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.