One of the greatest barriers to finding habitable worlds elsewhere in the cosmos is that we have only one example of such a world ever in existence: Earth.
Yet beyond our Solar System’s seven planets, we now know that there are thousands (if not millions) of other star systems with their own myriad planets — some of which may be quite similar to Earth. In the last three decades, scientists have discovered some 4,000 known exoplanets. A handful of these orbs are known to exist in the “Goldilocks Zone,” a special area of space both close and far enough from their host stars to have surface temperatures able to sustain liquid water, like Earth. Further, some exoplanets show faint signs of carrying water in their atmospheres — another key sign of habitability.
But these worlds are so far away, that spotting them is a painstaking task: Observatories in space and on the ground have to stare at stars for as long as possible to see if they can spot tiny aberrations in the star’s light. In these moments, exoplanets reveal themselves to eagle-eyed scientists and offer up a smattering of data to decipher about its orbit, size, and atmospheric composition.
“When you see the planet pass in front of the star indirectly, we see the effect the planet has on the star,” explains Knicole Colón, the James Webb Space Telescope Deputy Project Scientist for Exoplanet Science at NASA. Essentially, the planet blocks the star’s light — by passing in front of it — and bends and dips some of the light — both by getting in the way and by filtering the light through its atmosphere, if it has one.
“If the planet has an atmosphere, we can see the starlight that gets filtered by the atmosphere — we look for missing dips and see what wavelengths of light do they happen at and how does that correspond to different molecules that may exist to block light,” she says.
But now scientists like Colón have an opportunity: The James Webb Space Telescope. In the first year of science alone, the Webb will observe “about 70” different exoplanets, Colón says. These 70 planets have all been discovered by other observatories, but the Webb will transform astrophysicists’ ability to decipher their true nature — and even image them directly. The Webb could also discover new exoplanets itself, too. In fact, Colón says, one of the first scientific endeavors the Webb will undertake is to look closely at the star system closest to Earth, Alpha Centauri, and try to spot any planets that may be hiding there for the first time.
“That’s using direct imaging to see if we can find a planet there,” Colón says. “Like a giant planet around there.”
Inverse sat down with Colón to learn more about how the Webb can transform planetary science, discover new worlds, and help hone scientists’ concept of what makes a world habitable beyond what we know about Earth.
The following interview has been lightly edited for clarity and length.
Inverse: We’re still a few months out from the James Webb Telescope’s first science observations, but tell me a little bit about the first projects the Webb will carry out to do with exoplanets?
Knicole Colón: We have the first year of science targets selected and there’s a mix because exoplanets are predominantly going to be observed via the transit method. So when you see the planet pass in front of the star indirectly, you see the effect the planet has on the star. If the planet has an atmosphere, we can see the starlight that gets filtered by the atmosphere — we look for missing dips and see what wavelengths of light do they happen at and how does that correspond to different molecules that may exist to block light?
Most of the planets are going to be observed with the transit method. There are actually about 70 different transiting exoplanets that will be observed within Webb’s first year of science, and these are ones that have already been observed by other missions, like TESS [Transiting Exoplanet Survey Satellite]. TESS provided a big bulk of the targets and that is because TESS is finding small planets around small stars. Those are the most interesting for a few reasons. I mean, the small factor, right, because we want to find small planets, and TESS is finding a lot of them, and we’re feeding that to the Webb now.
The 70 planets run the full gamut from you know, around Earth-size, to larger than Jupiter, and everything in between. So they have a really nice diverse sample selected for the first year. And then there are also the planets that will be observed using direct imaging as well.
“You’d be surprised how much we can extract from a fuzzy blob.”
Tell me a little more about that — you will be directly imaging exoplanets with Webb?
So the majority of the time, at least right now, the focus will be on transits. But there is a fraction of time devoted to direct imaging as well. It’s kind of like two scopes, where, in some cases, people are using different instruments to observe planets that were previously discovered by, again, either a ground-based telescope or even Hubble, I think, discovered a planet via direct imaging, so these are already known, you know, we know they’re there. So Webb can look and specifically has special observing modes to block out the light from the central star somehow. So, we see a faint planet that’s like hiding in the glare of the star, we can get rid of the starlight basically and see the planet itself.
