Exoplanet

An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. As of 23 April 2026, there are 6,273 confirmed exoplanets in 4,694 planetary systems, with many systems having more than one planet.

There are many methods of detecting exoplanets, with transit photometry and Doppler spectroscopy being the most common. Scientists estimate that about 1 in 5 Sun-like stars may have an "Earth-sized" planet in the habitable zone, where liquid water could exist. This means there could be billions of potentially habitable planets in the Milky Way alone.

The least massive exoplanet known is Draugr, about twice the mass of the Moon, while the most massive known is HR 2562 b, about 30 times the mass of Jupiter. Some exoplanets orbit their stars in less than an hour, while others take thousands of years. The nearest exoplanets are 4.2 light-years from Earth, orbiting Proxima Centauri, the closest star to the Sun.

The discovery of exoplanets has increased interest in the search for extraterrestrial life, especially on planets in the habitable zone where liquid water might exist. The James Webb Space Telescope is helping scientists learn more about these distant worlds, their composition, and whether they could support life. There may also be many rogue planets in the Milky Way—planets not orbiting any star at all.

Definition

The International Astronomical Union has a special group that works on defining what an exoplanet is. An exoplanet is a planet that orbits a star outside our Solar System. There are some rules to decide if something is a planet or not. For example, it must be small enough to not glow on its own, usually less than 13 times the size of Jupiter, and it must orbit a star.

Some people think these rules should be different based on how the planet formed or how far it is from its star. Scientists use different methods to confirm that they have truly found a new planet, often needing more than one kind of observation to be sure.

Nomenclature

Exoplanets, which are planets outside our Solar System, follow a special naming rule set by the International Astronomical Union. When naming a planet that orbits a single star, we start with the star’s name and add a lowercase letter. The first planet found around that star is called "b", the next one "c", and so on. If several planets are found at the same time, the one closest to the star gets the next letter, followed by the others based on their distance from the star. Some exoplanets also have official names approved by the IAU, but most are known by these letter designations. There are also rules for naming planets that orbit two stars together, called circumbinary planets.

History of detection

For a long time, scientists and writers wondered if planets existed beyond our Solar System, but there was no way to prove it. Many early claims of finding such planets were later shown to be wrong.

The first confirmed discovery of an exoplanet was in 1992, when two planets were found orbiting a fast-spinning star called a pulsar. The first planet found around a normal star like our Sun was discovered in 1995. Most exoplanets are found indirectly by watching how they affect their stars, but some have been imaged directly by telescopes.

Early speculations

In the past, many thinkers believed other stars might have planets like our Sun does. For example, the Italian philosopher Giordano Bruno thought that stars were like our Sun and probably had planets orbiting them.

Discredited claims

Many early claims of finding exoplanets turned out to be mistakes. For example, some scientists thought they found planets around stars like 61 Cygni or Barnard's Star, but later observations showed these claims were wrong.

Confirmed discoveries

The first confirmed exoplanet around a normal star was found in 1995 orbiting the star 51 Pegasi. This discovery started a new era of finding many more exoplanets. Since then, over 6,000 exoplanets have been confirmed. These planets come in many sizes and orbits, and some even orbit two stars at once.

Candidate discoveries

Space telescopes like Kepler and TESS have found many possible planets that need more study to confirm. Some of these candidates are about the size of Earth and might be in the right place for life.

Detection methods

Main article: Methods of detecting exoplanets

We discover planets outside our Solar System, called exoplanets, using different methods. One way is direct imaging, where special cameras block out the bright light of a star so we can see the much fainter light from a planet far away. This works best for large planets that are far from their stars.

Most exoplanets are found using indirect methods. The transit method looks for tiny drops in a star's brightness when a planet passes in front of it. The radial velocity method detects a star's slight wobble caused by an orbiting planet. Other methods include measuring changes in a star’s position over time (astrometry) or watching how a star’s light bends when another star passes by (microlensing). Each method helps scientists find planets in different places and sizes.

