What signatures are scientists looking for in the search for alien life? What discoveries are realistically on the horizon? And why might cloudy, hazy planets turn out to be some of the best places to look for life beyond Earth?
These are some of the questions I recently asked astronomer Lisa Kaltenegger, founding director of the Carl Sagan Institute at Cornell University. Kaltenegger is at the forefront of studying exoplanets, and her research often focuses on innovative ways to detect signs of life in the atmospheres and on the surfaces of these distant worlds, a project she details in her 2024 book Alien Earths.
Adam Frank: Let’s start by zooming out. What should people be watching for in the search for life over the next 10, 20, or 30 years? Where is this field going, and when do you think we might actually have some kind of answer — even if that answer is, “Sorry, no life here”?
Lisa Kaltenegger: I think the first thing to remember is: We are right at the beginning of this adventure. There’s so much excitement that every little signal — every “wiggle” in a spectrum — gets people saying, “Oh! That might be life!” And then, on the other side, other people respond with, “I don’t see enough wiggles, so there’s probably not even an atmosphere. Dead planet. Move on.”
Both reactions are too fast.
We have to understand that a planet is an environment, not a carbon copy of modern Earth. We still have huge open questions, even about our own planet’s history. So, a spectral feature that would be interesting on an Earth-like world might be meaningless on, say, a gas ball planet, where the conditions for life as we know it aren’t even there.
Frank: You’re saying that context is everything.
Kaltenegger: Exactly. And that gets especially tricky with the kinds of stars we’re looking at right now. Many of the most promising targets for Earth-sized planets are tiny red stars — M dwarfs. They’re much more active than our Sun.
These stars are basically giant convective gas balls. Material is churning up and down from the interior to the surface. A star like the Sun has a radiative zone and a convective zone; its structure is more layered. With M dwarfs, the whole star is convective. That makes their surfaces very different and much harder for us to model and understand.
Frank: And yet M dwarfs are the main targets for astrobiology.
Kaltenegger: We love M dwarfs observationally, because if your planet is the size of Earth and your star is as small as possible, the contrast between the two is best. Your little planet blocks a bigger fraction of the star’s light, so you can study it more easily. So, we end up looking at these very small, very active stars that we don’t yet fully understand.
My advice for the next decade or so is a little careful. When you see big headlines saying, “We found signs of life!” — wait for confirmation. Ask: Do we really understand this star-planet system? Do we have enough data? And likewise, when you see doomsday declarations of, “We see no atmospheres; everything’s dead,” ask: Do we truly have enough signal, and do we really know what the star is doing?
Science moves forward with both detections and non-detections. We learn from each attempt — successful or not.
Frank: Given all those caveats, what’s realistically on the table in the near term? What’s the kind of result we might plausibly see in the next 10 or 20 years?
Kaltenegger: One very realistic near-term step is detecting key atmospheric features on small, rocky exoplanets. For example, seeing clear signatures of water vapor in the atmosphere of a planet in the habitable zone — like some of the planets in the TRAPPIST-1 system — that would be huge.
Even more exciting would be combinations of gases that are hard to explain without life. Methane on its own is interesting. Oxygen or ozone on their own are interesting. But the really big prize is seeing methane together with oxygen, or methane with ozone, in significant amounts.
On Earth, methane and oxygen don’t like to coexist — they react and form CO₂ and water. So, if you see them both present in large quantities at the same time, something must be constantly replenishing them. On our planet, that “something” is life. In that context, we don’t know any purely abiotic way to keep those gases in that kind of disequilibrium.
So that’s one of the combinations we’re really looking for. But again, the challenge is that the star is changing while we’re observing. Flares, spots, variability — all of that can mimic or mask signals. We have to model and correct for the star’s behavior very carefully before we can confidently interpret what we see in the planet’s atmosphere.
Frank: On a longer timescale, what are the big facilities coming that will really change the game?
Kaltenegger: We’re entering a really exciting era. On the ground, we’re building the Extremely Large Telescope (ELT) in Chile, with a primary mirror close to 40 meters in diameter. In space, NASA is planning the Habitable Worlds Observatory. Together, these telescopes will add a new way of looking for life.
Right now, with the James Webb Space Telescope, this amazing 6.5-meter space-based observatory, our main method for small planets is transit spectroscopy. When a planet passes in front of its star, some of the star’s light filters through the planet’s atmosphere. The atmosphere leaves fingerprints in the light, and we read those to figure out its composition — oxygen, methane, ozone, etc.
