The transit method consists of regularly measuring the luminosity of a star in order to detect the periodic decrease in luminosity associated with the transit of an exoplanet. The transit happens when a planet passes in front of its star. On the other hand, when the planet passes being the star, it is called an eclipse. The effect measured during a transit is quite small. For a star the size of the Sun, the transit of a Jupiter-size planet will cause a decrease in apparent luminosity of about 1%, while this decrease will be of about 0.001% for a planet the size of the Earth.
In other words, it is a photometric method that aims to indirectly detect the presence of one or more exoplanets in orbit around a star through covering a small portion of it.
This principle can be applied to more than just exoplanets detection and has even been used in our own Solar System.
The transit occurs when the planet passes in front of its star, whereas the eclipse occurs when the planet passes behind its star.
Most of the planets discovered by the transit method have been revealed through large field surveys. The aim is to study a large number of stars, without pre-selecting them, since there is no indication a priori which stars will have planets that are favourably aligned with respect to the Earth in order to be able to detect them by the transit method.
Launched in 2009 and having completed its mission in 2018, the Kepler Space Telescope has played a crucial role in the search for exoplanets using the transit method. It alone has observed 530,000 stars in the Cygnus constellation and it has confirmed the existence of more than 2,600 exoplanets, revolutionizing our vision of exoplanets. This abundance of planets has shown that there is a much greater diversity of planets compared to the planets in our solar system.