Finding water on exoplanets… a dream still unattainable

We will have to wait a long time before knowing whether exoplanets, or extrasolar planets, have oceans. Because, on this point, things are clear, “no instrument is able to detect the presence of liquid water on such distant worlds”, explains Franck Selsis, CNRS research director at the Bordeaux Astrophysics Laboratory. At most, we will perhaps one day succeed in developing telescopes powerful enough to capture starlight reflected by seas… However, no instrument of this type is on the program.

Until then, we will have to make do with clues. And there, things get complicated. Because to hope to deduce that a planet contains appreciable quantities of liquid water, we must be able to gather as much information as possible (mass, size, orbit, temperature, sunlight, characteristics and composition of the atmosphere) to test the models… However , each piece of information collected comes at a high price, recalls Alain Lecavelier des Etangs, CNRS research director at the Paris Institute of Astrophysics. “Everyone would like to know if exoterres are likely to harbor oceans or a form of life. Unfortunately, he explains, these planets of a size and mass close to that of Earth have so far only been detected around small dwarf stars which have proven to be very active. This poses problems for teams observing them, using the James Webb space telescope, particularly in the “Trappist 1” system, famous for having seven of them. »

Theories predict the existence of “ocean planets” with a mass five to ten times greater than ours, able, under certain conditions, to conserve 50% water. But these liquefied versions of “Super Earth” or “Mini-Neptune” only exist on paper. The latest announcement of this kind, relating to a star called K2-18b, is causing debate, between triumphant announcements on the signature of carbon molecules identified in its atmosphere thanks to the James Webb spectrometer and the divergent interpretations of these measurements.