Key Takeaways:
- The James Webb Space Telescope (JWST) and future powerful telescopes are enabling the search for biosignatures – chemicals indicating life – in exoplanet atmospheres using transmission spectroscopy, which analyzes starlight passing through planetary atmospheres.
- Laboratory experiments and computational simulations are crucial for interpreting exoplanet atmospheric data, specifically identifying molecules based on their unique infrared light absorption patterns. This was demonstrated by the detection of sulfur dioxide (SO2) in WASP-39b's atmosphere, a molecule not previously observed in exoplanets.
- The search for biosignatures focuses on rocky, Earth-sized exoplanets in habitable zones, particularly around M dwarf stars due to observational advantages. However, challenges include the active nature of M dwarf stars and potential atmospheric erosion.
- Future telescopes like the Extremely Large Telescope (ELT) and the Habitable Worlds Observatory will enhance the search for biosignatures, potentially detecting combinations of gases like oxygen and methane, and enabling direct imaging of habitable exoplanets. Computational methods are also improving the identification of unknown spectral features.
The next time you look at a star, imagine a rocky planet orbiting at just the right distance for a climate that supports liquid water on its surface. Picture thick forests upon vast landscapes teeming with wildlife. Or consider a water world devoid of land where strange marine life swims beneath a hydrogen sky. Maybe even more exotic life-forms inhabit oceans filled not with water but hydrocarbons like methane.
For the first time in human history, these musings are not idle fantasy: Scientists can search for signs of life in the atmospheres of alien planets by aiming powerful telescopes at them as they cross in front of and slip behind their stars, studying the telltale dimming of light.
These techniques are entering a new phase thanks to NASA’s James Webb Space Telescope (JWST) and even more powerful upcoming scopes. The hope is that scientists will uncover biosignatures — chemicals like oxygen or methane that can indicate life. But it takes great care to determine whether these signals are due to life and not some other phenomenon mimicking life’s chemical fingerprint.
To solve that puzzle, scientists are conducting experiments and computer simulations to evaluate what gases could exist in exoplanet atmospheres. If astronomers are going to find signs of life beyond our solar system, they will need synergy between observatories and laboratories — and perhaps a bit of luck.
Related: New study revisits signs of life on K2-18 b
The curious case of WASP-39 b
On July 10, 2022, researchers directed JWST toward a star slightly smaller than our Sun that hosts a gas giant known as WASP-39 b. With a mass similar to Saturn and a temperature of some 1,650 degrees Fahrenheit (900 degrees Celsius), the planet is hardly life-friendly. But as one of the first exoplanets that JWST studied, WASP-39 b was an important test of the telescope’s ability to perform a technique called transmission spectroscopy.
For about eight hours, JWST’s mirrors collected photons of infrared light, including a roughly three-hour period when the planet transited in front of its star. This meant some starlight filtered through the thin layer of atmosphere surrounding the planet — and certain wavelengths of infrared light were absorbed by its gases. When the rest of the light reached the telescope’s instruments, it was split into its individual wavelengths, akin to how a prism separates light into a rainbow. By comparing the data from the transit to those taken before and after, scientists obtained a spectrum showing the fraction of light blocked at different wavelengths during the transit by different atmospheric gases.
The role of spectroscopists
Spectroscopy is the study of how light interacts with matter. Single atoms, for example, absorb light that has wavelengths that match the exact energy required to move their electrons to higher energy levels. On a spectrum, the absorptions appear as distinct dark lines because that color of light disappears.
But molecules — assemblages of multiple atoms — are more complex. This is because they have the ability to absorb wavelengths of infrared light that match the frequencies at which their bonds vibrate (e.g., bend and/or stretch), which depend on their structure. Molecules can also rotate at the same time they vibrate, especially at higher temperatures, which broadens the wavelengths of light they can absorb. This makes the spectral features less distinct and harder to identify.
To determine which unique wavelengths cause a molecule to resonate, researchers fire infrared light at gases in laboratories and analyze the absorption data. To date, molecular spectroscopists have identified the vibrational frequencies of 100 molecules — a tremendous achievement, considering the complex behavior of molecules.
