Over 5,000 Exoplanets Have Now Been Discovered

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. If you look up at the night sky and find the constellation Gemini, look toward Pollux, one of the twins. You’re not just seeing a bright star. You’re also gazing at an exoplanet that was discovered in 2006.

It’s some 34 light years away, and it’s about twice the size of Jupiter. And it’s just one of 5,000 confirmed exoplanets tracked by NASA’s Exoplanet Archive. A group of planets added to the rolls this week took the catalog over this 5,000 milestone.

Today, it might seem commonplace. It’s a given that stars might have planets around them. But as recently as 1992, there were no known exoplanets. Joining me now to talk about the hunt for exoplanets is Jessie Christiansen. She’s the NASA Exoplanet Archive Project Science Lead at IPAC-Caltech in Pasadena, California. Welcome to Science Friday. Thanks so much for being here.

JESSIE CHRISTIANSEN: Hi, John. Thank you for having me.

JOHN DANKOSKY: So why is this milestone such a big deal for you?

JESSIE CHRISTIANSEN: Oh, it’s exciting for a number of reasons. One is that it’s a celebration, right? We tried for so long to find planets. And as soon as we started finding them, we started realizing they’re everywhere. So reaching a number like 5,000 just feels like real validation of the field. Like, we’ve worked so hard. Hurray! Another thing that’s exciting is what we can do with all of those planets, like the questions we can answer now about how planets form and evolve and migrate.

JOHN DANKOSKY: And it really is very interesting that we’ve got this big number now. We can learn so much. So how much do we know about each of these 5,000 at this moment?

JESSIE CHRISTIANSEN: Yeah, it turns out most of them, we don’t know very much. What we know about most of them is their rough size, and we know how long it takes them to orbit their star. So their year. And that tells us how hot the planets are. If you’re very close to your star, then you’re very hot. If you’re far away, then you’re very cold. So we know their rough size and their rough temperature. That’s it for most of the 5,000.

JOHN DANKOSKY: I want to take a step back and talk about how you actually find a planet that could be dozens of light years away. Just explain it to us, what it is that you’re looking for.

JESSIE CHRISTIANSEN: So the most successful planet hunting technique that accounts for 2/3 or more of the 5,000 planets is called the transit method. Now what we’re doing with that is we’re actually just monitoring the brightness of the star over and over and over again for years at a time sometimes, and we’re just waiting for little dips in the brightness, because that means a planet could have orbited in front of that star and blocked some of the light. So if you see these dips, then you look at the star with other telescopes and you confirm that you’re actually really seeing a planet.

JOHN DANKOSKY: So what are the other techniques? If that accounts for most of the planets that you’ve seen, what else have you used to find these planets?

JESSIE CHRISTIANSEN: Yeah. So there’s also something called the radial velocity method or the Doppler method. This relies on the fact that planets as they orbit their stars are actually tugging on their stars the same way the stars are tugging on their planets. So for instance, in our solar system, Jupiter is actually dragging our sun around the middle of the solar system on a roughly 12-year orbit.

So when we look at other suns, other stars in the sky, we can actually see them wobbling as well. So then you can measure from the size of the wobble and the duration of the wobble how big a planet must to be pulling on the star in that way.

We’ve also found planets using direct imaging. So that’s for the very nearby planets. If you’re very careful, you can block out the light from the star and actually search around the star for nearby faint glowing blobs, basically, which turn out to be hot young planets.

And another successful technique is microlensing, which relies on relativity, basically, that everything with mass bends space time. Planets bend space time. So if you carefully monitor some stars, you can actually see them warped by the planets that orbit between us and the star.

JOHN DANKOSKY: Wow. So some very direct methods and some pretty indirect methods in there in terms of how you find planets.

JESSIE CHRISTIANSEN: Yes, exactly.

JOHN DANKOSKY: So you say 5,000 confirmed exoplanets. What exactly does it take to confirm one? And how confident are you in all of those 5,000?

JESSIE CHRISTIANSEN: Right. So there’s two ways to get a planet into NASA’s Exoplanet Archive. One is to measure its mass and show that it’s truly a planetary mass object. It’s not big enough to be a star. And you can use the radial velocity method for that, for instance, or the microlensing method.

