# Exoplanet
An exoplanet, or extrasolar planet, is a planet orbiting a star other than the Sun, a brown dwarf, or a stellar remnant. The International Astronomical Union's working definition, amended in August 2018, sets the upper mass at the deuterium-fusion limit, about 13 Jupiter masses for objects of solar metallicity, and requires a mass ratio to the central object below about 1/25; planetary-mass objects orbiting brown dwarfs count if that ratio is met, and the definition applies no matter how the object formed[1](https://arxiv.org/abs/2203.09520). NASA's Exoplanet Archive currently lists 6,445 confirmed exoplanets[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html), a count that passed 6,000 only three years after passing 5,000[3](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/).
| Key fact | Detail |
|---|---|
| Confirmed count | 6,445 planets: 4,781 by transit, 1,203 by radial velocity, 292 by microlensing, 97 by imaging, 6 by astrometry, plus smaller timing and modulation categories[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html) |
| Definition | True mass below ~13 Jupiter masses (deuterium-fusion limit), mass ratio to central object below ~1/25[1](https://arxiv.org/abs/2203.09520) |
| Signal sizes | Earth analog: transit depth ~100 ppm, transit probability ~0.5%, radial-velocity signal ~10 cm/s[4](https://ar5iv.labs.arxiv.org/html/1505.06869) • [5](https://iopscience.iop.org/article/10.1086/598984) |
| Radius gap | Dearth of planets near 1.7–1.8 Earth radii dividing super-Earths (peak ~1.3 R⊕) from sub-Neptunes (peak ~2.4 R⊕)[6](https://www.mdpi.com/2075-4434/6/2/51) • [7](https://arxiv.org/abs/2401.16451) |
| Occurrence | η⊕ estimated at 0.37 (+0.48/−0.21) to 0.88 (+1.28/−0.51) Earth-like planets per Sun-like star[7](https://arxiv.org/abs/2401.16451) |
| Habitable zone | Roughly 0.75–1.4 AU around a Sun-like star, contingent on greenhouse effect, albedo, and carbon cycle[8](https://www.mdpi.com/2078-1547/5/2/296) |
| JWST | Chemistry analyzed for over 100 exoplanet atmospheres; JWST makes spectra possible for planets down to Earth size around the smallest, coolest stars[3](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/) • [9](https://www.pnas.org/doi/10.1073/pnas.2507109122) |
## History of detection
The field's effective beginning came in 1995, when a planet was found around the Sun-like star 51 Pegasi. Scientists found a dozen more in the next three years and about 175 in the first 10 years after that breakthrough; after 20 years the total was nearly 2,000, and it now exceeds 6,000[10](https://science.nasa.gov/exoplanets/30-years-of-exoplanets-compilation/). The first known exoplanet orbits closer to its star than Mercury is to the Sun, an architecture absent from the [Solar System](https://www.edgepedia.org/solar-system)[10](https://science.nasa.gov/exoplanets/30-years-of-exoplanets-compilation/).
[Detection capability](https://www.edgepedia.org/detection-capability) grew through steady instrumental improvement. Radial-velocity precision increased by a factor of 1,000 over 30 years, from 250 m/s in the 1970s to about 0.25 m/s at the time of one review[11](https://www.sciencedirect.com/science/article/pii/S1387647311000443).
## Detection methods
Five methods are in common use: transit photometry, radial velocity, microlensing, astrometry, and direct imaging, each with inherent observational incompleteness and complementary sensitivity to different orbital separations[4](https://ar5iv.labs.arxiv.org/html/1505.06869) • [12](https://science.nasa.gov/exoplanets/facts/). The methods can yield different quantities: astrometry, microlensing, and radial velocities can yield planet masses (radial velocities only the product of mass and the sine of the orbital inclination); transits can yield radii; direct imaging can yield intrinsic luminosity[13](https://link.springer.com/article/10.1007/s12210-025-01335-3).
**Why transits dominate.** About 75 percent of known exoplanets were found by the transit method and 19 percent by radial velocity[14](https://wisconsin.pressbooks.pub/astrobiology/chapter/exoplanets-discoveries/); together the two techniques account for 93 percent of all verified exoplanets[15](https://www.astronomy.com/science/explore-30-years-of-exoplanet-discoveries/). Transits win on scale: Kepler monitored about 150,000 stars from 2009 to 2013. The cost is a strong selection bias. A transit is only seen if the orbit is nearly edge-on, and the alignment probability falls as the orbit grows, so almost all transiting planets orbit closer than 0.1 AU[9](https://www.pnas.org/doi/10.1073/pnas.2507109122). [Radial velocity](https://www.edgepedia.org/radial-velocity) carries its own bias, returning only a minimum mass M sin i; if the orbital inclination is 30 degrees, the true mass is twice the Doppler-derived minimum[4](https://ar5iv.labs.arxiv.org/html/1505.06869). Combining a transit light curve, which gives the inclination, with radial velocities breaks that degeneracy[6](https://www.mdpi.com/2075-4434/6/2/51).
