Trappist-1 is a red dwarf, escorted by seven planets of the same size as the Earth (between a terrestrial radius of 0.75 and 1.15) that rotate around it in just a few days (from 1.5 to 19).

By observing the edge of the planet with a spectrometer as it passes in front of its star (i.e. the transit), the scientists found that the atmospheres of the planets closest to their star do not contain large volumes of hydrogen. Therefore, these planets are not giants like Uranus or Neptune. This discovery is in agreement with computer simulations of their atmospheric erosion. Moreover, this suggests that these planets may have lost their original hydrogen and produced another atmosphere, more favorable to the presence of liquid water on the surface. These "secondary" atmospheres may be more finely analyzed using the upcoming James Webb Space Telescope, which should provide a more precise indication of their composition in heavier gases such as nitrogen, methane and carbon dioxide.

Although the radius of these planets can be measured precisely during these transits, the same cannot be said for their mass. For this, the researchers must exploit the irregularity of the orbits – and therefore that of the transits – attributable to the mutual gravitational attraction of the planets. In this way, the hourly record of hundreds of transits has made it possible to narrow down the planetary masses with an uncertainty of 5 to 10%. The result: Trappist-1's planets are all less dense than the Earth! In particular, one of them has a water and volatile compound content that reaches 5% of the planet's mass, as compared to 0.1% for the Earth. These values need to be refined using measurements over longer periods, but the trend they show is that these planets can form with very large amounts of water.

These studies show the emergence of new disciplines such as comparative planetology and the study of habitability outside of our solar system. They result from a collaboration between the Laboratoire d'astrophysique of Bordeaux (CNRS/Université de Bordeaux), the Irfu and the Laboratoire de météorologie dynamique (CNRS/École Polytechnique/Sorbonne Université/ENS Paris).