The transit method is a photometric method that aims to indirectly detect the presence of one or more exoplanets in orbit around a star. In 1999, the method was used to confirm the existence of HD209458b, a planet that had been discovered almost at the same time by the radial velocity method. This discovery, published in 2000 in a study led by David Charbonneau, among others, was made using a 10-cm diameter telescope installed in the parking lot of a building in the United States and paved the way for a whole new field of research on exoplanets. The first new detection was OGLE-TR-56b, discovered in 2003.
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 happen when a planet passes in front of its star. On the other hand, when the planet passes behing 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.
This method makes it possible to determine the radius of the planet as well as its period of revolution. Furthermore, if the planet has already been detected using the radial velocity method, then its mass is known and it is possible to obtain a value for the bulk density of the planet.
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 an 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.
One of the difficulties of the transit method is the geometry of the problem. Indeed, for an exoplanet to transit from our point of view on Earth, the plane of the exoplanet’s orbit must be aligned with the Earth. The probability of observing a transit of a planet around a given star is proportionally related to the radius of the star and inversely related to the distance between the star and the planet. For a solar type star, the probability of observing a transit of a planet the size of Earth at the distance from Earth is 0.5%.
What is next?
The transit method as such does not allow a detailed study of the characteristics of the planet. However, the study of the atmospheres of planets discovered by the transit method can be carried out by combining this method with spectroscopy. This technique, called transit spectroscopy, allows the study of the composition and structure of the atmosphere of transiting planets. The James Webb Space Telescope will be used for transit spectroscopy and will provide a better understanding of the chemical composition of the atmospheres of exoplanets.
The end of Kepler’s mission coincided with the beginning of the mission of NASA’s Transiting Exoplanet Survey Satellite (TESS). The mission of the TESS is to discover new exoplanets orbiting the bright stars closest to the Sun using the transit method. After one year of operation, 1100 candidate exoplanets have been identified through TESS and confirmation of the candidates is underway.
The SPIRou instrument that is currently installed at the Canada-France-Hawai’i Telescope and the future NIRPS instrument that will be installed at the 3.6-m telescope in La Silla (Chile) will be able to confirm and study in more detail the candidates identified by TESS in order to better understand the exoplanets neighboring our Sun.
Want to challenge yourself and find planets by the transit method? You can do it here! https://www.zooniverse.org/projects/nora-dot-eisner/planet-hunters-tess (en anglais seulement)