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© From B.Eymann - Académie des sciences
350
years
of
science
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The exoplanet
β
Pictoris b is a giant gas planet orbiting at about 9 au
(1 au = the distance between Earth and the sun, which is 150 million
kilometres) from its star (here represented as a star at the centre of the
image). On the left, the image of the discovery of exoplanet
β
Pictoris b
in November 2003 (light dot on the upper-left side of the image) ; on the
right, 6 years later (in the bottom-right of the image).
© ESO- A.-M. Lagrange
await confirmation. Their characterization already reveals a vast range of masses (ranging from a few Earth
masses to several Jupiter masses), radii and orbital properties (planets with inclined or retrograd orbits, or
very remote ones), some of which
have no equivalent in the solar
system. Such a diversity may not
be explained without considering
the existence of several planet
formation scenarios. Moreover,
it appears that the individual
and dynamic histories of young
extrasolar planet systems may
be very complex, even more than
the history of the solar system:
indeed, the orbit of an exoplanet,
once the planet is formed, may
be considerably altered due
to interactions with the protoplanetary disk (which may account for the existence of “Hot Jupiters”) or
with other objects present in the system (which may explain why there are exoplanets with inclined or
retrograde orbits).
The variety of architectures of the extrasolar planet systems is probably the most surprising result obtained
in exoplanetology so far. It is probable that the interiors and atmospheres of the exoplanets, which we have
only started to probe, will also prove very diverse. The proprieties of the forming atmospheres result from
the capture of the gas present in the Primitive Nebula but also from outgassing that occurs on the forming
planets, and from potential external inputs (asteroids, comets) and
complex and multiple physico-chemical processes. Their evolution
depends on many factors (volcanism, light gases escape, stellar
radiation, etc.) – as many circumstances that are difficult to foretell
and variable parameters that make it very tricky to formulate any
prediction. Since the atmospheres play fundamental roles on the
conditions in which life appears and develops, considerable efforts,
in particular with the James Webb Space Telescope, the successor
to the Hubble Space Telescope, and with the future Extremely Large
Telescopes, will be devoted in the years and decades to come to
probing the atmospheres of giant, and then telluric, exoplanets.