Detecting exoplanets

The bigger an exoplanet and/or the closer it is to its star, the greater the induced tidal effects. “We believe that if an exoplanet bigger than Jupiter were to venture too close to its star, it would disappear into it. This would explain why we detect so few enormous planets gravitating around their star: they would disappear too quickly to be statistically numerous. WASP-18b is the first of this sort to be observed so close to its star, in 15 years of research and among hundreds of detections.” The majority of exoplanets that have been found up until now, are planets on tight orbits with a mass similar to Jupiter, or even bigger; however, they are further from their star.

Several methods are used to uncover exoplanets. The first one to bear fruit was the radial speed method: when a body gravitates around a star, the latter’s movement in the direction of the earth is modulated by an amplitude periodic signal, which increases with the mass of the body. This modulation can be shown by the spectroscopic monitoring of stars.

Initially, in the 1970s, the radial speed method was used to determine the masses of stars’ binary systems. In the 1990s, instrument accuracy was sufficient to begin exoplanetary research. Today, almost 300 of these planets have been detected this way. The first one dates from 1995. With a mass equivalent to half of that of Jupiter, it gravitates very closely around its sun in 4.5 days. These high-mass exoplanets that orbit close to their star are called “Hot Jupiters”.

A second method has also allowed dozens of exoplanets to be discovered. The year 2000 inaugurated the transit method: when a planet passes in front of its star’s disk, it causes a slight drop in the star’s light intensity. Ambitious projects, based on this method, quickly appeared, including Kepler or CoRoT (Read : Voyage to the centre of the stars), which track planetary transits from space. For instance, the CoRoT satellite detected a planet whose mass is equivalent to only five times that of our planet, with a radius twice that of earth. It is the smallest planet ever detected outside our solar system. Up until now, planetary transits have led to the discovery of some sixty exoplanets.