How can we detect life on distant planets?

Could look­ing for the absence rather than the pres­ence of cer­tain chem­ic­al com­pounds in the atmo­sphere of exo­plan­ets (plan­ets orbit­ing stars oth­er than the Sun) be our best chance of find­ing liquid water – and there­fore per­haps even life – on these far-off worlds?

This is what research­ers from the MIT and the Uni­ver­sity of Birm­ing­ham are pro­pos­ing, hav­ing shown that if an extra­sol­ar ter­restri­al (rocky) plan­et has much less car­bon diox­ide (CO₂) in its atmo­sphere com­pared to that of oth­er plan­ets in the same sol­ar sys­tem, this could indic­ate the pres­ence of liquid water on the sur­face of that plan­et. Such a chem­ic­al “sig­na­ture” should be read­ily detect­able with NAS­A’s James Webb Space Tele­scope (JWST), some­thing that has not been the case with oth­er obser­vat­or­ies and tele­scopes until now.

To date, astro­nomers have dis­covered more than 5,000 plan­ets out­side our sol­ar sys­tem. Anoth­er feat: for more than twenty years, they have been able to assess wheth­er a plan­et is in a “hab­it­able” zone. But they are still unable to determ­ine if it is actu­ally cap­able of host­ing life.

In our sol­ar sys­tem, research­ers detect the pres­ence of liquid oceans, for example, by look­ing for “glints” of sun­light reflec­ted off liquid sur­faces. This is the way they have suc­ceeded in observing large lakes on Titan, Sat­urn’s largest moon, for instance. Doing the same for exo­plan­ets will be dif­fi­cult, how­ever, even with advanced tele­scopes like the JWST.

A team led by Juli­en de Wit from MIT (USA) and Amaury Tri­aud from the Uni­ver­sity of Birm­ing­ham (UK) has now applied what we know about the CO₂ levels in the atmo­spheres of the ter­restri­al plan­ets in our own sol­ar sys­tem to exo­plan­ets. On Earth, most of the CO₂ in our atmo­sphere has been dis­solved into the oceans and has gradu­ally been bur­ied in the Earth’s crust (over very long geo­lo­gic­al times­cales). Our plan­et is there­fore very dif­fer­ent from Venus, whose atmo­sphere con­tains over 95% CO₂. Earth con­tains as much CO₂ as Venus, but this CO₂ can­not be “seen”, which shows just how effi­cient the pro­cess of CO₂ stor­age in the Earth’s crust has been.

“We pro­pose that a sim­il­ar pro­cess on exo­plan­ets would enable astro­nomers to deduce that there is liquid water on them,” explains Amaury Tri­aud. “Such plan­ets would appear poorer in atmo­spher­ic CO₂ com­pared to their non-hab­it­able neighbors.”

The strategy the research­ers pro­pose would work best for sol­ar sys­tems like our own, that is, those in which sev­er­al ter­restri­al plan­ets, all roughly the same size, orbit rel­at­ively close to each oth­er around their host star. This is the case for TRAPPIST‑1, a sev­en-plan­et sys­tem loc­ated 40 light years from Earth – which is rel­at­ively close in astro­nom­ic­al terms.

First, they would con­firm that the plan­ets indeed pos­sess an atmo­sphere. To do this, the experts would look for the pres­ence of CO₂ using the JWST, the only tele­scope cur­rently cap­able of meas­ur­ing the chem­ic­al con­tent of the atmo­sphere of rocky exo­plan­ets. CO₂ strongly absorbs light in the infrared part of the elec­tro­mag­net­ic spec­trum and could there­fore be eas­ily detec­ted by the tele­scope. Next, they would com­pare the CO₂ con­tent of the dif­fer­ent plan­ets in the sys­tem to determ­ine wheth­er any of them has sig­ni­fic­antly less CO₂ than the oth­ers. Sub­sequent obser­va­tions would con­firm how import­ant this defi­cit is, but also wheth­er it is due to bio­logy (bio­mass bury­ing the car­bon) or by geo­logy (CO₂ dis­solv­ing into water).

As we dis­cov­er more and more exo­plan­ets, it’s not just import­ant to find out wheth­er their size and mass are sim­il­ar to those of Earth. We would also like to know wheth­er the con­di­tions on their sur­faces are also sim­il­ar. “By meas­ur­ing reduced amounts of CO₂ on neigh­bor­ing plan­ets in an extra­sol­ar sys­tem, we would be more cer­tain that those plan­ets have sur­face con­di­tions sim­il­ar to Earth’s, before ini­ti­at­ing the search for evid­ence of bio­lo­gic­al activ­ity itself,” explains Amaury Triaud.

“The Holy Grail in exo­plan­et sci­ence is to find hab­it­able worlds oth­er than Earth and signs of life,” adds Juli­en de Wit. “To this end, the field has tra­di­tion­ally focused on look­ing for an addi­tion­al sig­nal com­ing from a giv­en plan­et: a glint com­ing from oceans or the absorp­tion sig­na­ture of a spe­cif­ic gas (oxy­gen, for example). But all such fea­tures have so far been bey­ond the reach of even the new­est observatories.”

“Here, we make the very import­ant point that it is not just about look­ing for what has been added by life or the pres­ence of water, but also (and per­haps more import­antly) about what has been removed by life or the pres­ence of water,” he adds. “It is only by examin­ing the ‘neg­at­ive space’ (the deple­tion) cre­ated by one or the oth­er that we can enable the search for hab­it­ats and life with cur­rent tech­no­logy (that is, with­in the next few years for a sys­tem like TRAPPIST‑1). That’s how power­ful that simple change in definition/perspective is.”

The research­ers say they will also con­tin­ue to search for rocky tem­per­ate worlds oth­er than those in TRAPPIST‑1 that they could point the JWST towards. “For that, our tele­scopes in Chile, the Canary Islands and Mex­ico, a net­work called SPECULOOS, are essen­tial. Indeed, these tele­scopes are already start­ing to find new rocky exo­plan­ets,” they reveal.

Interview by Isabelle Dumé

Ref­er­ences:

Atmo­spher­ic car­bon deple­tion as a tracer of water oceans and bio­mass on tem­per­ate ter­restri­al exo­plan­ets, Nature Astonomy 8 17–29 (2024)