ExTrA’s prime objective is to detect planets transiting bright nearby M dwarfs, including habitable-zone Earth-size planets, and to determine their structure and composition.

Discovering habitable Earth-like planets, and searching for biomarkers in their atmosphere, are amongst the main objectives of XXIst century’s Astronomy. That will place our Earth into context, as just the closest member of the habitable telluric class, and will establish whether life exists elsewhere in the Universe.

In that (uncertain) quest for inhabited planets, ExTrA shall detect many other planets and thus possibly address other fundamental questions about their physical properties and their formation history. What is the stellar mass dependance of planet formation ? What is the occurrence of planets, as a function of their mass and orbital period, down to Earth-mass planets ? What tells the eccentricity of short-period planets (which experience tidal circularization) about their structure ? What are the architecture and dynamics of multi-planet systems ? Does stellar composition or magnetic activity correlates with the occurrence of planets ? What is the original environment that lead to the formation of planets ? What are the bulk structure and composition of telluric exoplanets ? What are the molecules and elements in their atmosphere ? What is their temperature, wind or meteorological conditions ? etc...