Exoplanet detection methods

The first idea that comes to mind for detecting an exoplanet is to produce an image of the star’s environment and look for a faint spot next to the star. Unfortunately, the planet is so dim by comparison with its star and so close to it that the observer is “dazzled” by the star (there is approximately 1 photon ^? from the planet for ten billion photons ^?  from the star). Imagine looking for a glow-worm next to a lighthouse 20 km away... The first attempts at direct detection therefore all failed.

• The first exoplanet discoveries were obtained using indirect methods : we look for oscillations in the star’s movements due to the presence of a planet. These methods measure the radial velocity of the star (movement which causes the star to move towards/away from the observer) or changes in its position in the plane of the sky, using astrometry, timing, or Transit Time Variations (TTV).

Two indirect detection methods

• Several years were needed before direct detection of planets by imaging became possible, with technical developments such as interferometry, which are capable of “extinguishing” the light from the star. Imaging with the Extremely Large Telescope (ELT) will soon be able to detect planets “close” to their star, like Jupiter. For its part, the JWST will be able to detect even smaller planets.

• One extremely prolific method is to detect the planet’s transits in front of the star, leading to a slight dimming of the light from the star. Transit of the planets behind the star (secondary transits) is also a rich source of information.

• Researchers have imagined a whole range of methods to explore all the physical phenomena and all spectral domains, such as detection of transit time variations (TTV), the search for radio emissions from exoplanets, deviation owing to the gravitational lensing of the light from remote systems by a very massive object in the line of sight.