Exoplanets

EXOTRAPPIST: the TRAPPIST exoplanet program

 

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fter centuries of speculation about their existence, the first exoplanets were detected at the end of the last century.  Since then, thousands of these exoplanets have been discovered by ever more ambitious research projects supported by ever improving technology. Most of these exoplanets have been discovered by the so-called transit method. This is an indirect method, in which the planet is not visible on an image, but its existence is deduced by the effect it has on its star. In this case, this method is based on the measurement of the apparent fall in brightness of a star when one of its planets transits, i.e. passes in front of it.

When an exoplanet transits, we can accurately measure the ratio between its size and that of its star. As stellar sizes are now easy to estimate, transits give thus access to the measurement of a planet’s size. In addition, its mass can sometimes be measured by a complementary method. The combination of the measurements of the mass and the radius leads directly to an estimate of its average density, which allows to constrain its internal composition. Moreover, the particular geometrical configuration of the orbit gives the possibility to study directly the planetary atmosphere without having to spatially resolve the planet and its host star. Indeed, photometric and spectroscopic measurements obtained when the planet is hidden by its star allow to characterize its emission and to study its composition and atmospheric structure. Similar measurements, performed when the planet transits in front of its star, allow to detect some atoms and molecules in its atmospheric limb.

EXOTRAPPIST, the exoplanet program of TRAPPIST, is focused on the detection and study of such exoplanets in transit. Its different components are:

• The search for possible transits of exoplanets detected in radial velocities. This sub-program was the objective of the initial concept of TRAPPIST, inspired by the detection of transits of the first known transiting exo-Neptune, GJ436b, by a small amateur telescope of 60cm (Gillon et al. 2007) and by the subsequent observation of transits of this planet with a robotic telescope in Israel. This sub-program has notably led to the detection of transits of another "exo-Neptune" named GJ3470b (Bonfils, Gillon et al. 2010).
• The follow-up and confirmation of transiting exoplanet candidates identified by various transit search projects operating from the ground (WASP especially) and space (CoRoT, Kepler, and now TESS).  This sub-program has been and still is the main component of ExoTRAPPIST in terms of observation time and number of publications. It has thus contributed to the discovery of more than 200 transiting exoplanets.
• Between 2011 and 2018, ExoTRAPPIST included a prototype subprogram of the SPECULOOS project, a search for potentially habitable rocky planets around the smallest and coolest stars in the solar neighborhood. The goal of this prototype was to demonstrate the feasibility of SPECULOOS (Gillon et al. 2013), but it does much better than that by discovering TRAPPIST-1, an incredible system of seven temperate, Earth-sized rocky planets orbiting a star ten times smaller than the Sun at 40 light-years (Gillon et al. 2016, 2017).
• An in-depth study of selected transiting planets of high scientific interest. The data obtained by TRAPPIST have thus been used to determine precisely the size of some transiting planets or to measure their thermal emission. (for ex. Delrez et al. 2017)