So this lets us see a planet, but it’s still going to be like a fuzzy blob, just because these planets are still really far away. We won’t resolve like, surface features. But you’d be surprised how much we can extract from a fuzzy blob. We’re still collecting all the light from that planet basically. And what we’re able to do is take that and interpret it and get the spectrum of the planet’s atmosphere. And then we look for the same things we look for using the transit method. But a lot of those planets — just because it’s a really hard technique, direct imaging — are Jupiter size or Jupiter mass. They’re all giant planets.
But we’ll also be looking at stars if we don’t know that they have planets and Webb will be searching for some by direct imaging as well actually, so Webb might discover some new planets that we haven’t seen with other observatories yet.
I do think most of those would be like Jupiter or maybe like Saturn size, so still gas giants. But the point is that Webb will be discovering some planets this way, we expect, and I think that kind of often gets buried in the news.
Why is that?
I don’t know. It’s not a primary, you know, component, but it’s also partly because we kind of have to see how Webb operates. Like now that it’s in space, you know, and, and like, how Webb does, how sensitive it is. So once we learned that, the direct imaging program will grow.
“Webb... is going to be looking at Alpha Centauri, which is our closest star system.”
So will Webb be imaging stars in a less pre-targeted fashion, and instead try to find planets around stars that we never thought could be there in the first place?
The programs that are using Webb to search for planets are mostly focused on some of the closest stars. Like for example, there is a program that is going to be looking at Alpha Centauri, which is our closest star system, and that’s using direct imaging to see if you know we can find a planet there. Like a giant planet around there. And there are some programs looking at some of those just closest and smallest stars. So these are like cool and dwarf stars that are much smaller than the Sun, basically. There’s a program doing a survey of, I think it’s like five or 10, you know, but it’s a handful, and there might be, you know, a chance of finding something there.
I even have a different program doing a search around white dwarf stars, actually, that are these remnants of stellar objects that are really the end stage of life, but there could still be remnant giant planets or something smaller orbiting around them.
So essentially looking at solar graveyards?
Right. So we don’t know yet until we look what’s there, and that’s the whole point of having something like Webb, too, because Webb is sensitive to these regimes.
They’re pretty small surveys, right, each individual program. But you know, they’re basically starting off the first year of science to see what we can find all with direct imaging.
Could Webb also discover exoplanets using the transit method?
We really need these massive, you know, long timescale, all-sky surveys, like TESS or Kepler. I really don’t think we’ll find transiting planets with Webb. Unless it’s serendipitous, like maybe we're observing a known transiting planet, and then, Webb is so sensitive that we see another dip or something in the starlight that we weren’t expecting, you know. Then maybe that’s really another transiting planet in that system that we hadn’t detected, for whatever reason, with other telescopes. So there could be serendipitous observations, but, to my knowledge, nobody is planning a transit search because it’s a really inefficient use of Webb’s time.
But Webb is kitted out with the instruments that you would need to see a transiting planet if you wanted to?
“Webb images in infrared, it’s not like we can see with our eyes.”
Yeah, so we can see, I guess. But because Webb looks at one star at a time usually, versus TESS which looks at billions of stars at a time, it would be very hard. Unless you were pointing at a very special star — like one of these nearby stars. I feel like you’d have to have very special circumstances to use Webb to stare long enough at a star to try to find another planet crossing in front of it.
I think some people think Webb will give us images of exoplanets that look as clear as a photo of a planet close to us, like Neptune. So tell me more about what Webb’s images will actually show when it comes to exoplanets?
Webb images in infrared, it’s not like we can see with our eyes. So it’s hard to imagine, but what we can do, let’s say when we release these images to the public, you know, they won’t be colors that you know, we’re used to like, like, there won’t be any obvious color. But we assign colors to them and we can say, okay, you know, this is what we think is close enough. But that’s okay!
What Webb does depends on how it is being used, like using a camera and using a certain filter, just like you would use, say, a Snapchat filter on your phone. There’s a certain scale of colors of light that we can collect with Webb and we can take that snapshot and then dissect the light. Like, when we look at the Sun, we call that white light but it contains all the colors of the rainbow. If you take the Sun and put it in a prism, it splits the light into our rainbow so that we can see all the colors from red to green, right? Imagine infrared light is the same thing, where we’re taking our fuzzy dot and it looks like it’s just one color, like white let’s say, but actually when you split it up into its rainbow of infrared light, then we get all this information.
