Exoplanet: Definition, Types, Discovery, Methods, Facts

An exoplanet is a planet beyond the Solar System that orbits stars other than the sun.

Extrasolar planets range from massive gas giants larger than Jupiter to rocky planets half size of Earth. Scientists categorized exoplanets into gas giant, Neptunian, super-Earth, and terrestrial types; most common are super-Earths and mini-Neptunes.

Five methods used to discover exoplanets let researchers filter finds by type and by method used to discover.

What is the definition of an exoplanet?

An exoplanet is a planet outside of the Solar System, and planets that orbit around other stars are called exoplanets. Exoplanets orbit stars, but they are veiled by the bright glare of stars they orbit and are hard to see directly with telescopes. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and confirmed exoplanets are nearly 6,000.

What are the types of exoplanets?

The types of exoplanets are outlined below.

• Five exoplanet types refer only to size: rocky planets, super-Earths, mini-Neptunes, ice giants, gas giants
• Terrestrial exoplanets
• Free-floating exoplanets (rogue planets)
• Ocean planets and desert planets
• Neptunian or Neptune-like
• Super-Earths and mini-Neptunes
• Rocky planets are the smallest exoplanet type
• Gas giants are the biggest exoplanet type
• Hot Jupiters are big gassy exoplanet types
• Hot Neptunes are big gassy planets in tight fast orbits around their stars
• Super-Earths are larger exoplanet type than rocky planets
• Ice giants are larger exoplanet type than mini-Neptunes
• Mini-Neptunes are larger exoplanet type than super-Earths
• Mini-Neptunes are subcategory within Neptunian exoplanets

Other stars are orbited by types that do not exist in our solar system, like ocean planets and desert planets.

Are there any habitable exoplanets?

The Habitable Worlds Catalog, maintained by the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, currently identifies 70 potentially habitable planets. Out of nearly 6,000 worlds discovered so far, only 12 have been found in the habitable zone of their stars and are smaller than twice the size of Earth. Twenty-nine of the catalogued planets are likely rocky and within the circumstellar habitable zone, meaning they are either under 10 Earth masses or smaller than 2.5 Earth radii.

Kepler-1649c, Kepler-452b, and TRAPPIST-1e are among the most cited examples. Kepler-452b is located in the habitable zone of a Sun-like star, while TRAPPIST-1e is thought to be the most likely to support existence as we know it. HD 20794 d orbits at the right distance from its star to sustain liquid water on its surface, placing it within the habitable zone of the HD 20794 system. Proxima Centauri b also lies in the habitable zone of its stars, though it is exposed to extreme ultraviolet radiation and has an orbital period of just 11.2 days.

Some planets once believed promising, Kepler-577b and Kepler-1649b, are no longer classified as habitable based on new data. Earth-like planets remain a rarity, yet estimates suggest that 10 billion Milky Way stars host habitable exoplanets. Future missions will use instruments like the Extremely Large Telescope, Habitable Worlds Observatory, and LIFE to characterise atmospheres and search for biosignatures, expanding the list of targets across the galaxy.

When was the first exoplanet discovery?

The first confirmed detection of an exoplanet came in 1992, when Aleksander Wolszczan and Dale Frail announced the discovery of two terrestrial-mass planets orbiting the millisecond pulsar PSR B1257+12.

Earlier possible detections had been published but remained unconfirmed: a different planet was first detected in 1988, yet its status was not secured until 2002, and the first evidence of an exoplanet, orbiting Van Maanen 2, was recorded in 1917, though its importance was not recognized until 2016.

The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when 51 Pegasi b, a giant planet in a four-day orbit around the nearby star 51 Pegasi, had its existence verified.

What are the odd exoplanets discovered?

Some exoplanets look almost ordinary until their secrets are exposed. 51 Pegasi b was the first exoplanet discovered around a sun-like star, opening the flood-gate in 1995. HR 5183 b is a Super-Jupiter three times the mass of the solar-system’s largest planet; its orbit is so highly eccentric that it behaves like a cosmic whip-lash, helping astronomers solve the hot-Jupiter mystery. Kepler-16 b, found in 2011, is the first known circumbinary planet, a world where two suns set over the horizon.

