How Do Astronomers Find Exoplanets? Wiggles!

Dune. Star Wars. Alien. Science fiction movies love alien worlds, and so do we. But how do scientists find planets outside our solar system in real life? One way is by looking for the stars that wiggle. Historically, astronomers have measured those wiggles via the Doppler method, carefully analyzing how the star's light shifts. Thanks to new data from the GAIA telescope, scientists have a much better picture of distant stars' wiggles — and the exoplanets that cause them. Want to hear more about exoplanet discoveries? Send us an email at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:00Speaker 00

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• 0:21Regina Barber?

  Growing up, you might have learned the names of the planets. Mercury, Venus, Mars, Jupiter. But what about Beta Pictorus C? You probably didn't learn that one. I didn't either. That's because we only found out about it in 2019. And because it's an extrasolar planet, or an exoplanet.

• 0:39Speaker 02

  Well, an exoplanet is a planet, but it doesn't orbit the sun. It orbits some other star in the galaxy.

• 0:46Regina Barber?

  That's Josh Nguyen. He's an astronomer at Princeton University and an exoplanet hunter.

• 0:51Speaker 02

  And the study of exoplanets is one of the newest and most exciting areas of astronomy. It really only got going in the mid-1990s.

• 0:59Regina Barber?

  Scientists have found thousands of exoplanets since then by relying on a little trick of gravity.

• 1:05Speaker 02

  When a planet is orbiting a star, it's because the star's gravity is pulling on the planet. But forces come in pairs. If the star is pulling on the planet, the planet has to be pulling on the star with the same force.

• 1:20Regina Barber?

  Compared to the planet, the star is massive, so the pull of gravity from the planet doesn't make it move much.

• 1:27Speaker 02

  But nevertheless, it does cause it to move.

• 1:30Regina Barber?

  Basically, planets make their stars wiggle. But we haven't always been able to directly observe this wiggle. Our telescopes just haven't been sensitive enough to detect it. So in the past, we've mainly used other methods instead. And these methods...

• 1:44Speaker 02

  They're very good at finding planets that have small orbits, that are located close to the star, kind of like Mercury and Venus and the Earth.

• 1:54Regina Barber?

  But those planets are only part of the story. Without a way to find planets far from their stars, scientists haven't been able to paint a full picture of these solar systems, until now.

• 2:05Speaker 02

  This new method, which is called the astrometric method, is actually best at finding distant planets. Planets like Jupiter, Saturn, Uranus, and Neptune around other stars.

• 2:16Regina Barber?

  This new method lets us fill in the gaps of the picture, finding planets that astronomers couldn't detect before.

• 2:23Speaker 02

  This is the beginning of the next big phase of exoplanet discovery. A few years from now, we're going to be in a position to use this technique to find potentially thousands of new exoplanets. And they're going to be different from the ones that we already know about.

• 2:41Regina Barber?

  So today on the show, the next phase of exoplanet discovery, how scientists are filling in missing pieces of a solar system puzzle, and how this search has just begun. I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR.

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• 4:24Regina Barber?

  Okay Josh, so to start, can you tell me, how have we found exoplanets in the past? Like, what methods do we know work?

• 4:30Speaker 02

  Sure. We have two main methods that have led to most of the discoveries. As of today, there are about 5,800 known exoplanets.

• 4:39Regina Barber?

  Right.

• 4:39Speaker 02

  And about 4,000 of them come from a very clever trick, which is based on eclipses. If a planet's orbit happens to carry it directly in front of the star that it orbits, then it will block a little bit of that star's light, and we can tell, because the star appears to get slightly fainter for a few hours. That's called the transit method. We say the planet is transiting across the star. But the transit method, while it's a wonderful technique, it has this serious problem, which is that it requires a very special coincidence for the orbit to be oriented just right, so that from our vantage point, we see these eclipses.

• 5:23Regina Barber?

  Yeah.

• 5:23Speaker 02

  And so it misses most of the planets that are out there.

• 5:27Regina Barber?

  It makes me think of like a lighthouse, right? Like, if you're not at the right angle, if you were a helicopter above the lighthouse, you would not see like the beam of light hitting you. You could miss it.

• 5:36Speaker 02

  That's right. So if if there are aliens viewing our solar system from every possible direction, only one out of 200 of them would ever see the Earth go directly in front of the Sun. Now, the second best method, and really the first one that worked in the mid-1990s, is based on sensing the motion of the star. And we can detect the motion of the star using a trick called the Doppler effect.