We get the infrared equivalent of red, green, blue, and we can dissect that and see there will be some parts that are brighter, like maybe it’s brighter in the red, you know, versus the yellow... Depending on how bright or dim the different colors are, once we dissect them, that tells us okay, well we know water should be in the atmosphere at this color of light. But the light from the planet looks very dim there, so there’s no water there reflecting light, otherwise, it would be bright.
Ultimately, we can extract a lot from that little dot, we can break it up into a rainbow of colors, essentially, but in infrared, and then look for what those rainbows of colors tell us whether there’s methane, carbon dioxide, water, or other molecules.
How does that information help us home in on habitable worlds, though? What can the atmosphere tell us, say, if we know the planet is in the Goldilocks Zone and might sustain water at the surface, for example?
This is just the first year of science we have planned, but a lot of the first targets are small. And small means they’re potentially rocky — but we don’t know that until we look at their atmosphere and try to see what their atmosphere is made of because they could be physically small, like their radius or diameter, but they could have a really thick atmosphere, like Venus.
So if we get a small planet with an atmosphere, and we try to look for evidence of water or something and we don’t see anything, or we just see like a flat spectrum, like we can’t make out any signals, that is very suggestive that it has a thick atmosphere or that the atmosphere is continually contained by like thick clouds and hazes. If that’s really the case, it’s very likely there’s not a surface underneath the atmosphere that is livable.
If there is an atmosphere that is reasonably like Earth, Webb will have the sensitivity to start to detect this water vapor and, and, you know, I mentioned methane and carbon dioxide, so, one should be able to start detecting these molecules.
We could say, we know there’s water, we know there’s this, we know there’s that, and we can run our models and you know, try to say, ‘Okay, how much water like is it? Is it saturated in water to the point where maybe it's more of like an ocean planet? Or, or is it really saturated in something poisonous like carbon monoxide?’ We can start to do some of that with Webb.
So will it ever be possible to conclusively say any observation is a clear sign of habitability?
“We’ll be more confident about some of this potential habitability.”
We can do our best with the data we have. But if Webb happens to find something that indicates a potential biosignature... That is something where we have to be very careful to confirm it, right? And I think the biggest constraint there is that the signals are really tiny. So we’ll need a lot of data to get there with Webb. We just might get there with Webb, where we can say you know, we may think we have evidence of something, but I would be surprised if anybody actually claimed — with Webb alone — that we found a habitable world, because it’s going to require a lot of careful analysis and re-analysis. So we don’t get it from one observation, basically, we have to look at planets many times.
And as we’re doing that, we’re looking at the star too, just because that’s how the process works. And the star might be changing, which could affect our interpretation of the planet as well. So we just have to be really careful to account for everything, as we're digging for these small signals. But I think Webb is really kicking off this field, so it's going to start doing this reconnaissance, and I think, tentatively we’ll be detecting some of these signals that we’re looking for.
But then for the ultimate saying, something is habitable, I think what will happen is we end up with this crop of planets and we say okay, we’ve deemed that these are still some of the most viable targets to continue to pursue, right? And so then that builds into some of these future missions that are already in the works and that are designed more for that sensitivity, like specific to biosignatures that we would expect to see on a modern-day Earth. Basically, you need a lot of telescopes at different wavelengths to get all the information you need.
How many planets in the habitable zone will Webb study?
In the first year of science, we know Webb will observe at least four or five small planets that you know, we think are in this Goldilocks zone, as far as we can tell. So Webb will definitely be observing, like four or five of these, potentially Goldilocks planets.
And then, so what we learn from those observations, right, will, hopefully, they won’t just imply that they have a thick atmosphere like Venus, you know? But if they do at least we know that now.
Webb really provides the first opportunity to do this either way, even if it’s not what we hope to see. With Webb we will be confident, like, okay, this is more of a thick atmosphere potentially, like Venus, and then this one, okay, you know, we have signs evidence of water and other molecules. At least then we know we should focus all our efforts on this one and just forget the other four, you know?
It’s hard. Like it’s just literally a really hard measurement to make because these atmospheres are so thin, and the signals we’re looking for are so small, and Webb is going to reveal a lot of information, but though I think we’ll be more confident about some of this potential habitability. But then actually finding signs of life? That’s a whole other thing.