Other worlds are stranger still. PSR B1257+12 b is a doomed world, one of the first exoplanets ever detected, yet it circles a dead pulsar rather than a living star. OGLE-2016-BLG-1928 wanders the Milky Way as a rogue exoplanet, untethered to any star. TOI-3757 b is a marshmallow planet whose puffy density defies expectations.

Temperature and chemistry create further oddities. KELT-9 b is so hot that heat tears molecules apart on the dayside; it was discovered in 2017. KOI-55 b is hotter than the Sun, a rocky ember orbiting the remains of a stellar core. HD 189733 b, 64 light-years away, has a distinct chemical composition and rain glass; astronomers can practically detect its signature from across the galaxy. WASP-76b goes further, raining molten iron in eternal night.

Some planets challenge textbook theories of formation. 55 Cancri e, once billed as the universe’s most priceless exoplanet made of diamond, saw that claim later challenged; it was first discovered in 2004. GJ 1132 b grew a second atmosphere after its first was stripped, offering a natural experiment in atmospheric recovery. GJ 436 b is mysteriously missing methane, leaving chemists puzzled.

Scale and setting add final layers of peculiarity. BD+05 4868 Ab, Mercury-sized, orbits its star every 30.5 hours 140 light-years away in Pegasus. Kepler-36 b has bizarrely close neighbors, hosting tightly packed giants. TOI-849 b had its atmosphere destroyed by relentless stellar radiation, exposing a naked core. Kepler-16 b, Kepler-36 b, HR 5183 b, and their like continue to prove that the world of exoplanets is limited only by imagination.

What is a Kepler exoplanet?

A Kepler exoplanet is any planet found by NASA’s Kepler space telescope. Kepler was NASA’s first planet-hunting mission; launched in 2009, it searched a portion of the Milky Way galaxy for Earth-sized planets orbiting stars outside our solar system by detecting transits.

Kepler-1658 b was the first exoplanet candidate it spotted; this hot Jupiter later confirmed the mission’s detection power. During its first six weeks Kepler discovered five more planets: Kepler-4b, 5b, 6b, 7b and 8b and thereafter continued to discover large numbers of new exoplanets, eventually amassing over 2000 candidates and showing that our galaxy contains billions of previously unseen exoplanets.

Among the confirmed finds, Kepler-22b became the first known transiting planet to orbit within the habitable zone of a Sun-like star. Roughly twice Earth’s radius, this “water world” circles the Sun-like G5 star Kepler-22 located 640 light-years away in Cygnus with an orbital period of about 290 days. Kepler-186f, an Earth-sized exoplanet orbiting the red dwarf star Kepler-186, is the outermost of five such planets detected; its semimajor axis is 0.432 AU and equilibrium temperature 188 K. These discoveries demonstrate how Kepler proved that small rocky worlds are common, with 20%-50% of stars likely hosting Earth-size planets within their habitable zones.

What are the methods of exoplanet detection?

Transit photometry and the radial-velocity technique remain the two most common methods. Transit photometry detects a periodic dip in stellar light caused by a planet crossing in front of its star; from the depth and duration of the dip astronomers obtain orbital size, period and an estimate of planet radius. The radial-velocity method, also known as Doppler spectroscopy, measures the tiny wavelength shifts produced as starlight is alternately squeezed and stretched by the star’s wobble; the amplitude of the wobble yields the planet’s minimum mass. These two indirect strategies dominated early discoveries and still provide most of the known census.

