happened
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to Mars. Right. Mars lost its magnetic field
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and then lost its atmosphere. These are incredibly
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complex variables. And for exoplanets light years
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away, these are variables we usually cannot measure.
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Right. So the most important takeaway here is
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that the habitable zone just tells astronomers
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where to look. It does not guarantee what they
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will find. Okay, so if distance doesn't guarantee
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a habitable surface, And we obviously can't send
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a probe because it would take tens of thousands
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of years to get there. How will we ever know
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if there's life? Are we just stuck guessing forever?
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We are not. This is the really exciting part.
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We are actively writing the newest chapter of
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the science right now. Okay. Tell me more. We
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are moving from just finding planets to actually
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analyzing the air they possess. And to do that,
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we go back to the transit method, the planet
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passing in front of the star. The mini eclipse.
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Right. If that planet happens to have an atmosphere,
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something magical happens during that transit.
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A tiny, tiny fraction of the starlight doesn't
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just get blocked by the solid rock. It actually
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filters through the glowing fringe of the planet's
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atmosphere on its way to our telescopes. Oh,
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wow. So the starlight actually passes through
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the alien air. Exactly. And as it does, the gases
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in that atmosphere interact with the light. Every
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chemical element absorbs very specific colors
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of light. They basically eat those specific colors.
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Okay. So when... light finally reaches earth
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when it reaches earth it has microscopic chunks
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missing from its rainbow it's often described
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as a chemical barcode imprinted on the starlight
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a chemical barcode i love that yeah and if you
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read that barcode if you look at exactly which
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colors are missing you can work out exactly what
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gases the atmosphere is made of this is a process
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called spectroscopy and we are doing this right
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now We are. The James Webb Space Telescope, which
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is operating right now, is doing this for the
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first time with unprecedented precision. It uses
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massive gold -plated mirrors to catch that filtered
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starlight, and it is actively teasing out the
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barcodes for water vapor, carbon dioxide, and
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other molecules in the air of planets sitting
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trillions of miles away. That is just incredible.
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But what are they actually looking for in that
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barcode? Because finding carbon dioxide doesn't
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mean aliens, right? Right. The holy grail for...
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astronomers is finding what's called a biosignature.
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The dream isn't just to find random gases, it's
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to find a very specific combination of gases.
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Like what? The major example scientists look
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for is finding oxygen sitting right alongside
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methane in the same atmosphere. Oxygen and methane.
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Why those two specifically? It all comes down
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to the mechanics of chemistry. Oxygen is a highly
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aggressive, highly reactive element. It is essentially
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an electron thief. An electron thief, okay. It
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desperately wants to bond with other elements.
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It wants to rust iron. It wants to burn carbon.
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And it desperately wants to react with methane
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to form carbon dioxide and water. Because oxygen
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is so chemically aggressive, oxygen and methane
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will naturally destroy each other in an atmosphere.
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Ah, so they shouldn't. naturally coexist for
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very long. Exactly. Left alone, they wipe each
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other out. Okay. So if we look at a distant planet
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and see a strong signal for both oxygen and methane.
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Then it means something is actively, constantly
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pumping massive amounts of those gases back into
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the air faster than they can destroy each other.
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Oh, wow. It represents a massive ongoing chemical
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imbalance. And on Earth, the primary thing maintaining
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that kind of extreme chemical imbalance is life.
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Right. Plants and bacteria are constantly pumping
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out oxygen and methane. Exactly. Finding that
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imbalance on another world would heavily imply
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that something living is down there, breathing,
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metabolizing, and altering its environment. So
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that chemical imbalance is the biosignature.
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Yes. But we need to temper our expectations with
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a deep note of caution here. Fair enough. Have
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we found one? No, we have not found a biosignature
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yet. These measurements filtering starlight through
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an alien atmosphere are sitting at the absolute
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bleeding edge of what is physically possible
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with current technology. I mean, a missing slice
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of color in a beam of light that has traveled
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for 50 years isn't exactly a slam dunk. Not at
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all. The signals are incredibly faint and very
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easy to misread. It may take a very long time
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before we can definitively say we've found a
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biological chemical imbalance. And honestly,
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it's possible we may not find one in our lifetimes.
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That's sobering. It is, but the profound shift
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is this. For the very first time in human history,
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searching for alien life is a real, executable
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scientific experiment. Rather than just a philosophical
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daydream. Exactly. We are no longer just wondering,
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we are finally looking. What a journey it has
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been to get here. I mean, within a single generation,
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within the span of a few decades, our understanding
20:54
of the universe has been turned completely upside
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down. We went from a quiet, lonely family of
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eight planets to discovering a galaxy that is
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literally teeming with worlds. Worlds that outnumber
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the stars themselves. It's beautiful. And I think
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the most beautiful application of this knowledge
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is how it changes your everyday experience. The
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next time you walk outside... and look up at
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a point of light in the night sky, you don't
21:19
have to wonder anymore. You can be fairly sure
21:22
that you're looking at a sun with worlds of its
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own. By just patiently watching starlight flicker
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and shift color, we've realized that the sky
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is far more crowded and far more interesting
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than we ever knew. That is amazing. And I want
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to leave you, the listener, with one final thought
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to mull over, building on what we discussed earlier
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about planet sizes. We just learned that the
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most common type of planet in our entire galaxy,
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the super -Earth or mini -Neptune, is completely
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missing from our own solar system. Which still
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blows my mind. Right? It makes you wonder if
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alien astronomers are... pointing their own telescopes
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at our sun right now, looking for those familiar
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quote -unquote normal planets they see everywhere
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else, what assumptions might they be getting
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completely wrong about our unusual quiet little
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family of eight?