​At the beginning of 2016, observations from the European Southern Observatory (ESO) TRAPPIST telescope allowed for the discovery of three Earth-sized planets around a "dwarf" star, named TRAPPIST-1. A planet is detected as it "transits" periodically in front of the star, whose small size (only 12 Earth radii) and low luminosity (0.05% that of the Sun) are extremely favorable to this type of observation. The physicists deduce the radius of the planet in transit from the periodic variations in the star's luminosity, its distance to the star and, therefore, its insolation.

Since then, this system has been systematically monitored, with success. TRAPPIST-1 has at least seven planets, all of which are very similar in size to the Earth (within a range of 15%). Their average temperature varies between that of Mercury and that of Mars, and they are likely to always present the same face to their host star.

Numerical simulations of the planets' climates indicate that three of them have an insolation that is compatible with the presence of liquid water on their surface, while their respective atmospheres have a wide range of possible states. Among other planets, the presence of liquid water is more problematic. The three planets with the highest insolation, although too hot, could host liquid water on their night face. And although too cold, the seventh planet could have liquid water under a layer of ice, like Jupiter's moon Europa. Yet do they even have an atmosphere? NASA and ESA's future space observatory James Webb is expected to answer this question.

This international collaboration involves, in particular, researchers from CNRS, UPMC, the Laboratory of Astrophysics of Bordeaux (CNRS/Bordeaux University) and the Dynamic Meteorology Laboratory (CNRS/UPMC/École Polytechnique/École Normale Supérieure in Paris).