• 6:02Regina Barber?

  Right.

• 6:04Speaker 02

  Now, the term Doppler probably rings a bell because of Doppler radar that's used to measure the speed of a car or the speed of raindrops falling from the sky. It's the effect that you get whenever you have a source of waves. That's moving relative to the observer. And light is a wave. So if a star is moving towards us, then the light rays that it emits, by the time they reach the earth, appear to be shifted in their wavelength.

• 6:34Regina Barber?

  Right. I mean, one analogy I like to use is is sound instead of light. They're both waves, so we can do that. The siren of a fire engine, it's going to sound different when it's coming at you versus when it passes you.

• 6:45Speaker 02

  Right.

• 6:45Regina Barber?

  And that's the Doppler effect.

• 6:46Speaker 02

  Exactly, yeah. We can actually hear those changes. Like you said, if a car goes by, we hear it go.

• 6:53Regina Barber?

  Yeah.

• 6:54Speaker 02

  Now, the speed of light is huge compared to the speed of sound.

• 6:57Regina Barber?

  It's too fast, it's too fast.

• 6:59Speaker 02

  Yeah, so when we divide by the speed of light, we get this tiny, undetectable effect to our eyes.

• 7:04Regina Barber?

  Right.

• 7:04Speaker 02

  But as astronomers, our whole job is to figure out how to analyze starlight very, very precisely. So we have specialized equipment that can detect these tiny shifts in wavelength.

• 7:16Regina Barber?

  So you use this new method to measure how like stars wiggle, and it's called astrometry. Like, what is astrometry?

• 7:22Speaker 02

  Astrometry is actually one of the oldest techniques in astronomy. It means measuring the position of the star in the sky. So if you can measure exactly where it is on the sky, you are doing astrometry.

• 7:37Regina Barber?

  Oh, wow. Okay, okay. So it's just it's just straight up measuring where the star is?

• 7:41Speaker 02

  That's right. Measuring the coordinates of the star in the sky.

• 7:45Regina Barber?

  Yeah, and and that's what I thought we were actually measuring when I first learned about star wiggling, like, back in the 1990s. But you're saying that that was actually the Doppler method, and we were measuring like the speed of stars rather than like observing those stars move.

• 8:00Speaker 02

  Right.

• 8:00Regina Barber?

  Okay.

• 8:00Speaker 02

  The astrometric method is conceptually simpler. We're just seeing the star move in the sky, wiggling back and forth. Now, we can't, we can't literally see it with our eyes. These motions are way too small.

• 8:13Regina Barber?

  Okay, okay. So using this new method, like, what kind of planets do we expect to see?

• 8:17Speaker 02

  The big difference is that the astrometric method, where you're seeing the star wiggle on the sky, is better at finding distant planets, planets with very wide orbits.

• 8:29Regina Barber?

  So why is it?

• 8:31Speaker 02

  It's because the further away the planet is from the star, the larger its orbit, and that also makes the star's orbit wider, too.

• 8:40Regina Barber?

  Hmm, got it, okay.

• 8:41Speaker 02

  And since if we're trying to see the star wiggle, we want the star to be moving as far as possible.

• 8:48Regina Barber?

  Right.

• 8:49Speaker 02

  So the wider the orbits, the better.

• 8:52Regina Barber?

  So why is this method possible now, like when it wasn't before?

• 8:56Speaker 02

  Yeah, the big game changer was a European space mission called Gaia.

• 9:03Speaker 02

  They launched a telescope in 2013. It's actually two telescopes, and they're pointing in different directions. And the telescope is spinning around.

• 9:14Regina Barber?

  In space?

• 9:15Speaker 02

  In space, so it's a spinning platform with two telescopes. And what the telescope is doing is it's measuring the exact time at which a star crosses through the field of view of each telescope. So every time it rotates and sees a certain star, it clocks that moment. And if you have billions of such measurements, then you can calculate the exact positions of all of those stars with the utmost precision.

• 9:43Regina Barber?

  Okay, and so to summarize, this new method is only possible because Gaia is capable of doing these precise measurements, which in turn makes it possible to like really see these wiggling stars and like help us identify potential exoplanets.

