PLATO’s Mission To Discover Exoplanets Like Earth

CHARLES BERGQUIST: This is Science Friday. I’m Charles Bergquist, sitting in for Ira Flatow. One of the hottest fields in astronomy right now is the hunt for exoplanets. NASA’s exoplanet archive currently lists over 5,700 confirmed planets orbiting around distant stars, and more discoveries will be on the way.

An exoplanet spotting satellite called PLATO, which stands for the Planetary Transits And Oscillations of Stars, is planned for launch in late 2026. It’ll be the second of three dedicated exoplanet satellites planned for launch by the European Space Agency as part of its cosmic vision program. It’ll be specifically looking for exoplanets in the habitable zone of 200,000 sun-like stars. It’ll be able to determine the size and mass of new planets, as well as the age of the stars that they orbit.

So how can PLATO build on what we’ve already found, and what new things might it discover? Joining me to answer some of these questions is one of the scientists working on the telescope, Dr. Suzanne Aigrain, Professor of Astrophysics at Oxford University. Welcome to Science Friday.

SUZANNE AIGRAIN: Thank you for having me.

CHARLES BERGQUIST: So fill me in. What exactly is this whole mission about?

SUZANNE AIGRAIN: So PLATO is a satellite which will carry 26 small telescopes, which will all stare in roughly the same direction. And together, they will monitor, as you mentioned, 200,000 stars. And they will monitor their brightness.

And they will then discover planets using the transit method, which is when a planet passes in front of its host star, it hides a very tiny fraction of the light from that host star. So we see the brightness of the star briefly dip by a very small amount. That allows us to discover planets in orbit around distant stars, provided that their orbit is aligned along with our line of sight. And PLATO will not only do this, but it will also monitor the intrinsic brightness variations in the stars that it observes, and that includes oscillations of stars, if you like, little starquakes, which pretty much every star like the sun undergoes.

So sound waves inside the stars, which are excited just by turbulent motion on the interior of the star, resonate inside this giant cavity. And the way in which they propagate tells us about the inside of the star. And it’s these sound waves which we can use to very precisely measure the masses and radii of the stars. And because everything we learn about exoplanets we learn relative to the host star, it’s extremely important to also understand the host star and measure its properties really well.

CHARLES BERGQUIST: Does this telescope look at one star at a time, or is it looking at a big chunk of sky all at once?

SUZANNE AIGRAIN: So it actually looks at around 20% of the entire sky in one shot. So within that very large field of view, it will monitor of order 100-plus thousand stars at once. And it’s scheduled to spend two years monitoring one field, and then another two years monitoring another field. And then there’s some freedom as to how we use the rest of the time.

The lifetime of the mission as a whole is at least five years and up to eight years. So we will have the opportunity to then decide whether to return to the stars we already looked at, or to go and look at other parts of the sky.

CHARLES BERGQUIST: Yeah. So how long do you have to look at any given star to be able to say, yeah, there might be a planet here?

SUZANNE AIGRAIN: Well, generally, we consider that we need at least two transits to have been observed for a confirmed detection. Measuring two consecutive transits allows us to tell what the orbital period of the planet is. So the transit happens once per orbit, so we need to see at least two to know how far the planet is from its host star.

So in order to see at least two transits for planets which have orbits similar to the earth, we need to look for at least two years. And ideally, actually, we’d really like to have three transits because that allows us to be certain that we’ve got the right period, and it strongly increases the confidence in our detections.

CHARLES BERGQUIST: Is this mission following up on observations that have already indicated that there might be a planet around some given star? Or is this sort of blue sky– we’re looking for completely new things that nobody’s reported before?

SUZANNE AIGRAIN: It’s primarily a blue sky discovery mission, although it builds on a lot of heritage from previous space-based transit search missions. Kepler was the first large-scale transit search mission that had the capacity to find some planets similar to the earth. But it had a much smaller field of view, so it looked for planets around stars which are more distant from our own sun and which are fainter.

Where PLATO is very different is it uses smaller telescopes to look at a much wider field of view. That allows it to target stars which are nearer to our own sun, which are brighter, and for which we can learn a lot more, both about the stars and about the planets themselves.

And one of the critical things which we will do as part of PLATO is measure the masses of the planets. And we don’t do that using PLATO data. We do that using data that we then get as follow-up observations with ground-based telescopes. I should say, though, that among the 200,000 stars that PLATO will look at, a number will already be known to host planets.