Another is to show statistically that it’s much more likely to be a planet than anything else. So you look at all of the other possible scenarios that could have created this signal in your data, and you rule them all out one by one. You say, it couldn’t be a background star. It couldn’t be an instrument glitch. And then when you have odds of better than 100 to one that the signal that you see is a planet, then we say, OK, you’ve statistically shown it’s a planet, and it can go in the archive.

JOHN DANKOSKY: If we’ve found about 5,000, what does this tell us about how many there actually are out there? Is there any way to extrapolate these numbers and say, OK, well, we found 5,000 of them. That must mean we have x number more planets out there to find.

JESSIE CHRISTIANSEN: That’s the really overwhelming part of reaching this milestone, because we’ve really only searched our local solar neighborhood. We’ve only really looked around us in the galaxy. So if you extrapolate over the hundreds of billions of stars just in our Milky Way, that means there’s likely tens of billions of planets.

JOHN DANKOSKY: How many places like our own solar system with a range of planet types and sizes have we found?

JESSIE CHRISTIANSEN: You know, that’s really interesting. We might be more unique than we would have expected. When we look at planetary systems around other stars, what we see is that they mostly have similar planets around them. So a star will have a lot of small planets or a lot of big planets.

And it’s actually not as common to see a mix like we have in our solar system. We’re still extending our observations so that we can test that, but at the moment we really think that our solar system might be an uncommon arrangement of planets.

JOHN DANKOSKY: So is there an average planet in your collection? You’ve said that around many of these stars you’ll see planets that are often of a similar type. Are you finding a similar type of planet amongst these 5,000?

JESSIE CHRISTIANSEN: The most common kind of planet we’ve found is actually a surprise, because it’s not a kind of planet we have in our solar system. We call it a super Earth, and it’s up to two times as big and 10 times as heavy as our Earth. And that’s really interesting, because we don’t have one, so we don’t know what they’re like. We don’t know whether they’re big rocks. We don’t know whether their little ice giants, like scaled-down Neptunes. So they’re a big mystery, but they seem like they’re the most common kind of planet that we found so far.

JOHN DANKOSKY: Is there something about that size range that might make it easier for us to find? Like, is there a minimum viable size of a planet that you could actually see using any of these different techniques that you use?

JESSIE CHRISTIANSEN: That’s a really great question. And it is very difficult to find planets as small as Earth. NASA had a mission called Kepler, which was trying to do this, and still couldn’t quite get there.

In the scheme of things, Earth is really small. [LAUGHS] So it’s very difficult to find them. But we know enough to be able to extrapolate how common we think earths should be, and we still think that super Earths, which are, as you say, easier to find because they’re bigger, are more common.

JOHN DANKOSKY: In terms of Earth size, you say it might be hard to find. If we were standing on one of these other exoplanets, do you think we’d be able to see the Earth?

JESSIE CHRISTIANSEN: Yes. And actually, there was a really interesting result that just came out last year where a pair of astronomers actually looked at all of the stars that could possibly see us transiting, right? You know, this geometry that I talked about where a planet has to be lined up just right to block some of the light.

We know what stars could look and see us transiting. And those stars have actually been the subject of searches from like the SETI Institute to say, hey, if you can see us, maybe we could see you.

JOHN DANKOSKY: Amongst these planets, is there an average distance from the sun in temperature? Are you finding planets of a certain temperature out there amongst their suns?

JESSIE CHRISTIANSEN: That one’s harder to answer, because that really depends on how long we’ve been searching around a given star for. It’s easier to find the close-in hot planets, because for instance, they transit more often. Like if you were looking at our sun, Mercury would transit much more often than Venus and more often than Earth.

So we’re very sensitive to the close-in hot things. And we have found thousands of close-in hot things. We’re still incomplete in our searches out here around where Earth is at the cooler temperatures. So it’s a bit hard to say yet where, for instance, the peak of planet occurrence is and distance from the star, but that’s something we’re really trying to answer with our next generation telescopes.

JOHN DANKOSKY: Yeah. And this is something we talk about an awful lot. There’s this question of the Goldilocks zone, the distance from a star that would allow a planet like Earth to support life. How many of those are out there of the 5,000, do you think?

