‘Certain planets known today have at the minimum several Earth masses. These are thus ‘Super-Earths.’ They can be solid. For two planets, observed in transit in front of their stars, we have been able to measure their size, mass and deduce their structure. The first was discovered by the CoRoT satellite and is called CoRoT-7b. All the models indicate that it is solid. It has six times the mass of the Earth, and 1.7 times its size in diameter. The second is also six Earth masses but it is closer to a ‘mini-Neptune.’ It has a little envelope of gas and a diameter 2.7 times greater than our planet. It is called Gliese 1214b."

The TRAPPIST project also uses an indirect method, the transit method. It consists of observing and analysing the variation of a star’s light intensity when a planet or another heavenly body passes between the star and the observer and thus eclipses part of the former (read also the article Voyage to the Centre of the Stars). The final outcome will be not only detecting new planets, but to also obtain more information about the stars whose existence was already known. ‘With TRAPPIST we cannot split up the sky into grids blindly in looking for planets in transit. We will instead focus on systems which might potentially contain them on the basis of information received beforehand. Automatic search programmes exist but they cover large fields. We for our part will study and measure with a lot more precision the regions in which we think there is a possibility of transiting planets.’ That is the reason why they will work jointly with other groups, such as the Geneva observatory, with its ESO HARPS spectrometry programme, or the CoRoT satellite. ‘We will offer assistance in confirming the planetary nature of objects detected by CoRoT, for example, and by characterising them.’

There is just a large quantity of information which can be obtained by bringing together the transit and radial velocity techniques. They enable a planet’s size, atmosphere, structure and composition to be demonstrated. The knowledge the astrophysicists have concerning the exoplanets to a large extent thus comes from a few planets only that have been observed in transit (10% of the known exoplanets). Thus, besides the several dozen gas giants, three ice giants similar to Neptune and two ‘Super Earths’ have been observed. And the outcome aimed at by TRAPPIST is inscribed very clearly in its connections to this research. As Michaël Gillon explains: ‘our objective is indeed to discover planets with similarities to our own, in the perspective of one day maybe being able to affirm that we are not alone in the Universe.’