Some of these gases in the spectrum were straightforward to identify. For instance, scientists know that carbon dioxide (CO2) molecules absorb light at a wavelength of 4.26 microns, thanks to lab experiments and calculations conducted by molecular spectroscopists. “Our basic way of building a model of how molecules absorb light is from how it works at room temperature and then performing the calculations for higher temperatures,” says molecular spectroscopist Jonathan Tennyson of University College London. This is helpful for studying hot worlds like WASP-39 b.
Astronomers were also able to identify water vapor (H2O), carbon monoxide (CO), and sodium gas (Na) in the JWST spectrum of the gas giant’s atmosphere.
But they also found a slight dimming at around 4.05 microns that they could not link to any molecule. Their models hadn’t predicted any gases in the planet’s atmosphere that would absorb at that wavelength. But when it showed up again in another JWST spectrum, “we started to feel comfortable that it is real,” says astronomer Jacob Bean of the University of Chicago.
After conducting an exhaustive search of known molecular features, they honed in on a candidate: sulfur dioxide (SO2), a molecule never before seen in an exoplanet spectrum. The JWST observations were a match to lab work and calculations by molecular spectroscopists. But to be confident of their detection, the team had to revisit their models and determine whether it made sense for sulfur dioxide to exist in WASP-39 b’s atmosphere. These models take into account characteristics of an exoplanet like its temperature, pressure, and chemical composition.
But it turned out they were missing a key process: High-energy ultraviolet photons streaming from the WASP-39 host star can break apart water molecules — similar to how sunlight splits oxygen molecules in Earth’s atmosphere to help form the ozone layer. On WASP-39 b, this initiates a series of reactions that could indeed produce sulfur dioxide — and in quantities large enough to explain the spectral feature detected by JWST.
Identifying a gas in an exoplanet atmosphere resulting from complex photochemistry and verifying its plausibility with models “was a spectacular achievement,” says Bean. “The prediction of SO2 required a lot of different pieces of information from different directions.”
In many ways, it also highlights the process that researchers would follow if they were to identify gases that might indicate life.
Strange worlds
To find such gases, researchers are interested in rocky, Earth-sized exoplanets that lie in their host stars’ habitable zones. Though no such planets have yet been detected around Sun-like stars, some have been found orbiting less luminous stars called M dwarfs.
These planets have several observational advantages. They orbit very close to their stars, in some cases completing one round trip in one or two days, allowing for more frequent observations to search for atmospheres. And because their stars are smaller and cooler, any life-indicating gases will absorb a higher fraction of their star’s light and have a better chance of showing up in JWST’s infrared spectra.
Researchers have already pointed JWST at the two innermost planets orbiting the M dwarf known as TRAPPIST-1. These planets, designated b and c, lie too close to be inside the habitable zone and are too hot for liquid water — but their quick orbits make them low-hanging fruit to see if they have atmospheres.
In addition to transits, JWST has also observed secondary eclipses, when a planet disappears behind its host star. Astronomers can search for signs of an atmosphere by measuring the heat from a planet’s warm dayside just before and after the eclipse. But so far, they have come up empty-handed.
This may be because M-dwarf stars are extremely active, erupting with bouts of high-energy particles and radiation that can erode planetary atmospheres. M dwarfs are “very exciting systems,” says planetary scientist Eliza Kempton of the University of Maryland, “but they’re maybe not the most hospitable stars to put planets around.”
The early results have some astronomers wondering whether rocky planets around these stars are capable of holding on to their original atmospheres, made of gases that they accreted when they formed. If they are, it seems they must orbit farther out.
In the next couple of years, astronomers plan to aim JWST at TRAPPIST-1 d, e, and f, which orbit in their star’s habitable zone. Some models show there’s a chance that TRAPPIST-1 d — on the inner edge of its star’s habitable zone — has liquid water on its surface. As for the next two planets outward, “I think there’s a chance that [e and f] could have nitrogen-dominated atmospheres like Earth’s,” says planetary scientist Howard Chen of the Florida Institute of Technology.
Researchers are also intrigued by the potential for life on other classes of exoplanets that have sizes in between Earth and Neptune. Known as super-Earths and mini-Neptunes, they are the most common types of exoplanet discovered to date.
Yet, there are no known examples in our solar system, leaving scientists unclear as to their true nature. “Is it just as simple as taking Earth and making it bigger or making Neptune smaller, or is there more going on here and are these objects entirely new or different?” asks Kempton.
