William BalmerOct 11, 2023

How astronomers search for life on exoplanets

One of humanity’s biggest questions remains: “Are we alone?” 

For nearly half a century, astronomers have looked for messages from extraterrestrial intelligences that might reach Earth. Many other astronomers hope to answer this question by taking detailed observations of exoplanets: planets orbiting distant stars. But how can astronomers hope to answer this kind of question by observing planets they'll never get to visit? And how (or, when) will we know if a claim of a sign of life is believable?

Looking for signs of life in light

For the past century, astronomers have used spectroscopy to learn more about stars, galaxies, super massive black holes, and the planets in our own Solar System. Only in the last two decades, with powerful new telescopes, cameras, and computers, have we finally achieved the precision necessary to measure the spectra of exoplanets. The first spectrum of an exoplanet, published in 2002, was taken using the Hubble Space Telescope and showed evidence of vaporized sodium in the atmosphere of the exoplanet HD 209458 b.

There are major prospects in the next few decades to observe the spectra of planets and hunt for potential “biosignatures.” Biosignatures are the spectral markers of molecules that might be necessary for, or produced by, life as we know it. 

There are three ways to measure a planet’s spectrum: 1) look for light bouncing off the planet’s surface or atmosphere (reflection spectroscopy), 2) observe the light produced by the heat of the planet itself (thermal emission spectroscopy), or 3) watch light pass through the planet’s atmosphere (transmission spectroscopy).

Spectra from different Earth-like planets
Spectra from different Earth-like planets Of the four terrestrial planets in our Solar System (Mercury, Venus, Earth and Mars) the last three possess atmospheres. These are the kinds of spectra we would expect when searching for Earth-like planets in other solar systems.Image: ESA

New technologies for seeing exoplanets 

Direct exoplanet imaging, which involves blocking the light from a bright star and resolving the light from a nearby, fainter planet, is an excellent avenue for both reflection spectroscopy and thermal emission spectroscopy. With the star out of the picture (no pun intended), astronomers can measure how hot and dense a planet’s atmosphere is, and can tell which molecules are present.

Recently, the James Webb Space Telescope (JWST) demonstrated its exquisite precision with this technique, observing sandy clouds in the atmosphere of a strange gas giant planet, and detecting hot carbon-monoxide and water molecules in multi-color images of another gas giant planet. Still, JWST is only able to image gas giant planets.

NASA’s next flagship space telescope, the Habitable Worlds Observatory (HWO) will seek to directly image Earth-like planets in the 2050s. The impressive mission concept hopes to observe signatures of oxygen, water vapor, carbon dioxide, and ozone. Oxygen and ozone are produced by and sustain life on the Earth, so they are natural biosignature candidates, but some astronomers are worried that oxygen on other planets could be produced by complex, but ‘abiotic’ chemical interactions too. Even if oxygen is detected in the spectrum of a directly imaged planet, it will likely take multiple lines of evidence (detecting multiple molecules and modeling where they come from), to rule out any false positives.

Atmospheres of TRAPPIST-1 planets
Atmospheres of TRAPPIST-1 planets The Hubble observations revealed that the planets do not have hydrogen-dominated atmospheres. The flatter spectrum shown in the lower illustration indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmosphere.Image: NASA / ESA / Z. Levy (STSci)

The prospects for observing biosignatures in the short term may improve using transmission spectroscopy. This technique has been leading exoplanetary atmosphere science for the past two decades, because direct imaging of exoplanets is so difficult.

Unfortunately, we don’t know of any transiting Earth-twins yet, even after missions like Kepler and TESS, we haven’t been able to stare at enough stars for long enough with enough sensitivity to pick out these tiny signals. The transiting planets that do orbit in their star’s habitable zone (where surface liquid water could be sustained) are all tidally locked in close orbits around dim, red dwarf stars. Depending on who you ask, that might be good or (very) bad news for the hunt for signs of life.

