Exoplanet

An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed exoplanet was found in 1992 around a pulsar. The first one found around a main-sequence star was detected in 1995. A different planet, first detected in 1988, was confirmed in 2003. In 2016, scientists realized that possible evidence of an exoplanet had already been noted in 1917, a precovery. As of 8 October 2026, there are 6,445 confirmed exoplanets in 4,834 planetary systems. Of these systems, 1,070 have more than one planet.

There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most. However, these methods are better at finding large planets that are close to their star. It is estimated that about 1 in 5 Sun-like stars has an "Earth-sized" planet in the habitable zone. If there are 200 billion stars in the Milky Way, then there could be about 11 billion Earth-sized planets in the Milky Way that might be able to support life. This number could rise to 40 billion if planets orbiting the many red dwarfs are included.

The least massive exoplanet known is Draugr, which has about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b. It has about 30 times the mass of Jupiter. However, some definitions of a planet are based on the nuclear fusion of deuterium. Under those definitions, this object is too massive to be a planet and might be a brown dwarf.

Known orbital times for exoplanets range from less than an hour for those closest to their star to thousands of years. Some are so far from their star that it is hard to tell whether they are still held by its gravity. The nearest exoplanets are 4.2 light-years (1.3 parsecs) from Earth. They orbit Proxima Centauri, the closest star to the Sun. At the other extreme, there is evidence for extragalactic planets, which are exoplanets located in other galaxies.

The discovery of exoplanets has increased interest in the search for extraterrestrial life. Scientists are especially interested in planets that orbit in a star's habitable zone, sometimes called the "goldilocks zone." In this zone, liquid water could exist on the surface. Liquid water is needed for life as we know it. The study of planetary habitability also looks at many other factors. The James Webb Space Telescope (JWST) is expected to help scientists learn more about exoplanets. It will work with ground-based and other space-based observatories. It will study things like their composition, environmental conditions, and habitability.

Rogue planets are planets that are not part of any planetary system. They are usually treated as a separate category from planets, especially if they are gas giants, which are often counted as sub-brown dwarfs. There may be billions of rogue planets in the Milky Way.

Definition

The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System, so it does not apply to exoplanets. The IAU made a working definition for exoplanets in 2001, changed it in 2003, and updated it in August 2018. Under the current version, an exoplanet has a true mass below the limit for deuterium fusion. This limit is about 13 Jupiter masses for objects with solar metallicity. The object orbits a star, brown dwarfs, or stellar remnants. Its mass compared with its central object is below the L₄/L₅ instability. Its minimum mass and size should be the same as those used in our Solar System.

Some scientists prefer to tell planets apart from brown dwarfs by how they form. Giant planets are thought to form through core accretion. Brown dwarfs form like stars, from collapsing clouds of gas. Objects that orbit their stars at hundreds or thousands of astronomical units likely formed as brown dwarfs. The 13-Jupiter-mass limit (¹³ M_Jup) does not have a precise physical meaning, and some catalogues use different limits. The Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses in 2011. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses, and the NASA Exoplanet Archive includes objects up to 30 Jupiter masses. NASA's archive confirms an exoplanet when different observation methods reveal features that only a planet could explain.

Nomenclature

Exoplanets are named using a system that builds on the way the International Astronomical Union (IAU) names star systems with more than one star. A planet that orbits one star gets the star's name plus a lowercase letter. The letters follow the order in which the planets were found around that star. The first planet found is "b" (the star is "a"), and later planets get the next letters. If several planets in one system are found at the same time, the one closest to the star gets the next letter. The others follow in order of how far they orbit from the star.

The IAU has a temporary standard for naming circumbinary planets. These are planets that orbit two stars. Only a few exoplanets have IAU-approved proper names. Other naming systems also exist.

History of detection

NASA graphic of present and future exoplanet missions as of 2022.