You mentioned how some of this data needs to be followed up on by other instruments, other telescopes. Tell me a little bit about what happens when you find a promising planet. You get– the machine goes bing, or you get an email saying, hey, we’ve got a candidate? What are the next steps?

So PLATO is very much a survey mission, which means that everything it does in theory is fully automated, including the analysis of the data and triggering an alert that we may have something that looks like a planetary transit. The pipeline is set up so that it will first identify events that look like planetary transits, and we will find many hundreds of those, even in the first few months of data. A significant fraction of those will not actually be real planets, but will be other things whose signals look like planets, including, for example, binary stars.

There’s actually a lot we can do to rule out these, which we call, astrophysical false positives just from the PLATO data itself– so from the brightness monitoring of the stars– together with catalog information from lots of previous surveys that have already taken place. So we cross-correlate all our data with this catalog information. It tells us lots about the stars and the part of the sky we’re looking at.

And then on that basis, we’ll reduce the candidates to just the ones that are still likely to be planets. And that’s the point at which we will have an algorithm that triggers the follow-up– so that says we now need to go and get additional observations with ground-based telescopes.

And there will be a number of different types of observations we need to do. First, aiming to confirm that the star which has the transit happening is really the one we think it is, and then looking for the gravitational pull of the planet on the star, which is what allows us to measure the mass of the planet.

And we look for that gravitational pull by taking time series of spectra of the stars. So we take the light of the star. We disperse it according to its constituent wavelengths of color or colors. And we look for very, very tiny shifts in the wavelength of the light from blue to red and back again.

And that indicates that the star is moving in a little dance about what we call the center of mass of the system. It’s called the Doppler or the radial velocity method. And it’s how we measure masses for the PLATO planets.

But that process– for the most interesting planets, the ones that are similar to the Earth, that process could take up to 10 years just to gather enough data. And the reason it could take that long is because of the intrinsic variations of the stars themselves. That’s actually the area that I specialize in.

So both in order to find the transits and in order to measure the masses of the planets, we have to overcome the variability of the stars themselves, which is usually much bigger than the signals from the planets we’re interested in. And so we use machine learning to learn the properties of that variability and to separate it from the signals of the planets.

The signals of the planets are actually really quite simple. They’re fully determined just by geometry and by Newton’s laws of motion. Whereas, the signals from the stars are really complex. So we can’t predict those a priori. We have to use lots and lots of data to learn their properties from the data.

CHARLES BERGQUIST: Are there things that you would like to be able to learn from this that you just can’t, even if you collect 10, 20 years of data?

SUZANNE AIGRAIN: I think that there are some things which we will not be able to do for PLATO’s planets straightaway. And perhaps the most exciting thing to do would be to look at what’s in the atmosphere of these planets, and in particular, to search for signs of biological activity in those planets.

So if you take a spectrum of the atmosphere of the earth, you will see a mixture of carbon dioxide, methane, and water vapor. These are the most common spectroscopically active molecules in the atmosphere of the Earth. But you will also see strong signatures from oxygen– molecular oxygen– or ozone, depending on which wavelength range you look at.

And normally, you would not expect to see this unless there was something producing oxygen at a very high rate. Because if it wasn’t continuously being produced, the oxygen would react with some of the other molecules in the planet’s atmosphere. So that’s the sort of thing we would look for in another planet, and it would tell us that it’s likely that there is life on that planet.

Now, in order to do that, we need instrumentation, which we don’t yet have and will not have in the next 10 years. We will probably need to wait another 20 years before we have that instrumentation.

In particular, NASA is in the process of studying large telescope concept for launch in the 2040s, which will involve a big space telescope equipped with a coronagraph, so that we can block out the light of the central star, isolate the light of the planet, and take a spectrum of the planet, which is what you need to do that kind of measurement. But that will take a significant amount of time to develop. So what PLATO will do for us is it will find the best targets for these future telescopes, and it will also tell us much better than we know today how common these planets are.

The other thing that I’m particularly excited about PLATO for is the fact that it will measure the ages of the planetary systems. I mentioned that by looking inside the stars, we can measure their masses and radii very precisely. The other thing we can measure very precisely is how much of their fuel, the hydrogen that they burn and convert to helium, has already been consumed. And that is a very precise, or a comparatively precise, chronometer for the lifetime of the star.