**Signal magnitudes.** A typical hot Jupiter, with radius at least Jupiter's and a roughly 3-day period around a solar-type star, shows a transit depth of about 1 percent, a geometric transit probability of about 10 percent, and a duration of about 3 hours[4](https://ar5iv.labs.arxiv.org/html/1505.06869). The super-Earth and sub-Neptune population found by Kepler is typified by a transit probability of about 2.5 percent, a depth of about 0.1 percent, and a duration of about 6 hours[4](https://ar5iv.labs.arxiv.org/html/1505.06869). An Earth-sized planet in an Earth-like orbit around a solar-type star presents a transit probability of about 0.5 percent, a depth of about 0.01 percent (100 parts per million), and a duration of about 15 hours[4](https://ar5iv.labs.arxiv.org/html/1505.06869); it induces radial-velocity variations of about 10 cm/s and a planet-star contrast ratio of 10⁻¹⁰ to 10⁻⁷[5](https://iopscience.iop.org/article/10.1086/598984).
**Astrometry and microlensing.** [Astrometry](https://www.edgepedia.org/astrometry) detects planets through tiny changes in a star's sky position, a precision that is extremely difficult to achieve; fewer than 10 planets have been found this way as of 2025, though [Gaia](https://www.edgepedia.org/gaia-spacecraft) is the most sensitive astrometric survey ever performed[16](https://sci.esa.int/web/exoplanets/-/60655-detection-methods) • [17](https://www.space.com/astronomy/exoplanets/6-000-and-counting-the-next-30-years-in-the-search-for-exoplanets). Microlensing has yielded 292 confirmed planets[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html).
## Planet properties and classification
Exoplanet categories have no Solar System equivalents at small sizes. One size-based classification places super-Earths between about 1.2 and 2.8 Earth radii, and mini-Neptunes (sub-Neptunes) between about 2.8 and 4 Earth radii[14](https://wisconsin.pressbooks.pub/astrobiology/chapter/exoplanets-discoveries/). The California Kepler Survey found two distinct populations among roughly 2,000 Kepler planets with precise radii: super-Earths peaking near 1.3 Earth radii and sub-Neptunes near 2.4, with a dearth near 1.8 Earth radii suggesting an evaporation valley[6](https://www.mdpi.com/2075-4434/6/2/51). In occurrence-rate space the dip for short-period planets is centered near 1.7 Earth radii and is often used as the dividing line between the two classes; super-Earths are often interpreted as rocky while sub-Neptunes have low densities consistent with a few-percent hydrogen/helium envelope, possibly a stripped atmosphere[7](https://arxiv.org/abs/2401.16451). Sub-Neptunes, spanning 1.5 to 4 Earth radii, are the most abundant known exoplanets, and their bimodal radius distribution suggests distinct formation classes[9](https://www.pnas.org/doi/10.1073/pnas.2507109122). A related feature is the paucity of close-in planets with radii of 4 to 10 Earth radii, sometimes called the hot Neptune desert, which appears to be real and possibly tied to the formation or longevity of planets this size[14](https://wisconsin.pressbooks.pub/astrobiology/chapter/exoplanets-discoveries/).
**The mass–radius degeneracy.** Of the 6,445 confirmed planets, 4,822 have measured radii but only 2,192 have measured masses[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html), so most planets are classified by radius alone. Even when both are known, composition can remain ambiguous: a planet with an atmosphere of 1 to 10 percent of its mass can have the same mass and radius as a water-rich planet[8](https://www.mdpi.com/2078-1547/5/2/296). Empirical mass–radius relations find transitions at about 2 Earth masses, dividing rocky from Neptunian planets, and about 0.41 Jupiter masses, dividing Neptunian from Jovian[6](https://www.mdpi.com/2075-4434/6/2/51).
## By the numbers
The archive's radius bins show where the population sits: 558 planets at or below 1.25 Earth radii, 1,149 at 1.25 to 2, 2,092 at 2 to 6, 788 at 6 to 15, and 235 above 15[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html). Mass bins run from 113 planets at or below 3 Earth masses and 386 at 3 to 10, up to 822 above 300 Earth masses[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html).
By mission, Kepler accounts for 2,787 confirmed planets, K2 for 550, and TESS for 1,008[2](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html). Kepler used a 1-meter space telescope to monitor about 150,000 stars from 2009 to 2013 with 6-hour photometric precision around 10⁻⁴, sufficient to detect about 4,000 planets[7](https://arxiv.org/abs/2401.16451); it was designed as a demographics mission for planets within 1 AU and to measure the frequency of Earth-like planets[18](https://iopscience.iop.org/article/10.3847/1538-3881/acebc8). TESS, performing an all-sky survey of brighter stars, supplied nearly half (about 490) of roughly 1,000 planets in a recent tally[19](https://exoplanetarchive.ipac.caltech.edu/docs/2025news.html).
