Exoplanets or the quest for other worlds elswhere in the Universe



fter centuries of speculation about the existence of other worlds in the Universe, the first extrasolar planet (exoplanet) orbiting a solar-type star was detected in 1995. This gas-giant planet, 51 Peg b, was detected indirectly by measuring the variations of the radial velocity of the host star as it orbited the centre-of-mass of the star-planet system. 51 Peg b lies at a distance of only 0.05 astronomical unit of its star. At such a close distance, the planet is strongly heated by the stellar radiation, making it the prototype of what is now called a hot Jupiter.


Since 1995, more than 500 exoplanets have been detected with the radial velocity method. Thanks to the constant improvement of their sensitivity, radial velocity surveys are now able to detect planets of a few Earth masses, the so-called super-Earths (loosely defined as planets between 2 and 10 Earth masses). Unfortunately, the radial velocity method alone leads to a rather limited knowledge of a detected planet, including a minimal value for its mass (if the stellar mass is known), while nothing is learned about its size, structure or atmospheric properties. Still, the large harvest of planets brought by this method has revealed the diversity of the planetary systems in our Galaxy and the importance of the migration through the protoplanetary disks during their formation.


In 1999 was discovered the first exoplanet transiting (i.e. eclipsing) its host star. This detection granted access for the first time to the physical properties of an exoplanet. Indeed, much can be learned from a planet that transits its parent star. First, one can measure accurately the orbital inclination and the planet-to-star radius ratio. Assuming that the planet is also detected by radial velocity and that an estimate of the stellar mass is available, one gets then directly access to the absolute mass and radius of the planet. The deduced average density combined with planetary models allows one to infer constraints on the internal composition of the planet. Furthermore, the special geometrical configuration of the orbit gives the opportunity to study directly the planetary atmosphere without the challenging need to spatially resolve its light from that of the host star. Indeed, comparing photometric and spectroscopic measurements obtained when and before (and/or after) the planet is hidden by its star allows one to map the thermal emission of the planet and to assess its atmospheric composition and physics, while doing the same when the planet passes in front of the star makes possible the detection of atomic and molecular features in the planetary atmosphere. Transiting planets have opened a new field of research: exoplanetology.


Our research group uses the TRAPPIST telescope for detecting and characterizing extrasolar planets by using high-precision differential photometry. More specifically, the main exoplanet science drivers of TRAPPIST are the following.

  • The search for the transits of planets detected by radial velocities, mostly by the HARPS and CORALIE Doppler surveys taking also place at La Silla Observatory. For the red dwarfs monitored by HARPS, TRAPPIST is able to detect the transits of terrestrial planets with a size close to that of our Earth.
  • The photometric follow-up of planet candidates found by the transit surveysSuperWASP and TESS
  • the goal being to confirm the transit signal and to discriminate bona fine planets from eclipsing binaries.
  • The characterization of confirmed transiting planets by high-precision eclipse photometry. Transit light curves obtained by TRAPPIST are used to determine precisely the size of the planet. For hot Jupiters that are extremely irradiated by their host star, the planetary emission that is blocked out during the occultation can be detected in the near-infrared by TRAPPIST. Such measurements can constrain the temperature, heat distribution efficiency, composition and albedo of the planetary atmosphere.
  • In combination with HARPS, detecting low-mass planets around red dwarfs.
  • The search for other planets in transiting systems.
Michaël GILLON

Principal Investigator (exoplanets programs)

Group Exotic


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