Astronomers have used JWST to place strong upper limits on the size and composition of the atmospheres of rocky planets transiting red dwarf stars, like LHS 475b, GJ 486b, and recently TRAPPIST 1b. But JWST hasn’t convincingly detected atmospheres on these rocky planets, because these red dwarf stars can also have vaporized water in their own atmospheres that can mimic a planetary spectral signal. In order to combat this, astronomers will have to learn a lot more about these strange stars before they can tackle the spectra of their planets.

Even then, in order to convincingly detect many biosignatures with the transit technique, astronomers would need to use up an outlandish fraction of the observing time on JWST, without a guarantee of success. Optimistic estimates indicate ten or more transits of a single planet should suffice (a week or more of observations, spread over years), but realistic or pessimistic estimates can range from twenty to hundreds of transits (each of which are many hours long). With so much feasible, groundbreaking astronomy to be done with JWST, the likelihood of the telescope being able to dedicate enough time to find convincing signs of life appears very slim.

A chance of life on gaseous worlds

Signs of life might not be relegated to Earth-sized planets, however. For a few years some astronomers have suggested that so called “mini-Neptunes,” with masses many times the Earth’s, could have large oceans hidden under puffy atmospheres of hydrogen and helium. The big problem is that astronomers don’t yet have precise enough measurements, or rigorous enough models, to prove definitively what a mini-Neptune’s interior is made of. And with all the uncertainty surrounding the birth and evolution of life on rocky planets, there is even more we don’t know about how life could come to be within a massive, high pressure ocean.

While very uncertain, this provocation is enticing, because the puffy, hydrogen and helium dominated atmospheres of mini-Neptunes are much easier to observe with transmission spectroscopy than the thin, oxygen and carbon dense atmospheres of terrestrial planets. Within this soup of hydrogen and helium, it’s possible that water vapor, methane, carbon dioxide, and even biosignature gasses could float up high enough to be detected with more reasonable observing programs.

Exoplanet K2-18 b
Exoplanet K2-18 b This artist’s concept shows what exoplanet K2-18 b could look like based on science data.Image: NASA / CSA / ESA / J. Olmsted (STScI) / Science: N. Madhusudhan (Cambridge University)

A group of astronomers recently used JWST to observe the transmission spectrum of the mini-Neptune K2-18b. Their spectrum, while fairly noisy, showed signatures of methane and carbon-dioxide, an impressive first for studies of mini-Neptune atmospheres. The observation of these molecules, given careful consideration and additional, rigorous modeling, could begin to build support for the mini-Neptune surface ocean hypothesis.

Unfortunately, the paper appeared, to many astronomers, to jump the gun. It controversially claimed a “detection” of dimethyl sulfide (DMS), a molecule that, on Earth, is produced by life. DMS doesn’t produce a distinct spectral feature in their data, like carbon dioxide or methane do, but their models of the planet’s atmosphere allow for DMS, and sometimes their models with more DMS appear to match the data well. In reality, their noisy data is likely overfit by DMS, since the spectrum can be explained just as easily without DMS as with it.

This claim propelled the result to the headlines, the most provocative of which read “NASA says planet 8.6 times bigger than Earth could support life” and “Possible hints of life found on distant planet”. As associate professor at the Japan Aerospace Exploration Agency (JAXA), Elizabeth Tasker put it, “We have not discovered life on K2-18b… All we can say is that the planet does not… not… have an ocean.”

In the near future, JWST observations of the transmission spectra of mini-Neptunes with surface oceans could be fascinating testbeds for hypotheses about life in the Universe… or they could become another case of astronomical clickbait. While JWST continues to make strides towards more precise observations of rocky planets around red dwarfs, astronomers will need to contend with contaminating signals from these stars before claims about water, or life, are to be believed. In the long run, however, all these transmission spectra might become eclipsed (pun intended) by direct imaging with HWO, and its measurements of oxygen on rocky planets orbiting Sun-like stars.

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