What Is an Exoplanet? Definition and How They Are Found

An exoplanet, or extrasolar planet, is a planet that orbits a star other than the Sun. Thousands are now known, ranging from gas giants larger than Jupiter to rocky worlds close to Earth in size, and nearly all were found by indirect methods that detect a planet through its effect on its star rather than by seeing it.

This page explains what counts as an exoplanet, the main ways of finding one, and why the numbers keep changing.

Definition: what counts as a planet elsewhere

The International Astronomical Union (IAU) adopted a definition of “planet” in 2006 that applies to the Solar System, and it has a working position on objects beyond it. In general terms, an exoplanet is an object below the mass at which deuterium fusion starts (roughly 13 times the mass of Jupiter) that orbits a star or stellar remnant. Heavier objects are usually called brown dwarfs.

The boundaries are fuzzy. Some objects drift freely without a star; these “rogue” or free-floating planets are discussed separately because they do not orbit anything.

The first discoveries

The first confirmed exoplanets were found around a pulsar in 1992 by Aleksander Wolszczan and Dale Frail. The first around a Sun-like star, 51 Pegasi b, was announced in 1995 by Michel Mayor and Didier Queloz, who shared the 2019 Nobel Prize in Physics for the work, according to the Nobel Prize organisation.

51 Pegasi b surprised astronomers because it is a giant planet orbiting extremely close to its star, a type later called a “hot Jupiter”. Nothing like it exists in our Solar System.

How astronomers find exoplanets

Each method has strengths and biases, which is why different surveys find different kinds of planets.

Transit method

When a planet passes in front of its star as seen from Earth, it blocks a small fraction of the light. A telescope that measures stellar brightness over time sees a regular dip. The depth of the dip gives the planet’s size relative to the star, and the spacing between dips gives the orbital period. Space missions such as Kepler and TESS were designed around this technique.

Radial velocity method

A planet’s gravity makes its star move in a small orbit of its own. When the star moves toward us, its light shifts slightly to the blue; when it moves away, to the red. Measuring that shift in the spectrum reveals a minimum mass for the planet. Combined with a transit, it gives both size and mass, and therefore an estimate of density.

Direct imaging

Blocking the star’s glare with a coronagraph or other technique can reveal a very faint planet. It works best for young, hot, massive planets far from their stars, because they glow brightly in infrared light.

Gravitational microlensing

If a star with a planet passes in front of a more distant star, its gravity bends and magnifies the background light. A planet adds a short extra spike. This method can find planets far away, but the alignment never repeats, so the planet cannot be re-observed.

Astrometry

Astrometry measures the tiny sideways motion of a star caused by an orbiting planet. It is technically demanding, but space missions such as Gaia are designed to improve it.

Comparing the methods

Method	What it measures	Best at finding
Transit	Dip in starlight	Close-in planets, planet size
Radial velocity	Doppler shift of starlight	Massive planets, minimum mass
Direct imaging	Light from the planet	Young, wide-orbit giants
Microlensing	Brightness spike from bending light	Distant planets, cold orbits
Astrometry	Star’s position wobble	Massive planets on wide orbits

Confirmed versus candidate

A signal is first a candidate. Astronomers then check that it is not caused by a binary star, instrument noise or stellar activity. Only after follow-up observations, often with a second method, is it called confirmed.

The count of confirmed worlds grows steadily. NASA reported passing 5,000 confirmed exoplanets in March 2022, and its public catalogue is updated continually, so check the NASA Exoplanet Exploration pages for the current total instead of relying on any fixed figure.

What astronomers learn after detection

Finding a planet is the beginning. Once the orbit, size and mass are known, observers ask what the planet is made of and whether it has an atmosphere. That is the subject of reading exoplanet atmospheres, where light passing through a planet’s air is split into a spectrum to reveal molecules. Technical papers on such measurements are gathered on a publications list.

Frequently asked questions

How many exoplanets are there?

Thousands are confirmed, and statistical studies suggest most stars host at least one planet. The exact count changes as new discoveries are verified, so use NASA’s exoplanet archive for an up-to-date number.

Can we see exoplanets directly?

Rarely. Most are too faint and too close to their stars. Direct imaging has captured a small number of young, hot giants, but nearly all known planets were found indirectly.

What is the nearest exoplanet?

Proxima Centauri b, which orbits the closest star to the Sun, is the nearest known one, at about 4.2 light years. Its conditions are still being studied, and it is much closer than any other known planet.

Are any exoplanets like Earth?

Some are similar in size and receive comparable starlight, but size alone does not mean a planet is habitable. Astronomers need atmospheric and surface information, which is difficult to obtain for small rocky worlds.

In brief

An exoplanet is simply a planet around another star, and almost every one we know was inferred from a faint effect on starlight. The field moved from a handful of curiosities in the 1990s to a large catalogue, and the next challenge is characterising what these worlds are actually like. Start with the home page for an overview of the subject.