Scientists first recorded possible evidence of an exoplanet orbiting Van Maanen 2 in 1917. They did not recognize it as a planet until 2016. Over the years, many early claims of planets were later shown to be wrong. The first planet that scientists accepted was announced in 1992. Aleksander Wolszczan and Dale Frail found two planets orbiting the millisecond pulsar PSR B1257+12. In 1995, a giant planet was found orbiting the nearby star 51 Pegasi. It was the first confirmed planet around a star like the Sun. Most exoplanets have been found by indirect methods, such as the transit method and the radial-velocity method. A few have been imaged directly by telescopes.

In 2018, researchers used the Chandra X-ray Observatory and a technique called microlensing. They found evidence of planets in a distant galaxy. They estimated that the number of planets in that galaxy was more than a trillion.

Main article: Discoveries of exoplanets

Detection methods

Main article: Methods of detecting exoplanets

Planets are extremely faint next to their stars. A Sun-like star is about a billion times brighter than the light reflected from a planet orbiting it, and the star's glare often hides the planet. Blocking the star's light is a major challenge that requires extreme optothermal stability. Almost all exoplanets that have been directly imaged are large (more massive than Jupiter) and far from their stars. Special instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will image dozens of gas giants, but most known exoplanets were found using indirect methods.

Scientists have found planets in several indirect ways. In the transit method, a planet passes (transits) in front of its star, and the star gets a little dimmer. The Kepler telescope used this method. In the radial velocity or Doppler method, a star wobbles slightly as its planet orbits it. The star's movement toward or away from Earth can be seen in shifts of its spectral lines, caused by the Doppler effect. Small changes in the timing of a planet's transits can reveal another planet, as with Kepler-19b and the non-transiting Kepler-19c. Changes in transit length have helped confirm many planets that orbit two stars, but they have not found new planets. In gravitational microlensing, a star's gravity acts like a lens and magnifies the light of a distant star. This method works best for planets about 1 to 10 AU away from Sun-like stars.

Astrometry measures a star's position in the sky and how it changes over time. The motion caused by a planet is very small, so this method did not produce many results until the 2020s, and it has only a few confirmed discoveries. A pulsar is a small, dense remnant of a star that exploded as a supernova, and it gives off regular radio waves. Planets orbiting a pulsar change its distance from Earth, which shifts the timing of its radio pulses. This method was used for the first confirmed discovery of a planet outside our solar system. As of 2011, it had found five planets around three pulsars. Some other stars show regular activity, and a planet can cause changes in it. Other methods study the light from a planet and its star, including reflection and emission changes, relativistic beaming, ellipsoidal variations, and polarimetry. Finally, dust disks around stars glow in infrared light and may hint at planets, but this is not considered a sure detection method.

Formation and evolution

Planets may form a few to tens of millions of years after their star forms. We can only see the planets of the Solar System as they are today. But by studying planetary systems of different ages, scientists can see planets at different stages of growth. Some systems are young protoplanetary disks, where planets are still forming. Others are more than 10 billion years old.

When planets form in a disk of gas, they gather hydrogen and helium around them. Over time, this gas cools and shrinks. Depending on the planet's mass, some or all of it can drift off into space. So even rocky planets can start out very large if they form early. For example, Kepler-51b has only about twice the mass of Earth, but it is almost the size of Saturn. Saturn has about a hundred times Earth's mass. Kepler-51b is still quite young, at only a few hundred million years old.

Planet-hosting stars

Main article: Planet-hosting star

Most stars have at least one planet. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Most known exoplanets orbit stars that are a lot like the Sun. These are called main-sequence stars of spectral categories F, G, or K. Smaller stars, called red dwarfs, are less likely to have planets big enough to be found by the radial-velocity method. Still, the Kepler space telescope has found several tens of planets around red dwarfs. It used the transit method to spot smaller planets.

Data from Kepler shows a link between a star's metallicity and its chance of having a giant planet, which is similar in size to Jupiter. Stars with higher metallicity are more likely to have planets, especially giant ones. Some planets orbit one star in a binary star system. Several circumbinary planets orbit both stars. A few planets are known in triple star systems. One is known in the four-star system Kepler-64.

