How exoplanets are detected indirectly

Artist’s impression of Kepler 186f — the first rocky planet to be found within the habitable zone. Source: NASA Ames/JPL-Caltech

Over 3,500 exoplanets have been discovered so far, over 2000 of which are found by the Kepler Space Telescope, launched in 2009. Besides exoplanets, we have found rouge planets, sub-brown dwarfs and even exocomets.

More than 97% of the exoplanets are detected by indirect methods. This means we don’t exactly look for the exoplanet, we observe the impact it might have on, say the luminosity of the star, or the ‘wobble’ of a star due to the gravity of the exoplanet. Primarily, there are two indirect detection technique that have proved to be effective:

1. Radial velocity

Also known as Doppler spectroscopy, this method relies on the fact that a star ‘wobbles’ or moves around in a small circle or ellipse, due to the gravity of its companion. It is important to note that planets do not orbit the star, rather they orbit the center of mass of the planetary system. These movements change the normal light spectrum of the star.

If the star shifts towards Earth, its spectrum is slightly blue-shifted, whereas if it shifts away from Earth, the spectrum gets red-shifted. If these shifts are regular and periodic, it can be said that a body is in orbit around the star. The…