Transiting exoplanets
An exoplanet is a planet that orbits another star than the Sun. The first ones were detected in 1992. Since then, thousands have been discovered.
Most of them have been detected by the transit method. This method is based on a very simple principle: when an exoplanet passes in front of its star, this latter will appear a little dimmer because a part of its disk is occulted by the planet. For this purely geometric phenomenon to occur, the exoplanet's orbit must be almost parallel to the line connecting the star to the observer. The probability of such a configuration is always low, and even lower the further the planet orbits from its star. Thus, even if we now know that the majority of stars harbor a planetary system, only a small fraction have one or more transiting planets seen from Earth. It is therefore necessary to observe a large number of stars to hope to detect exoplanets by this method. Moreover, it is necessary to observe at the right time, the passage of the planet in front of the star lasting only a few hours at most and occurring only once per orbit. And it is necessary to be able to measure the brightness of the stars with a VERY high precision, because a planet is very small compared to its star and thus hides only a tiny part of it during the transit. Thus, a hypothetical extraterrestrial astronomer around a randomly chosen nearby star would only have a one in 200 chance that the Earth is transiting the Sun as seen from its planet, and even if this were the case, he or she would have to be able to detect a drop of brightness of one hundredth of a percent, occurring only once a year and lasting only a few hours.
The principle of detecting exoplanets by observing their transits is therefore simple, but putting it into practice poses many challenges. This is why the first transiting exoplanet was only detected in 2000. Since then, more and more ambitious projects have been launched to track down transiting exoplanets, either from Earth or from space, and they have discovered thousands of exoplanets whose study has revealed an extreme diversity of planetary systems on the scale of the Galaxy.
When an exoplanet transits, we can measure its size, its mass, its orbital parameters, but also study its atmosphere. Indeed, during the transit, a tiny fraction of the light emitted by the star in our direction passes into the atmosphere of the planet and is filtered. With a sufficiently powerful telescope, we can measure the impact of this filtration on the star's light, and thus study the planet's atmosphere (composition, structure, extent, etc.). Other methods even allow to refine this atmospheric study. So far, it is mainly giant planets with short orbital periods that have had their atmospheres studied with this method, but in the long run, it should be possible to extend such studies to rocky and even potentially habitable exoplanets, which could lead to the detection of chemical traces of life beyond our solar system.
The EXOTIC team focuses its work on the detection and study of these transiting exoplanets, with a particular attention on potentially habitable exoplanets. Its researchers have led or contributed to the discovery of several hundred transiting exoplanets, including the first potentially habitable exoplanets suitable for the search for life.