Gravitational microlensing, astrometry and direct imaging extend the inventory to planets that eclipse rarely or never. Microlensing relies on chance alignment: a foreground star-planet system briefly warps and amplifies the light of a distant background star, betraying the planet’s presence through a characteristic light-curve anomaly. Astrometry, the oldest proposed technique, records the minute ellipse that a star traces on the sky as it orbits the common centre of mass with its unseen companion; the Gaia mission is now supplying the precision needed to complete orbital solutions already hinted at by radial-velocity surveys. Direct imaging, still less common, blocks the glare of the host star to record infrared light emitted or reflected by young, self-luminous giant planets; advanced data-processing methods like angular differential imaging and the ANDROMEDA maximum-likelihood algorithm separate the faint planetary signal from instrumental speckles.

Specialised timing and spectroscopic tricks further enlarge the parameter space. Pulsar timing detects planets through microseconds-level delays in the radio pulses of dead stars, while eclipse-timing variations of close binaries betray circumbinary worlds. Transmission and emission spectroscopy read starlight filtered through, or emitted by, a planet’s atmosphere, revealing chemical species: water, helium or barium in ultra-hot gas giants. Variants transit-timing duration, relativistic beaming, ellipsoidal variations, circumstellar-disk kinematics exploit every measurable modulation light or position can offer, all contributing to the growing catalogue of known exoplanets.

Why can’t we see exoplanets with a telescope?

To see an exoplanet directly requires us to separate it from the contrast-destroying presence of the host star it is orbiting and to spatially resolve the disc of the planet. Stars look like tiny points of light even through strong ground- or space-based telescopes, yet they are millions of times brighter than their planets. The major problem astronomers face is that planets are very difficult to spot next to their bright host stars because any light reflected off of the planet is drowned out by massive amounts of radiation coming from the star.

Consequently, we can’t see the exoplanet. Even the biggest exoplanet is not resolved at all, and imaging an exoplanet at the resolution the scientists describe requires a telescope twenty times wider than Earth or an enormous telescope with a mirror kilometers across. A telescope must be placed at least fourteen times farther away from the Sun than Pluto to capture an exoplanet image through the solar gravitational lens, a distance further than humans have ever sent a spacecraft. Scientists align the telescope, the Sun, and the exoplanet so that the gravitational field of the Sun magnifies light from the exoplanet, allowing a Hubble-sized telescope to image the planet with enough power to capture fine details on its surface.

Until such extreme technology is deployed, scientists rely on indirect methods: starlight dims when an exoplanet passes in front of its star, and careful observations can reveal an exoplanet’s orbit size and shape even though we never see the planet itself.

What is the difference between a planet and an exoplanet?

The difference between a planet and an exoplanet is explained in the table below.

An exoplanet is the same kind of body, but it adds a single Greek syllable: “exo”, meaning outside. Because the prefix signals that the object orbits another star, every exoplanet is a planet, yet not every planet is an exoplanet.

The International Astronomical Union’s formal definition of planet was drafted only for the Solar System, so it has no authority once we step across the border of our own planetary family.

What are some facts about exoplanets?

Some facts about exoplanets are presented below.

• Exoplanets are extremely common and extremely diverse over various properties: sizes, masses, and orbital architectures.
• Exoplanets have masses ranging from twice the mass of the Moon to 30 times the mass of Jupiter.
• Known exoplanets fall along a range of sizes, masses, and orbital positions.
• Exoplanets vary in both size and composition, from rocky Earth-like planets to gas-rich giants.
• Exoplanets are extremely common and extremely diverse.
• Thousands of exoplanets have been confirmed orbiting other stars.
• Some exoplanets have been imaged directly by telescopes.
• Most exoplanets discovered so far are in a relatively small region of the Milky Way.
• Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.
• Exoplanets have orbital architectures ranging from tight packing to wide separations.
• As of 14 August 2025 there are 5,983 confirmed exoplanets in 4,470 planetary systems.
• Nearly 6,000 exoplanets have been confirmed.
• Some exoplanets are rogue planets.
• Exoplanets are confirmed by observing several transits that have the same dip in star light, time to transit the star, and amount of time between successive transits.
• Astronomers estimate that there could be trillions of planets around other stars.
• Exoplanets thus exist outside the solar system.
• The nearest exoplanets are located 4.2 light‑years (1.3 parsecs) from Earth and orbit Proxima Centauri.
• At least one exoplanet has been found to have an exomoon.
• The closest exoplanet to Earth is Proxima Centauri b.
• Exoplanets can vaporize while orbiting too close to its star.
• Exoplanets can leave behind trail of material while vaporizing.
• The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b.