On how common Earth-like planets are, the most recent η⊕ estimate (Bryson et al. 2021) ranges from about 0.37 (+0.48/−0.21) to 0.88 (+1.28/−0.51) Earth-like planets per Sun-like star, depending on extrapolation assumptions[7](https://arxiv.org/abs/2401.16451).
## Habitability and biosignatures
The habitable zone around a Sun-like star is conventionally placed between roughly 0.75 and 1.4 AU, the region where liquid water can exist on a planetary surface. The definition is contested in scope because the greenhouse effect, albedo, the carbon cycle, and orbital properties also influence a planet's surface conditions[8](https://www.mdpi.com/2078-1547/5/2/296).
JWST's sensitivity, broad wavelength coverage, and detector stability make spectra possible for planets down to Earth size around the smallest, coolest stars[9](https://www.pnas.org/doi/10.1073/pnas.2507109122), and the telescope has analyzed the chemistry of over 100 exoplanet atmospheres[3](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/). Its infrared transit spectroscopy of K2-18 b, a habitable-zone super-Earth, characterized the atmosphere, with dimethyl sulfide discussed as a possible signal[14](https://wisconsin.pressbooks.pub/astrobiology/chapter/exoplanets-discoveries/). That claim is disputed: applying three criteria for biosignature claims, Seager and colleagues conclude that the claimed dimethyl sulfide detection fails each test[9](https://www.pnas.org/doi/10.1073/pnas.2507109122). More broadly, high-fidelity atmospheric observations of tens of exoplanets have opened the era of comparative exoplanetology, and detection of a biosignature is considered conceivable over the next decade[20](https://www.annualreviews.org/content/journals/10.1146/annurev-astro-081817-051846).
## How it compares with the Solar System
Exoplanets occupy nearly all of the parameter space where detection is possible, including close-in super-Earths, mini-Neptunes, and hot Jupiters, unlike the Solar System, where massive planets orbit only in the outer regions[8](https://www.mdpi.com/2078-1547/5/2/296). [Orbital eccentricity](https://www.edgepedia.org/orbital-eccentricity) separates the two samples sharply: Doppler surveys show exoplanet orbits from circular to cometlike, spanning eccentricities from 0 to 0.93 with a median of 0.24 for semimajor axes beyond 0.1 AU, whereas Solar System giant planets have nearly circular orbits[5](https://iopscience.iop.org/article/10.1086/598984) • [8](https://www.mdpi.com/2078-1547/5/2/296).
## Recent developments
The confirmed count passed 6,000 after having passed 5,000 only three years earlier, and stood at 6,022 at the beginning of October 2025[3](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/) • [17](https://www.space.com/astronomy/exoplanets/6-000-and-counting-the-next-30-years-in-the-search-for-exoplanets). JWST-era atmospheric studies now span hot giant planets through temperate sub-Neptunes and rocky planets[21](https://link.springer.com/article/10.1007/s10509-026-04624-x), and in 2025 the archive added JWST secondary-eclipse spectra for the ultra-short-period planet [TOI-561](https://www.edgepedia.org/toi-561) b, with a radius roughly 1.4 times Earth's, plus new spectra for [TRAPPIST-1](https://www.edgepedia.org/trappist-1) d[19](https://exoplanetarchive.ipac.caltech.edu/docs/2025news.html).