• 9:57Speaker 02

  Yeah.

• 9:58Regina Barber?

  And I mean, there's lots of stars out there. Like, you found this one exoplanet with this new method. How did you pick which stars to like look at?

• 10:06Speaker 02

  So the team that operates the Gaia telescope prepared a list of about 75 stars that appeared to be wiggling back and forth. And so our idea was, okay, these appear to be new planets from the astrometric method. Let's use the Doppler method to see if we can confirm them.

• 10:25Regina Barber?

  Okay.

• 10:26Speaker 02

  And as it turns out, most of those 75 objects are not exoplanets. They are something else, yeah. So the one that we found came from a long kind of sifting through of these candidates, ruling out most of them, and arriving at so far just one that we're pretty sure is an exoplanet.

• 10:48Regina Barber?

  And this one exoplanet, which has like since been named like Gaia 4b, can you talk a little bit more about that planet? Like, what do we know about it?

• 10:56Speaker 02

  Sure, it is an unusually massive planet. It's almost 12 times the mass of Jupiter.

• 11:03Regina Barber?

  Oh, wow.

• 11:03Speaker 02

  So it's, you might call it a super Jupiter.

• 11:06Regina Barber?

  Yeah, I do like those.

• 11:08Speaker 02

  It's orbiting around its star every 571 days.

• 11:13Regina Barber?

  What?

• 11:14Speaker 02

  It's lower in mass and redder and less luminous than the Sun.

• 11:19Regina Barber?

  Mm-hmm.

• 11:20Speaker 02

  And it is located about 240 light-years away.

• 11:23Regina Barber?

  That's so close.

• 11:24Speaker 02

  It's pretty close by galactic standards.

• 11:26Regina Barber?

  Yeah, I mean, for me, as someone who's studied galaxies that were like hundreds of millions of light-years away, but I also, I love Jupiter, so like, super Jupiters sound incredible. All of this Gaia data, like, just sounds so awesome.

• 11:41Speaker 02

  The Gaia mission actually just ended.

• 11:44Regina Barber?

  Just ended.

• 11:45Speaker 02

  Yeah, it ran for more than 10 years, diligently collecting data and measuring the locations of all of these billion-or-so stars. But eventually, you know, all good things must come to an end. It ran out of fuel, and it won't be conducting any more observations.

• 12:02Regina Barber?

  But there's all this data that we haven't even looked at yet, right?

• 12:05Speaker 02

  Exactly, yeah. So about a year and a half, maybe two years from now, we're going to have a much larger set of data that they're busy processing right now. And that should lead to the detection of at least hundreds and probably thousands of new exoplanets.

• 12:21Regina Barber?

  So tell me more about how, like, using astrometry is going to, like, basically give us a more complete picture. Like, why do you think it's important for people to know more about these other solar systems, these other planets around other stars?

• 12:34Speaker 02

  Well, imagine what it would be like if we didn't know about Jupiter, or Saturn, or Uranus, or Neptune in our own solar system. We would have a really incomplete picture of what's going on around the Sun.

• 12:50Regina Barber?

  Mm-hmm.

• 12:51Speaker 02

  And we want to have... The same kind of complete knowledge of exoplanetary systems. We want to know about the little planets, the big planets, the nearby planets to the star, and the more distant planets, so that we can see what are the relationships between them? What are the patterns that hold in those systems? And how do they compare to what we observe in our system?

• 13:14Regina Barber?

  Yeah.

• 13:15Speaker 02

  It's hard to draw any conclusions when you're basing all your conclusions on one example, the solar system. You really want thousands of data points.

• 13:24Regina Barber?

  Right.

• 13:24Speaker 02

  And you want complete knowledge of those systems before you can be confident about any patterns that might exist.

• 13:31Regina Barber?

  Yeah.

• 13:35Regina Barber?

  Josh, thank you so much. I can't wait to see how many you find when you start analyzing that Gaia data.

• 13:41Speaker 02

  It's been a pleasure. Thanks for having me.

• 13:44Regina Barber?

  If you like this episode, make sure you never miss a new one by following us on whatever podcasting platform you're listening from. And if you have a science question you'd like us to investigate, send us an email at [email protected]. This episode was produced by Hannah Chen and edited by Burleigh McCoy. Tyler Jones checked the facts. Kwasi Lee was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Short Wave from NPR.

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