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Satellites, black holes, exoplanets: when science extends beyond our planet

How can we detect life on distant planets?

Julien de Wit, Associate Professor of Planetary Sciences at MIT and Amaury Triaud , Professor of Exoplanetology at University of Birmingham
On May 21st, 2024 |
3 min reading time
de Julien de Wit
Julien de Wit
Associate Professor of Planetary Sciences at MIT
Amaury Triaud
Amaury Triaud
Professor of Exoplanetology at University of Birmingham
Key takeaways
  • Usually, it is by detecting certain chemical compounds in the atmosphere that allows exoplanets to be identified.
  • A new approach is being considered: looking for a low concentration of CO2 in the atmosphere of exoplanets.
  • On Earth, most of the CO2 has been dissolved in the oceans and then buried in the Earth's crust. A small proportion of atmospheric CO2 would therefore be a chemical “signature” of the presence of water.
  • This method could be facilitated by NASA's James Webb Space Telescope.
  • The ultimate goal: to determine whether the surface conditions on exoplanets are similar to those on Earth, so that we can look for signs of life.

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.

Solar systems like ours?

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).

Earth-like conditions on distant worlds?

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


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

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