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La Lettre
© Science Photo Library - Alamy
The Primitive Earth (artist view)
Given that close – not mentioning
in situ
– explorations are impossible, it is the detailed study of the
atmospheres that will probably allow for the first signs of life to be detected on exoplanets. The first step
is to identify which exoplanets are most fit for the development of life. Drawing on an analogy with Earth,
a preference is given to telluric planets located at such distance to their stars as to allow liquid water
to be on the surface. The presence of liquid water is assumed to be a necessary condition for life to
appear on our planet. Yet it is probably not sufficient. Other factors indeed may have played an important,
or even necessary, role for life to develop, including: the proprieties of the sun itself, moon, comets,
asteroids, magnetic field of Earth, intern activity of our planet (volcanism, outgassing), composition of the
atmosphere (greenhouse effect).
What signatures of life are to be looked for on the exoplanets? This question is highly complex. Drawing
on an analogy with our planet, one preferred criterion is the presence of dioxygen (and ozone, which
derives from it) in an atmosphere that would moreover have proprieties quite similar to those of Earth. The
presence of dioxygen in our atmosphere results indeed from photosynthesis and it would not be possible
to explain it by simple physical ou chemical abiotic processes. However, is this criterion still valid in the
case of the atmosphere of an exoEarth bearing a different physico-chemical history, and orbiting a star
that is different from the sun?
Whether Earth should play a potential role as a reference model is indeed a very central question. So