Orbital and physical parameters

General features

Main article: Ring system § Rings around exoplanets

Main article: Exomoon

How bright a planet looks depends on how far away you are (apparent magnitude), how much light it reflects, and how much light it gets from its star. A planet that reflects little light but is close to its star can look brighter than a reflective planet that is far away. In 2013, scientists first worked out the color of an exoplanet. HD 189733b looks deep dark blue, and GJ 504 b looks magenta. Kappa Andromedae b would look reddish up close. Helium planets are expected to look white or grey. The darkest known planet is TrES-2b, a hot Jupiter. It reflects less than 1% of its star's light, so it is darker than coal. Scientists do not know why it is so dark. For gas giants, the geometric albedo changes with temperature, metals in the air, and clouds.

In 2014, scientists found a magnetic field around HD 209458 b. They discovered it by watching the hydrogen escaping from the planet. The field is about one-tenth as strong as Jupiter's. One day, exoplanet magnetic fields might be found through auroral radio signals picked up by telescopes such as LOFAR, but none have been found yet. Earth's magnetic field comes from its flowing liquid metal core. Different materials may form inside super-Earths. Hot Jupiters are larger than expected, possibly because stellar wind heats them through Joule heating.

In 2018, scientists reported that deuterium can turn into metallic hydrogen. This may help explain the strong magnetic fields of giant planets like Jupiter. Earlier claims that magnetic fields cause stellar flares were shown to be false in the HD 189733 system. In 2023, astronomers saw what might be the first sign of a magnetosphere around a rocky exoplanet in the YZ Ceti system.

Scientists also disagree about plate tectonics on super-Earths. If a super-Earth has more than 80 times as much water as Earth, it becomes an ocean planet, with all its land underwater. Large temperature changes on 55 Cancri e may come from volcanoes. Gravitational tugs from nearby planets might cause volcanic activity on rocky exoplanets.

Some exoplanets may have rings. In 2007, the star V1400 Centauri was blocked by an object called J1407b. It was once thought to have a huge ring system, larger than Saturn's rings. Later studies suggested it might be a disk instead. HIP 41378 f likely has rings, since its measured size is too large for its mass.

Moons around other planets, called exomoons, may exist, but none has been confirmed. Candidates have been reported around WASP-12b-type planets, a 0.5 M🜨 moon in the MOA-2011-BLG-262 event, and Kepler-1625b. Sodium found in hot Jupiters such as WASP-76b, HD 189733 b, and WASP-49b may be due to an Io-like moon.

Atmospheres have been found around several exoplanets. The first was HD 209458 b, in 2001. By February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres had been observed. An atmosphere was also found on Tau Boötis b. Kepler-1520b is a small rocky planet leaving a dusty tail like a comet. GJ 436 b's atmosphere has been evaporating into a long tail.

Planets tidally locked in a 1:1 spin-orbit resonance would have one hot spot facing their star and a freezing opposite side. Planets with an eccentric orbit can be locked in other patterns.

Surface

Scientists can tell a planet's surface apart from its air by comparing its light using transmission spectroscopy. Mid-infrared light may show rocky surfaces. Near-infrared light may show magma oceans, very hot lava, wet silicate surfaces, and water ice. This is a clear way to tell rocky planets from gas planets.

Artist's illustration of temperature inversion in an exoplanet's atmosphere, with and without a stratosphere — Artist's illustration of temperature inversion in exoplanet's atmosphere.

Scientists can estimate an exoplanet's temperature by measuring how much light it gets from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of about −220 °C (50 K). These estimates may be quite wrong. They depend on the planet's albedo, which is usually unknown. Factors such as the greenhouse effect may also add unknown effects. A few planets have had their temperature measured by watching how their infrared light changes as they move in their orbit and go behind their parent star. For example, the planet HD 189733b has an estimated average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.