The evidence so far is intriguing. Super-Earths have densities similar to our planet, suggesting they are rocky bodies. Mini-Neptunes have lower densities, which would seem to imply they are ice-covered and swaddled in a thick atmosphere of hydrogen and helium. But according to some models, a hydrogen atmosphere could sustain a global water ocean. These theoretical planets have been dubbed “hycean” worlds — a portmanteau of hydrogen and ocean. “You could imagine microbial life persisting in the upper layers of this ocean,” says astrobiologist Edward Schwieterman of the University of California, Riverside. And if that life interacts with the atmosphere, it could produce signatures that JWST could detect, he adds.
In April, a team led by astronomer Nikku Madhusudhan of the University of Cambridge, U.K., claimed to have found a signal in a JWST spectrum of the candidate hycean world K2-18 b. They attributed the purported signal to dimethyl sulfide or dimethyl disulfide, the former of which is produced by phytoplankton in Earth’s oceans. But some scientists have expressed skepticism — including Schwieterman, who says he is not yet convinced that the current data are of sufficient quality to confirm the detection of these gases.
Hunting for atmospheres
An increasingly important set of techniques to study exoplanet atmospheres involves taking observations when a planet slips behind its host star — an event called a secondary eclipse. Just before and after the star eclipses the planet, the planet shows its warm dayside to our vantage point. Although JWST cannot resolve the disk of the planet, scientists can subtract the light from the star alone — when the planet is behind it — to isolate and measure the infrared light that the planet emits from its dayside. If the planet has no atmosphere, the dayside will be scorching hot; but an atmosphere will circulate some of the heat around to the nightside, cooling the dayside.
For this technique, JWST observes in the 15-micron band, which is absorbed by carbon dioxide. By comparing observations to models of predicted spectra with and without atmospheres, researchers can gain insight into whether an atmosphere is present and, if so, what it is made of.
JWST’s operator, the Space Telescope Science Institute in Baltimore, Maryland, has set aside 500 hours of director’s discretionary time to search for atmospheres around rocky planets in more than a dozen nearby M-dwarf systems with this technique. The Hubble Space Telescope will assist by monitoring the host stars’ activity. Scientists hope this initiative, called the Rocky Worlds DDT Program, will settle the question of whether Earth-sized worlds around M dwarfs can hang onto their atmospheres.
An ensemble act
To assess the range of possible gases in exoplanet atmospheres that could be produced by life, scientists lean on what they know from Earth. For instance, oxygen is mainly produced by photosynthesis, and metabolizing microbes release most of the methane and nitrous oxide present. But proving from afar that these gases are the result of life — and not some other nonliving process — is not simple. For example, carbon dioxide and methane (CH4) can be produced by volcanic outgassing.
Therefore, researchers are looking to identify combinations of gases in exoplanet atmospheres that could strengthen the case that life is producing them. “Really, it’s about looking for gases that shouldn’t be there at high abundances,” says Schwieterman.
For example, finding methane and carbon dioxide in a rocky exoplanet atmosphere would be appealing because the former is readily converted to carbon dioxide by reactions with light if not continually replenished — perhaps by life. This pair would be even more appealing if accompanied by an absence of carbon monoxide, which is consumed by organisms for energy. “It’s very hard to get those two species and no carbon monoxide through abiotic sources,” says planetary scientist Maggie Thompson of the Carnegie Institution for Science in Washington, D.C., and ETH Zurich in Switzerland.
A rocky methane-rich planet would also be reminiscent of early Earth, when microbes produced thousands of times more methane than present in our atmosphere today. “It would be interesting if life evolves on other planets in a similar way to how it evolved on Earth,” says Thompson.
Another way to boost the case that a potential biosignature is caused by life is to see if it changes seasonally. On Earth, the abundances of gases like methane and carbon dioxide vary as plants grow and fade with the changing amount of solar radiation. “It wouldn’t be the first thing you would look for because it’s very hard to detect,” says Schwieterman, “but it could be something that tells us for sure that there’s life after you have already identified the planet as an interesting target.”
There is also the possibility that exoplanet atmospheres host different forms of life. On Earth, for instance, elements essential for life are sourced from chemical reactions between predominantly silicate rocks and water. But that could change if the mineralogy is different, says Thompson.
