Exoplanets

What is an exoplanet?

An exoplanet, or extrasolar planet, is a planet that orbits a star other than the Sun. For centuries people guessed they existed, but they are so faint and so close to their bright stars that finding one seemed nearly impossible.

Today we know planets are common. Studies suggest most stars in the Milky Way have at least one, which means our galaxy likely holds hundreds of billions of planets.

The first discoveries

In 1992, radio astronomers Aleksander Wolszczan and Dale Frail found planets around a pulsar, the spinning core of an exploded star. Tiny changes in the pulsar's very regular radio pulses gave them away.

In 1995, Michel Mayor and Didier Queloz announced 51 Pegasi b, the first planet found around a Sun-like star. It was a shock: a gas giant about half Jupiter's mass circling its star in just over four days.

Planets like this became known as hot Jupiters, and they showed that other planetary systems can look very different from ours. Mayor and Queloz shared the 2019 Nobel Prize in Physics for the discovery.

How we find them

Transit method: when a planet passes in front of its star from our point of view, the star dims very slightly. Measuring how much it dims tells us the planet's size, and repeating dips give the length of its year. This method has found the majority of known exoplanets.

Radial velocity method: a planet's gravity makes its star wobble a little. That wobble shifts the star's light toward blue and then red as it moves toward and away from us. The size of the wobble reveals the planet's minimum mass.

Direct imaging: blocking the star's glare with special instruments lets telescopes photograph some planets directly. It works best for large, young, hot planets far from their stars.

Other techniques include gravitational microlensing, where a planet briefly bends and brightens the light of a more distant star, and precise timing, as with the pulsar planets.

How many are there?

As of 2026, the NASA Exoplanet Archive lists more than 6,000 confirmed exoplanets; the tally passed that mark in late 2025. Thousands more candidates are still being checked.

They come in many kinds: gas giants, ice giants like Neptune, rocky worlds, and super-Earths and sub-Neptunes that sit in a size range missing from our own Solar System. Those in-between planets appear to be among the most common types in the galaxy.

The habitable zone and TRAPPIST-1

The habitable zone is the band of orbits around a star where a rocky planet could, in principle, keep liquid water on its surface. Too close and water boils away; too far and it freezes. Being in the zone does not guarantee water or life; the atmosphere matters a great deal.

TRAPPIST-1 is a small, cool red dwarf about 40 light-years away with seven roughly Earth-sized planets, all orbiting closer to their star than Mercury does to the Sun. Three or four of them lie in or near the habitable zone.

Because red dwarfs are dim, their habitable zones are tight. They also flare often, which may strip planets of their air. JWST measurements so far suggest the innermost TRAPPIST-1 planets have little or no thick atmosphere, and studies of the others continue.

The missions

NASA's Kepler telescope (2009–2018) stared at one patch of sky and found more than 2,600 confirmed planets by the transit method. It showed that small planets are common.

TESS, launched in 2018, surveys almost the whole sky, focusing on bright, nearby stars. Its planets are easier to study in detail from the ground and from space.

The James Webb Space Telescope studies exoplanet atmospheres. When a planet transits, a little starlight filters through its air, and the colors absorbed reveal gases such as water vapor, carbon dioxide and methane. Future missions aim to image Earth-like planets directly.