Habitability

As more planets are found, exoplanetology keeps growing and becomes a deeper study of worlds beyond our Sun. In the future, it will look at whether life on planets could exist outside the Solar System. Life that is very far away could only be found if it changed a planet in ways that normal physical and chemical processes cannot explain. For example, O₂ in the atmosphere of Earth comes from photosynthesis by living plants and tiny living things. So it can be an indication of life on other planets, although small amounts of oxygen can also form without life. If water is a requirement for life, a habitable planet must have water. It must orbit a stable star at a distance where planetary-mass objects with enough atmospheric pressure can keep liquid water on their surfaces.

Main article: Habitable zone

The habitable zone is the area around a star where the temperature is right for liquid water to exist on a planet's surface. It is not so close that the water evaporates, and not so far that it freezes. Stars give off different amounts of heat depending on their size and age, so the habitable zone is at different distances for different stars. The atmosphere matters too. Desert planets, with very little water, may keep water closer to their star than Earth is to the Sun. A planet's rotation rate affects the circulation of the atmosphere and its clouds. Slowly rotating planets can be habitable closer to their star, because their thick clouds reflect more sunlight. Venus may have lost its oceans to a runaway greenhouse effect. Tidally locked planets may also be habitable closer to their star because of clouds. Planets around stars with low metallicity may be better for complex life on land. Even rogue planets could have liquid water deep underground. In an earlier era of the universe, the cosmic microwave background would have allowed rocky planets to have liquid water wherever they were. Jupiter-like planets might not be habitable, but they could have habitable moons. Planets can also freeze over, but a large axial tilt makes this less likely. Cooling can also stop when carbon dioxide weathering stops and carbon dioxide builds up again. Extra energy from tidal heating can keep planets with eccentric orbits warm. Some of these could have a hidden ocean below the surface, like Europa's, as in the Upsilon Andromedae system. Using a 2015 review, Kepler-62f, Kepler-186f and Kepler-442b are among the best candidates for being potentially habitable. They are 1000, 490 and 1,120 light-years away. Proxima Centauri b, about 4.2 light-years away, is the nearest candidate. Red dwarfs may also have many Earth-size planets in their habitable zones. In 2015, Kepler-452b became the first near-Earth-size planet found in the habitable zone of a Sun-like G2-type star. In 2020, TOI-700 d became the first Earth-size planet in the habitable zone found by TESS.

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 are often part of planetary systems. A planetary system has many planets that orbit one star. The planets pull on each other with gravity. Sometimes their orbits follow simple whole-number patterns, which is called a resonant system. The Kepler-223 system has four planets in an 8:6:4:3 orbital resonance.

Some hot Jupiters orbit their stars in the opposite direction to the way their stars spin. One possible explanation is that hot Jupiters often form in crowded groups of stars. In these groups, perturbations are more common. Perturbations are small pushes and pulls that change orbits. Sometimes a planet can even be caught by the gravity of a nearby star.

Search projects

ANDES is a planned tool for finding and studying planets. It will likely be placed on ESO's ELT 39.3m telescope. It used to be called HIRES. That tool was made when the teams behind the earlier CODEX and SIMPLE ideas joined together.
CoRoT is a space telescope. It found the first rocky planet that was seen passing in front of its star.
ESPRESSO is a tool for finding rocky planets and watching stars closely. It is attached to ESO's 4 × 8.2 m VLT telescope. That telescope sits on the flat top of Cerro Paranal in the Atacama Desert of northern Chile.
HARPS is a tool for finding planets. It is installed on the ESO's 3.6m telescope at La Silla Observatory in Chile.
Kepler was a mission that looked for many exoplanets. It did this by watching for dips in starlight when planets passed in front of their stars.
TESS is a mission that searches for new exoplanets. It was active from 2018 to 2020 and then started looking at stars across the whole sky. As of 22 March 2025, TESS had found 7,525 candidate exoplanets. Of these, 618 had been confirmed.