But in that geometry, we don’t see the planet’s surface directly. Any light that hits the surface just bounces away; it doesn’t go through the atmosphere in a way JWST can capture during a transit.
The ELT and the Habitable Worlds Observatory will mostly look at reflected light — the starlight that bounces off the planet’s surface and atmosphere. That’s a different, complementary view. And this is where our work on biopigments — the colors of life — becomes crucial.
What you really want is two independent lines of evidence. First, you detect atmospheric gases that could be produced by life — say, that methane-oxygen combination. Then, in addition, you detect surface or cloud-top colors that match what biopigments might produce: a characteristic way life reflects and absorbs light.
If you have both — gases in the atmosphere and biopigment signatures in the reflected light — you have two completely different methods pointing to the same conclusion. I’d be thrilled with one strong line of evidence. Two would be even better!
Frank: And that brings us to something you’re working on right now — this idea of life in the clouds, right?
Kaltenegger: Yes! This is one of the things I’m really excited about. You might remember Carl Sagan’s old idea about life in the clouds of Venus. We’ve always had this notion of “cloud worlds,” but we often dismissed fully cloud-covered planets as hopeless for detecting life. Too cloudy, too hazy — we’re blind.
Our new work says: Maybe that’s not true.
We have a paper that looks at the biopigments — the colors of life — in Earth’s atmosphere. Specifically, we studied microorganisms living in the air, in the clouds. This is the first systematic look at the spectral signatures of aerial life in our own atmosphere.
On Earth, we don’t see dramatic, colorful clouds made by life, mainly because it’s relatively dry and the biomass in the air is low and patchy. But imagine a planet with much higher humidity, and therefore much more cloud coverage. Our biological collaborators tell us that such an environment could support much more abundant aerial communities.
So suddenly, those planets we thought we’d never be able to study — because clouds hide the surface — might actually be some of the best places to look for signs of life. Life could be living in the clouds themselves and broadcasting its presence in brilliant colors.
Frank: That’s such a great way to flip the script. The thing that used to be the problem — clouds — becomes the opportunity.
Kaltenegger: Exactly. Clouds and hazes have long been the bane of exoplanet observers: “Oh, it’s cloudy. We can’t see anything.” But if clouds host an aerial biosphere, then they are the thing we want to see.
In our study, we worked with seven strains of airborne microorganisms — tiny life forms collected by collaborators in Florida. They weren’t originally gathered for their colors, but we asked for them to measure their spectra. What we found is that many of them are rich in carotenoids — biopigments that are often reddish or yellowish, not green like chlorophyll.
These pigments make sense biologically. Biopigments often protect against radiation and harsh environments. If you’re a microbe drifting in the upper atmosphere, exposed to UV light, you’d want protection. So, pigments are a natural adaptation.
Now, imagine a planet with a thick, humid atmosphere and extensive clouds full of such organisms. You could have large, persistent communities of aerial biospheres. They might alter the way the clouds reflect light enough that a sensitive telescope could detect their combined color signature.
We’re not claiming we can see this now or that it will be easy. What we’re saying is that fully cloud-covered planets — a part of parameter space we used to treat as “we’re blind here” — may actually host detectable biosignatures in their clouds.
I like to picture it as flying into a gorgeous bank of clouds, and instead of just white and gray, you see subtle hues — reds, yellows, maybe other colors — because the clouds are filled with life. It’s a beautiful and slightly mind-bending image.
Frank: OK, that is a really cool idea! And I love that this search for life elsewhere is now forcing us to look more carefully at our own planet.
Kaltenegger: Yes! That’s one of the things I find most satisfying. By thinking about how life might show up in extreme or unfamiliar environments elsewhere, we realize how little we know about similar niches on Earth — like the aerial biosphere.
The same goes for our planet’s history. When we think about what oxygen means as a biosignature on an exoplanet, we have to dig deeply into how, when, and why oxygen rose in Earth’s atmosphere. That forces us to re-examine assumptions we’ve been making almost unconsciously.
Frank: Right. We did that paper pushing back on the “hard steps” model, and it required us to really look at the evidence for oxygen on Earth. It was incredibly useful for thinking about other planets, but it also changed how we think about our own story.
Kaltenegger: Exactly. There are these basic assumptions hiding in the background, and by posing the exoplanet question — “Would this be a sign of life somewhere else?” — you suddenly notice them. You ask, “Why am I assuming that’s always true? Under what conditions would it fail?”
That’s a kind of intellectual liberation. You’re free to think about biology and planets in a broader way.
This article Aerial aliens: Why cloudy worlds might make detecting life easier is featured on Big Think.