JESSIE CHRISTIANSEN: That’s the million, billion, trillion dollar question.

JOHN DANKOSKY: [LAUGHS]

JESSIE CHRISTIANSEN: So so far, we haven’t found any planets like the Earth in the Goldilocks zone, of a star like the sun. We have found planets like the Earth in the Goldilocks zone of much smaller, much cooler stars called M dwarfs. And actually, most of the stars in the galaxy are M dwarfs. So it might be that habitable Goldilocks zone real estate is common throughout the galaxy.

But there’s a big question, which is, can planets around M dwarfs, which are a very different kind of star than our sun, actually support life? And we don’t know the answer to that yet.

JOHN DANKOSKY: Are there certain types of stars around which you find more planets than others?

JESSIE CHRISTIANSEN: Yeah. These M dwarfs actually seem like they’re really, really good at making rocky planets, for instance. Which was a surprise, because I think a lot of us expected that bigger stars would make more planets. They’re starting from a bigger amount of gas and dust, the bigger protoplanetary disk with material to form planets.

But it seems like maybe those bigger stars, they put out a lot of radiation. Maybe they blow a lot of the gas and dust away, and they’re not as efficient at turning that into planets. Whereas small stars seem like they’re very good at converting their protoplanetary disks into actual planets. So we see many more planets around small stars than around big stars, which is a surprise.

JOHN DANKOSKY: Amongst your colleagues and the people who do this work, are there differences of opinion on how you define a planet or how you should define a planet?

JESSIE CHRISTIANSEN: [LAUGHS] That’s a very timely question. Just earlier this week, the International Astronomical Union, famous for demoting Pluto in 2006, decided to put out a proposal for what the definition of an exoplanet should be. So an exoplanet is a planet around another star. And we’re already fighting about what the definition of planet is around our own star, so we haven’t really settled yet.

There is debate. And what I’ll say is that the different archives online that try and keep track of these things, we all have our own criteria that we’ve kind of settled on scientifically and politically, like, this is our box that we’re going to fill. So there are definitely different criteria. There is not consensus yet about, for instance, if you have a planetary mass object just free floating in the galaxy, is it a planet? Because it’s not orbiting a star, but it’s planetary mass. So it’s one of the open questions right now. Do you call that a planet or not?

JOHN DANKOSKY: Interesting. OK. So what are you most excited about right now in exoplanet research? What’s the next big thing?

JESSIE CHRISTIANSEN: Well, everyone is really excited about James Webb. So this just launched in December. It’s currently going through its commissioning right now. And one of the most exciting things about the James Webb Space Telescope is that it’ll give us the ability to find out so much more about the planets.

So remember I said at the start, we really only know their sizes and their temperatures. So James Webb will let us peer into the atmospheres and onto the surfaces of these planets to look for things like clouds and structures and surfaces and surface materials and compositions. It’s super exciting. They’ll turn from just dots on a plot into real 3D worlds with data associated with them, which I’m really excited about.

The NASA Test mission is flying right now. That’s NASA’s current planet hunting mission. It’s doing an all-sky survey for planets transiting the very brightest stars. And those planets will end up being the targets for James Webb for further characterization. So yes, Test is a very prolific mission which is being very successful right now that I’m also working on.

JOHN DANKOSKY: I’m talking with Jessie Christensen. She’s the NASA Exoplanet Archive Project Science Lead at IPAC-Caltech in Pasadena, California. I’m John Dankosky, and this is Science Friday from WNYC Studios. Do you have a favorite exoplanet?

JESSIE CHRISTIANSEN: [LAUGHS] That’s like asking a mother to choose between her children.

JOHN DANKOSKY: Between her 5,000 children.

JESSIE CHRISTIANSEN: Between her 5,000 children. My favorite is always the next one, right? Like, people are like, it’s 5,000. Don’t you have enough? And it’s not just a number. It’s not just 5,000. Every one of these is a whole new planet, a new world.

Like, think about the diversity just in our solar system, right? And each world has personality and has features and is different. So of these 5,000, they’re all amazing and incredible and rich worlds that I just can’t wait to learn more about.

