Exoplanet

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An exoplanet, or extrasolar planet, is a planet that orbits a star other than the Sun. As of October 15, 2026, the NASA Exoplanet Archive, a catalogue run for NASA by the California Institute of Technology, listed 6,451 confirmed exoplanets.^[1:1]

The first exoplanets were confirmed in 1992, orbiting a pulsar, a dead star that sends out radio pulses at extremely regular intervals.^[2:1] The first planet around a star like the Sun, 51 Pegasi b, followed in 1995.^[3:1]

Exoplanets are faint and sit very close to much brighter stars, so only a small fraction have been photographed directly. More than nine in ten known exoplanets were found indirectly, either by the slight dimming of a star when a planet passes in front of it or by the slight wobble a planet's gravity causes in its star.^[1:2] Surveys suggest that planets are the rule rather than the exception: on average, every star in the Milky Way has at least one.^[4:1]

Definition

The International Astronomical Union (IAU), the body that names and classifies objects in space, has only a working definition of an exoplanet, last amended in August 2018. Under it, an object counts as a planet regardless of how it formed if it meets three conditions:^[5:1]

• It is not too heavy. Its mass is below the point at which an object can fuse deuterium, a heavy form of hydrogen, in its core. This limit is about 13 times the mass of Jupiter. Heavier objects are classed as brown dwarfs, bodies between planets and stars.
• It orbits something. It circles a star, a brown dwarf, or a stellar remnant (what is left after a star dies).
• It is much lighter than what it orbits. Its mass is less than about 1/25 of the object it orbits.

Not every catalogue applies the IAU's mass limit. The NASA Exoplanet Archive includes objects of up to 30 Jupiter masses, more than twice the IAU threshold.^[6:1] Published counts of exoplanets therefore depend partly on which definition the counter uses.

History of discovery

Pulsar planets (1992)

In January 1992, Aleksander Wolszczan of the Arecibo Observatory and Dale Frail of the National Radio Astronomy Observatory announced two planets orbiting the pulsar PSR B1257+12.^[2:1] They found them by timing the pulsar's radio pulses: the pulses arrived slightly early or late in a repeating pattern, as the pulsar was tugged back and forth by orbiting bodies. Each planet has about four times the mass of Earth. A third, much smaller planet, about twice the mass of the Moon, was found in 1994.^[7:1]

The system has stayed unusual. A 2025 review counted only eight confirmed pulsar planets around six pulsars, and several large searches for more have found none.^[2:2]

51 Pegasi b (1995)

Michel Mayor and Didier Queloz used their spectrograph ELODIE, an instrument that splits starlight into its colours precisely enough to measure how fast a star moves towards or away from Earth. With it mounted on a 1.93 m telescope at the Haute-Provence Observatory in southern France, they surveyed 142 stars. On October 6, 1995, Mayor announced a planet orbiting the star 51 Pegasi.^[3:1]

The planet surprised astronomers. It has roughly the mass of Jupiter, yet it circles its star every four days at a distance of about 8 million km, far closer than any giant planet in the Solar System.^[8:1] A rival team led by Geoffrey Marcy and Paul Butler soon confirmed it.^[3:1] Mayor and Queloz received half of the 2019 Nobel Prize in Physics for the discovery.^[9:1]

Which came first

Sources disagree on wording. The Nobel Prize press release calls 51 Pegasi b "the first discovery of a planet outside our solar system".^[9:1] The pulsar planets, however, were announced three years earlier.^[2:1] The two claims are reconciled by what each discovery is credited with: the pulsar planets were the first confirmed exoplanets, while 51 Pegasi b was the first found around a star like the Sun. The pulsar planets probably formed differently from most planets, and their dead host star is unlikely to support life, which is why the Nobel Prize recognised 51 Pegasi b.^[3:2]

Detection methods

Most exoplanets are too faint to see next to their stars, so astronomers usually detect them through their effects on the star. Each method finds some kinds of planets more easily than others, so the methods complement each other rather than compete.^[10:1]

Counts are confirmed planets by discovery method in the NASA Exoplanet Archive as of October 15, 2026; "timing methods" combines transit, eclipse, pulsar and pulsation timing variations. Two other methods account for the remaining 10 planets.^[1:2] How each method works follows the Nobel Prize committee's and NASA's descriptions;^[8:1][11:1] which planets each finds most easily follows a review by Jason Wright and Scott Gaudi.^[10:1]

These preferences shape what is known. The two most productive methods favour large or massive planets close to their stars, so small planets far from their stars, like Earth, are among the hardest to find.^[10:1]