So for the first time, we will be able to measure the ages of a very large number of stars to precisions of order 15%. And any planets that we find around those stars will also have the same precision on their ages. Whereas, until now, most of the planets we know orbit around stars that we think are sort of middle-aged. And that means we know their ages to a precision of maybe two billion years out of four or six. So it’s really very imprecise.

CHARLES BERGQUIST: Right. I think when a lot of people hear somebody say telescope, they picture the big tube in somebody’s observatory. This is not that. Tell me a little bit about what the satellite physically looks like.

SUZANNE AIGRAIN: Each of the telescopes that PLATO carries, the biggest lens in the camera is only 12 centimeters across. And so it has 26 of these 12-centimeter cameras. And each of those has a detector at the back of it.

So 24 of the cameras are our normal science camera. They sit in four groups of six, and they look at partly overlapping parts of the sky. So any given star will be looked at by at least six cameras, and at most 24, depending on whether it’s at the center or the edge of the field of view.

And then we have an additional two cameras which will observe at a faster cadence and have a color filter in front of them. So they’ll give us some information on the color variations for the very brightest stars.

And all of these cameras sit on a kind of– I like to think of it as the seats in a stadium around an arena. So they sit on a sort of staggered staircase in rows of six. Then they are surrounded at the back by the solar panels of the telescope, which also extend out to the side.

And the entire satellite is around 2.5 meters by 2.5 meters by 2 meters. So that gives you its kind of size. And then when the solar panels are fully deployed, then they extend to about 6 meters in length in total.

CHARLES BERGQUIST: We mentioned that this is the second of a series of three satellites. How do they all connect together? And how does the PLATO mission fit into the wider exoplanet research plan at ESA.

SUZANNE AIGRAIN: There’s a current ESA mission called Cheops, which is just one smallish telescope. It’s about 30 centimeters across. And that one carries out targeted follow-up of individual planetary systems in order to very precisely measure the planet’s radii or the times of the individual transits. If you have multiple planets in a system, you can have what we call transit timing variations.

And, of course, these specialist exoplanet missions, transit search missions, for example, are then closely related with the big observatory class missions, like JWST, which I’m sure your listeners have heard of and which has been producing some amazing results, both in terms of measuring atmospheric spectra of exoplanets, and also doing direct imaging, finding planets directly by masking out the light of the star and looking in the vicinity of the star.

After PLATO, the next mission in ESA’s so-called Cosmic Visions program is also an exoplanet mission. It’s called Ariel, and it will use the transit method, just like PLATO does. But it will be equipped with a spectrograph. And so it will take spectra of exoplanets, much in the same way that JWST currently does. But it will do this systematically for a sample of 1,000 exoplanets.

So it’s a smaller telescope, but it will obtain a very wide wavelength coverage in a single shot. And it will do that for a very large number of planets. So it will give us a really good statistical sample of exoplanet atmospheres.

CHARLES BERGQUIST: I love talking about exoplanets. It taps into that whole wonder and discovery thing. But some listeners always write in and say, this is great, but why do we care? Like, how can this help people here on Earth?

SUZANNE AIGRAIN: For me, the top level motivation is always trying to place ourselves, our civilization, but also our planet, and even our solar system, in context– so to understand where we sit within the universe. And from the response that I get when I speak to members of the public, I think that is a motivation that touches not just an exoplanet specialist like myself, but also everybody who thinks about the problem.

I guess one way you might like to think about it is as follows. If you look at the night sky, and you look at any star, we now know that there’s a very good chance that that star is orbited by a planetary system. And that is something we didn’t even 20 years ago.

Now, imagine looking ahead 10 years down the line and being able to say that bright star here has a planet like the Earth that might have life. And then looking ahead maybe 20, 25 years down the line and being able to say that star there has a planet which has signs of life on it. And that, I think, would completely change the way in which we think of ourselves and our place in the universe.

And it’s kind of paradoxical because it probably wouldn’t change the daily lives of most of us very much at all. And yet, it would profoundly changed the way we think about our own species.

CHARLES BERGQUIST: Dr. Suzanne Aigrain is a Professor of Astrophysics at Oxford University. Thanks so much for taking the time to talk with me today, and good luck with the mission.

SUZANNE AIGRAIN: Thank you very much. It’s been a pleasure.