## Future prospects
Four instruments are expected to reshape the field in the coming decade: the PLATO mission (ESA, launch planned for 2026), the [Nancy Grace Roman Space Telescope](https://www.edgepedia.org/nancy-grace-roman-space-telescope) (NASA, scheduled for mid-2027), the ARIEL mission (ESA, late 2020s), and the [Extremely Large Telescope](https://www.edgepedia.org/extremely-large-telescope) (ESO)[13](https://link.springer.com/article/10.1007/s12210-025-01335-3) • [15](https://www.astronomy.com/science/explore-30-years-of-exoplanet-discoveries/). PLATO will use 26 telescopes of 20 cm class, collecting photons like a 1-meter-class telescope, aiming to find about 10 habitable-zone Earth-sized planets around Sun-like stars and to determine radii of Earth-like planets to better than 3 percent for a core sample of about 15,000 stars[7](https://arxiv.org/abs/2401.16451) • [13](https://link.springer.com/article/10.1007/s12210-025-01335-3). ARIEL will census roughly a thousand Neptune- and Jupiter-sized atmospheres rather than rocky Earth-like planets[17](https://www.space.com/astronomy/exoplanets/6-000-and-counting-the-next-30-years-in-the-search-for-exoplanets). Roman's coronagraph technology demonstration aims to directly image a planet the size and temperature of Jupiter orbiting a Sun-like star at a similar distance[3](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/). Beyond these, the Habitable Worlds Observatory is planned as a mission built specifically to find and observe Earth-like exoplanets and search for signs of life, requiring new technology to block the starlight in which orbiting planets hide[12](https://science.nasa.gov/exoplanets/facts/) • [22](https://www.smithsonianmag.com/smart-news/exoplanet-discoveries-pass-the-6000-mark-shedding-light-on-how-our-solar-system-compares-to-the-rest-of-the-universe-180987390/). Gaia's astrometric catalog could add thousands of candidates by the end of 2026, and a China-based Earth 2.0 transit survey covering the original [Kepler field](https://www.edgepedia.org/kepler-field) has been proposed[17](https://www.space.com/astronomy/exoplanets/6-000-and-counting-the-next-30-years-in-the-search-for-exoplanets) • [7](https://arxiv.org/abs/2401.16451).
## References
1. [The IAU Working Definition of an Exoplanet (New Astronomy Reviews / arXiv)](https://arxiv.org/abs/2203.09520)
2. [Confirmed Exoplanet Statistics, NASA Exoplanet Archive](https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html)
3. [NASA's Tally of Planets Outside Our Solar System Reaches 6,000, NASA](https://www.nasa.gov/universe/exoplanets/nasas-tally-of-planets-outside-our-solar-system-reaches-6000/)
4. [Exoplanet Detection Techniques (arXiv book chapter)](https://ar5iv.labs.arxiv.org/html/1505.06869)
5. [International Year of Astronomy Invited Review on Exoplanets, PASP](https://iopscience.iop.org/article/10.1086/598984)
6. [Exoplanets: Past, Present, and Future, Galaxies](https://www.mdpi.com/2075-4434/6/2/51)
7. [Planet Occurrence: Doppler and Transit Surveys (arXiv, 2024)](https://arxiv.org/abs/2401.16451)
8. [The Detection and Characterization of Extrasolar Planets, Challenges](https://www.mdpi.com/2078-1547/5/2/296)
9. [Characterization of exoplanets in the James Webb Space Telescope era, PNAS](https://www.pnas.org/doi/10.1073/pnas.2507109122)
10. [30 Years of Exoplanets, NASA Science](https://science.nasa.gov/exoplanets/30-years-of-exoplanets-compilation/)
11. [From 51 Peg to Earth-type planets, ScienceDirect proceedings](https://www.sciencedirect.com/science/article/pii/S1387647311000443)
12. [Exoplanets — In Depth / Facts, NASA Science](https://science.nasa.gov/exoplanets/facts/)
13. [The discovery of exoplanets: towards understanding planet formation, Rendiconti Lincei (2025)](https://link.springer.com/article/10.1007/s12210-025-01335-3)
14. [Searching for Habitable Planets – Cosmic Origins, University of Wisconsin pressbooks](https://wisconsin.pressbooks.pub/astrobiology/chapter/exoplanets-discoveries/)
15. [Explore 30 years of exoplanet discoveries, Astronomy.com](https://www.astronomy.com/science/explore-30-years-of-exoplanet-discoveries/)
16. [Exoplanet detection methods, ESA Science & Technology](https://sci.esa.int/web/exoplanets/-/60655-detection-methods)
17. [6,000 and counting: The next 30 years in the search for exoplanets, Space.com](https://www.space.com/astronomy/exoplanets/6-000-and-counting-the-next-30-years-in-the-search-for-exoplanets)
18. [A Unified Treatment of Kepler Occurrence to Trace Planet Evolution. I., AJ](https://iopscience.iop.org/article/10.3847/1538-3881/acebc8)
19. [2025 Exoplanet Archive News, NASA Exoplanet Archive](https://exoplanetarchive.ipac.caltech.edu/docs/2025news.html)
20. [Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects, Annual Review of Astronomy and Astrophysics](https://www.annualreviews.org/content/journals/10.1146/annurev-astro-081817-051846)
21. [Insights into the diversity of exoplanet atmospheres in the era of JWST, Astrophysics and Space Science](https://link.springer.com/article/10.1007/s10509-026-04624-x)
22. [Exoplanet Discoveries Pass the 6,000 Mark, Smithsonian Magazine](https://www.smithsonianmag.com/smart-news/exoplanet-discoveries-pass-the-6000-mark-shedding-light-on-how-our-solar-system-compares-to-the-rest-of-the-universe-180987390/)
---
*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Cosmology and observation › Observational techniques: astrometry, photometry, spectroscopy*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license