Direct imaging

Photographing an exoplanet is hard because the planet sits very close to its star on the sky and is far fainter. In 2008, a team led by Christian Marois used the Keck and Gemini telescopes, with adaptive optics to correct for the blurring of Earth's atmosphere, to photograph three planets orbiting the star HR 8799. The planets lie 24, 38 and 68 times Earth's distance from the Sun and weigh an estimated 5 to 13 Jupiter masses. Images taken at different times show all three moving around the star.^[12:1]

Space telescopes

The Kepler space telescope, which NASA built to watch stars for the tiny dips in brightness caused by transits,^[11] found more than 2,600 exoplanets before it ran out of fuel and was retired in October 2018.^[13:1] Its successor, the Transiting Exoplanet Survey Satellite (TESS), watches at least 200,000 relatively bright stars with four wide-field cameras. It targets nearby, bright stars on purpose: their planets are easier to study in detail afterwards, including with the James Webb Space Telescope.^[14:1]

Types of exoplanets

NASA groups exoplanets into four broad types:^[15:1]

• Gas giants resemble Jupiter and Saturn but are often larger and closer to their stars. Hot Jupiters orbit so close that their temperatures reach thousands of degrees.
• Neptunian planets are similar in size to Neptune or Uranus. Mini-Neptunes are smaller than Neptune but larger than Earth.
• Super-Earths are larger than Earth and typically rocky, with or without atmospheres.
• Terrestrial planets are Earth-sized or smaller and made of rock, silicate, water or carbon.

The most common planets found by Kepler have no counterpart in the Solar System. Planets between Earth and Neptune in size are, by a wide margin, the most frequent in Kepler's sample.^[16:1] Measuring 2,025 of them precisely, Benjamin Fulton and colleagues found that these planets fall into two groups with few in between: rocky super-Earths up to about 1.5 times Earth's width, and sub-Neptunes about 2 to 3 times Earth's width. The authors interpret both groups as rocky cores, with the larger planets wrapped in different amounts of light gas.^[16:2]

How common planets are

Two independent lines of evidence suggest that most stars have planets.

• Microlensing. A study led by Arnaud Cassan used microlensing events observed from 2002 to 2007, which can reveal planets orbiting 0.5 to 10 times Earth's distance from their stars. It estimated that 17% of stars host a planet of roughly Jupiter's mass, and that smaller, cooler planets are more common still: Neptune-like planets around 52% of stars and super-Earths around 62%. The authors concluded that stars are orbited by planets "as a rule, rather than the exception". The estimate rests on only 10 detected planets, so its uncertainties are wide.^[4:1]
• Transits. Kepler's data indicate that the galaxy contains more planets than stars.^[13:1] Analysing 42,000 Sun-like stars, Erik Petigura and colleagues found 603 planets, 10 of them Earth-sized and receiving a similar amount of starlight to Earth. After correcting for planets the search would have missed, they estimated that 11% of Sun-like stars have an Earth-sized planet receiving one to four times Earth's starlight.^[17:1]

Habitability

The habitable zone is the range of distances from a star at which a planet could have liquid water on its surface. Extrapolating their Kepler results to longer orbits, Petigura and colleagues estimated that about 22% of Sun-like stars have an Earth-sized planet in the habitable zone, and that the nearest such planet may be within 12 light-years of Earth.^[17:1] The authors note the limits of this figure: the extrapolation carries less weight than a direct measurement, habitability depends on each planet's own properties, and their habitable zone also includes Venus and Mars, neither of which has surface water today.^[17:2]

The nearest known example is Proxima b, announced in 2016. It orbits Proxima Centauri, the closest star to the Sun, every 11.2 days at about 7 million km, and has at least 1.3 times Earth's mass. Because its star is much fainter than the Sun, that close orbit places it within the habitable zone. Its conditions are still uncertain: ultraviolet and X-ray flares from the star may strongly affect its surface, and one side of the planet probably faces the star permanently.^[18:1]

Atmospheres

When a planet passes in front of its star, a little starlight filters through the edge of its atmosphere. Each gas absorbs particular colours, so measuring how much light is lost at each colour reveals what the atmosphere contains. This technique is called transmission spectroscopy.

On July 10, 2022, the James Webb Space Telescope observed the gas giant WASP-39b, which has about the mass of Saturn and orbits its star every four days. The resulting spectrum shows a strong carbon dioxide signal. Earlier observations by the Spitzer Space Telescope had hinted at carbon dioxide on the planet but could not confirm it.^[19:1]

Wikidata: Q44559