Exoplanets are extremely common; astronomers estimate that trillions circle stars in the Milky Way alone, and billions exist across the galaxy. Sizes stretch from rocky globes twice the Moon’s mass to gas-rich behemoths thirty times Jupiter’s mass, while the least massive known, Draugr, and the most massive catalogued, HR 2562 b, exemplify the extremes. Mini-Neptunes, slightly smaller than Uranus and Neptune, fill the gap between Earth-scale rocks and giant planets. Orbital architectures are equally diverse: many systems are tightly packed, with planets orbiting closer than Mercury’s distance to the Sun, whereas others occupy wide separations so extreme that some worlds are rogue planets detached from any star. Tight-orbiting hot Jupiters, for example WASP-76 b, discovered in 2016 by the transit method, vaporize and leave behind trails of material. TrES-2b, another hot Jupiter, reflects less than one percent of starlight, making it the darkest known planet. Some exoplanets have had their color measured: HD 189733 b appears deep dark blue, while HR 2562 b tips the scales at roughly thirty Jupiter masses. Confirming a single exoplanet requires about 1,000 people-hours of observation and analysis, illustrating the painstaking effort behind every addition to the growing catalog.

What is the surface of an exoplanet like?

Exoplanets have solid surfaces. Some exoplanets are likely to be rocky planets similar to Earth, Mars, Mercury or Venus. An exoplanet has a shallow surface like Earth, an intermediate surface like Venus, or a deep surface like a gas giant. Exoplanets are airless like Mercury, or they have thin atmospheres like Mars, substantial atmospheres like Venus, or life-and-water-friendly atmospheres like Earth. Mid-infrared spectroscopy of exoplanets detects rocky surfaces with certainty. Near-infrared identifies magma oceans or high-temperature lavas, hydrated silicate surfaces, and water ice with a high degree of certainty. Surface features are distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Tracking changes in the atmosphere over time gives insight in processes occurring on the surface of these exoplanets. Small differences in received sunlight during the planet’s orbit determine how reflective the planet’s surface is. The presence of clouds in the exoplanets’ atmosphere is revealed this way. Spectroscopy measurements will tell the basic colour of the exoplanet. The missions Cheops, Plato and Ariel will use this technique to reveal the surface colours of exoplanets. OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (−364 °F). Exoplanets have liquid oceans, lakes made of water, liquid methane or ammonia, and water ice.

What are the timelines for exoplanet formation?

The timelines for exoplanet formation are given in the table below.

Planets form within a few to tens of millions of years of their star forming. The existence of a planetary system is viewed as three phases: circumstellar disk, protoplanetary disk, fully formed planetary system. Each phase is characterized by different physical processes and different durations.

The core accretion model of planet formation begins with the growth and settling of dust grains, followed by the formation of planetesimals that accrete to form a planetary core. Usually planet formation is a bottom-up scheme: small objects build up to form a bigger planet. Jupiter-like exoplanets were initially thought to take nearly 3 to 5 million years to fully form, yet recent observations now suggest the process is closer to about 1 to 2 million years. A large amount of solids is only found when a system is younger than 2 million years; the availability of these building blocks significantly decreases over millions of years.

ALMA’s early observations of young protoplanetary disks, some only about one million years old, reveal surprisingly well-defined structures including prominent rings and gaps, hallmarks of planets. If the giant planet does not migrate, late accretion extends over about 5 × 10⁵ years; rapid inward migration occurs on time scales ~10³ yr. Researchers found that the early formation of these exoplanets aligns with recent evidence suggesting Jupiter formed earlier than previously thought.