And life could be based on solvents other than water, like methane or ethane. “We definitely need to be able to recognize things that are like Earth,” says Schwieterman, “but life could take on different characteristics.”
Europe’s star power
The European Southern Observatory’s 39-meter Extremely Large Telescope, pictured here under construction in Chile in January 2025, is more than halfway complete. When it begins operations — scheduled for 2030 — it will be the largest optical telescope in the world, with an ability to resolve and directly image exoplanets that surpasses that of the James Webb Space Telescope.
The U.S. is funding two telescopes of comparable size — the Giant Magellan Telescope in Chile and the Thirty Meter Telescope (TMT) atop Maunakea in Hawaii. But amid rising costs and budget uncertainty, in 2024, the board of the National Science Foundation (NSF) recommended cutting its support for one of them.
For now, both scopes remain in limbo. A December 2024 report commissioned by NSF did not recommend either facility for advancement to the final design phase, nor did it recommend which one to cut. The report also acknowledged that the future of TMT hinges on the approval of local and Native Hawaiian communities.
Editor’s note appended Sept. 9, 2025: The Giant Magellan Telescope was advanced to the final design phase in June 2025, after this issue went to press. See “Giant Magellan Telescope gets NSF nod” on p. 12 of our November issue for more.
Future directions
The current cycle of JWST observations includes a program focusing on observing secondary eclipses of rocky exoplanets around M dwarfs. Researchers aim to gather enough data to get a good sense of whether such planets can retain atmospheres at all.
If one or more planets are found to host atmospheres, scientists will likely plan additional observations, combining data and searching for spectral signatures. Up to 200 hours of stacked observations may be necessary to be confident any features are potential signs of life and not due to instrument noise or variations in their stars’ outputs. “As the years go by, we will start pushing the limits of the sensitivity of the telescope and really doing the harder things,” says astrophysicist Macarena Garcia Marin, the deputy project scientist in the Webb Mission Office at the Space Telescope Science Institute.
Then, starting in 2030, the European Southern Observatory’s Extremely Large Telescope (ELT), currently under construction in Chile, will join the hunt. With a 39-meter primary mirror to JWST’s 6.5-meter mirror, the ELT will surpass JWST’s ability to resolve exoplanets directly and conduct high-resolution spectroscopy of their atmospheres. The ELT may even be capable of detecting the most suggestive ensemble of biosignatures — the pairing of oxygen and methane — in the atmospheres of rocky planets around M-dwarfs.
Scientists are also looking forward to NASA’s next flagship observatory in planning — the Habitable Worlds Observatory, expected to launch in the 2040s. The telescope will be powerful enough to directly image habitable exoplanets, analyzing the starlight they reflect. It is being designed to detect oxygen and other potential biosignatures in planetary atmospheres. “I think we’re going to be able to look at systems that are much like our solar system and have true Earth twins,” says Chen.
As these facilities come online, spectral features that don’t correspond to known vibrational frequencies will likely appear. “This is a problem we’re going to start seeing more and more as we gather more data from telescopes,” says computational spectroscopist Juan Camilo Zapata Trujillo. That means molecular spectroscopists will need to conduct time-intensive lab experiments.
One complementary approach that Zapata Trujillo worked on during his Ph.D. at the University of New South Wales in Sydney is using computational quantum chemistry to produce approximate vibrational frequencies for thousands of molecules.
This approach has already borne fruit. For example, his approximations were able to narrow down an unknown 4.5-micron signal in JWST’s spectrum of WASP-39 b to a stretching of carbon-nitrogen bonds in one of three molecules. “It’s easier to run calculations or take measurements for three or 10 molecules instead of 1,000,” says Zapata Trujillo.
The more gases that scientists can identify in an exoplanet’s atmosphere, the better they will understand how that planet formed and evolved, helping to assess the likelihood that any detected spectral features are biosignatures.
Humans have long wondered about the possibility of life outside the solar system. That scientists have the ability to probe that question is in large part to strengthening bonds between observatories and laboratories. And it is a development that amazes even the scientists who are working on it, like Kempton. “It’s an absolute marvel that we are able to disentangle signatures of planetary atmospheres from those burning, bright stars in the sky,” she says.