Formation and evolution

See also: Accretion (astrophysics), Nebular hypothesis, and Planetary migration

Planets can form within just a few million years after their star is born. We can study planets at different ages by looking at various planetary systems. This helps us see how planets change over time. Young planets often start with thick layers of gas, but these layers get smaller as the planets get older. Even small planets might have started out much larger before losing their gas layers. One example is Kepler-51b, a planet only twice the size of Earth but as big as Saturn when it was young.

Planet-hosting stars

Main article: Planet-hosting star

Scientists have discovered that on average, there is about one planet for every star. Around one in five stars similar to our Sun have a planet about the size of Earth in a zone where water could exist. Most of the planets we know are around stars like the Sun, but some have also been found around smaller, cooler stars called red dwarfs. These smaller stars are harder to study, but space telescopes like Kepler have found many planets around them. Stars that have more heavy elements are more likely to have big planets, like Jupiter, orbiting them. Some planets even orbit two stars at once, and a few have been found around systems with three or four stars.

Orbital and physical parameters

Scientists study how far exoplanets are from their stars and how big they are. These details help us understand what kinds of worlds these planets might be. By looking at these measurements, researchers can guess whether an exoplanet could possibly support life or what its weather might be like.

General features

The brightness of an exoplanet — a planet outside our Solar System — depends on its distance from us, how reflective it is, and how much light it receives from its star. Planets that are closer to their stars or less reflective can appear brighter than more distant, highly reflective ones.

In 2013, scientists determined the color of an exoplanet for the first time. The planet HD 189733b appears deep dark blue, while another, GJ 504 b, looks magenta. Some exoplanets, like TrES-2b, reflect almost no light, making them darker than coal.

Surface

Surface features of exoplanets can be studied by comparing different types of light signals. Scientists use special tools to tell apart the surface from the atmosphere. This helps them figure out if a planet is rocky or full of gas.

We can guess how hot an exoplanet is by looking at how much light it gets from its star. For instance, the planet OGLE-2005-BLG-390Lb is thought to be very cold, about −220 °C. But these guesses can be tricky because we don’t always know how much light the planet reflects or how its atmosphere affects the temperature. Some planets, like HD 189733b, have had their temperatures measured by watching changes in infrared light as they orbit their stars.

Habitability

See also: Astrobiology, Circumstellar habitable zone, and Planetary habitability

Scientists are excited about finding planets beyond our solar system that might be able to support life. To have life as we know it, a planet needs to have liquid water. This means the planet must be at just the right distance from its star—not too hot and not too cold—so that water can stay liquid. The distance where this can happen is called the habitable zone.

The habitable zone changes depending on the type of star. Some planets might stay warm enough for liquid water even far from their star because of special conditions like thick atmospheres or tidal heating, which generates heat through gravitational forces. Scientists have found several planets in habitable zones, such as Kepler-62f, Kepler-186f, and Kepler-442b, but we are still learning more about whether these planets could truly support life.

Notable Exoplanets – Kepler Space Telescope
Comparison of small planets found by Kepler in the habitable zone of their host stars.

Planetary system

Main article: Planetary system

Exoplanets often belong to systems with multiple planets orbiting the same star. These planets can influence each other with their gravity, sometimes creating patterns where their orbits fit into simple ratios. For example, the Kepler-223 system has four planets with orbital times that match an 8:6:4:3 pattern.

Some large planets, called hot Jupiters, spin around their stars in the opposite direction of the star's own spin. Scientists think these planets may form in crowded areas where their paths can be changed by the gravity of nearby stars.

Search projects

Several projects help scientists find exoplanets. CoRoT is a space telescope that discovered the first rocky planet passing in front of its star. Kepler was a mission that looked for many exoplanets by watching for stars that dim when a planet passes in front of them.

TESS is another mission that searched for new exoplanets. It observed stars all over the sky from 2018 to 2020 and found many possible exoplanets, with 618 confirmed by March 2025. Other tools like ESPRESSO and HARPS are powerful instruments on big telescopes that help study these distant worlds.