Exoplanet
An exoplanet—or extrasolar planet—is a planet that orbits a star other than the Sun. For a long time, their existence was merely hypothesized. Since the first confirmation in 1992, discoveries have been piling up: today there are more than 5,000 confirmed ones, and several billion are estimated to exist in the Milky Way alone.
How are they detected?
Exoplanets are too small and too distant to be photographed directly in most cases. Astronomers use indirect methods. The most common is the transit method: when a planet passes in front of its star, it blocks a tiny portion of the star’s light, causing a slight dip in brightness. By measuring this variation precisely, we can deduce the planet’s size, orbital period, and sometimes its atmospheric composition.
Another method, radial velocity, detects the micro-movements of the &star caused by the gravity of its planet. Other techniques, such as direct imaging or gravitational microlensing, round out the arsenal of exoplanet hunters.
A wide variety of worlds
The exoplanets discovered are surprisingly diverse. There are hot Jupiters, gas giants orbiting very close to their star in just a few days. Super-Earths, rocky planets larger than our own. Mini-Neptunes, with no equivalent in our solar system. And perhaps Earth-like planets in habitable zones—the region around a star where liquid water could exist on the surface.
The Question of Life
The search for exoplanets is closely linked to the question of extraterrestrial life. Several systems are being studied in particular: TRAPPIST-1, 40 light-years away, has seven rocky planets, three of which are in the habitable zone. The James Webb Space Telescope is now analyzing their atmospheres in search of biosignatures—chemical clues that may indicate the presence of living organisms.
Did you know?
The first confirmed exoplanet, 51 Pegasi b, was discovered in 1995 by Michel Mayor and Didier Queloz, who were awarded the Nobel Prize in Physics in 2019. This planet is a hot Jupiter that completes its orbit in just 4 days—a configuration that no one had anticipated.
Frequently Asked Questions
How many exoplanets have been discovered?
More than 5,500 exoplanets have been confirmed to date, with thousands more candidates awaiting confirmation. Estimates suggest there could be several hundred billion in our galaxy alone.
Can we see an exoplanet with the naked eye?
No. Exoplanets are far too small and too distant to be visible without instruments. Only the most powerful telescopes, such as the James Webb or the VLT, can sometimes image them directly under very specific conditions.
Are there any habitable exoplanets?
Some are in the habitable zone of their star, which means liquid water could exist there. But being in the habitable zone does not guarantee life—it also requires a suitable atmosphere, a favorable geological composition, and the absence of excessively intense radiation.