And then personally, there’s a system called K2-138, which I love. It was found by citizen scientists. So that’s people just like your listeners at home on their computers, looking through NASA data and helping us find planets. It’s got six planets around it. And the inner five planets play music. They’re in a resonance. And they actually– if you put that to music, it actually plays “Twinkle Twinkle Little Star.” It’s really sweet.

JOHN DANKOSKY: [LAUGHS] That’s so amazing. It’s just amazing to think about, isn’t it?

JESSIE CHRISTIANSEN: Yeah, it’s so much fun. It’s so much fun.

JOHN DANKOSKY: So if you were to go to one of these planets to take an up-close look, though, and do tests and learn a little bit more about, is there one of these planets that you think about an awful lot?

JESSIE CHRISTIANSEN: Yeah. So one of our big holdouts is Kepler-452b. So I mentioned that Kepler was our planet hunting telescope that we were trying to find Earth-like planets with, and we just couldn’t quite get there. And Kepler-452b is as close as we got.

So we think it’s about one and a half times the size of the Earth, which is where we start to get worried that it’s not going to be predominantly rocky with a thin atmosphere anymore like Earth, but something more like a scaled down Neptune. So an ice giant sort of thing. So we’re not sure if it’s rocky, so that’s one mystery I would love to solve, just to get there and to know whether it’s rocky.

We know it’s in the habitable zone of a star like the sun, but the other mystery about Kepler-452b is whether it’s actually there or not. [LAUGHS] So the signal we see in the Kepler data is a lot like a type of noise that we also see in the Kepler data. So either it’s the very closest thing we’ve found to an Earth-like planet with Kepler if it’s rocky and has a thin atmosphere, or it’s not there. [LAUGHS] So I would really love to go there and solve that mystery, because it has plagued us for a decade at this point.

JOHN DANKOSKY: [LAUGHS] Do you think that we will double this number of exoplanets? Triple it? I mean, how many more exoplanets will we find over the course of, I don’t know, say, your career?

JESSIE CHRISTIANSEN: Oh, hundreds of thousands is the prediction. So for instance, NASA is launching the Nancy Grace Roman Space Telescope in five years or so, and it’s going to do a survey of the center of the galaxy where most of the stars are. And it’s expected to find 100,000 planets just on its own.

Planets are everywhere. That’s the big discovery of the last 30 years, is everywhere we look we see planets. And so as our technology and our instruments improve, we’re just going to find more and more. And I’m hoping we find more and more interesting planets, and I’m hoping we find more Earth-like planets.

JOHN DANKOSKY: Almost all the people I would think who are listening to Science Friday right now get really excited about the idea of a search for new life, a search for new planets. But there are a lot of people who say, look, we’ve got a lot of problems here on Earth. Why are we spending so much money and time and all these bright scientific minds looking for planets that we’ll never be able to get to? And I’m sure people say that to you too. What do you tell them?

JESSIE CHRISTIANSEN: Yeah, so usually I have two answers. One is that the question of are we alone is one of humanity’s most fundamental, earliest, oldest questions. In this whole vast universe and all of its potential and all that’s a possibility, could it possibly just be us? Are we alone? That’s such a huge question to answer.

And the second is if we can study the planets around us, we’ll work out what our fate will be. You hear that in five billion years, the sun will expand to become a red giant and expand out to the orbit near Earth. And then after that, it’s going to slough off its outer layers and become a white dwarf, which is just a little cooling ball of carbon and oxygen, and it’s just going to cool forever.

And there’s been a lot of open questions about, what does that mean for Earth? Like, what does that mean for us? What’s our fate? And one of the discoveries of the last five years is planets around white dwarfs, it’s possible to survive that phase of a star going into a red giant and then becoming a white dwarf.

And that means that the trillions of years that our sun will spend as a white dwarf, it might be possible that there’s a second stage of planet life after this red giant phase. So I think that’s a really important question as well. Not just like, where did we come from and are we alone, but where are we going?

JOHN DANKOSKY: That is a good place to leave our conversation. Jessie Christensen is NASA Exoplanet Archive Project Science Lead at IPAC-Caltech in Pasadena, California. Congratulations on this milestone, and thanks so much for joining us.

JESSIE CHRISTIANSEN: Thank you. It was a pleasure.

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