Fomalhaut b (Dagon), 25 light-years away, with its parent star Fomalhaut blacked out, as pictured by Hubble in 2012.[1] In 2020 this object was determined to be an expanding debris cloud from a collision of asteroids rather than a planet.[2]
Distribution of nearest known exoplanets as of March 2018
There are 4,160 known exoplanets, or planets outside the Solar System that orbit a star, as of January 1, 2020; only a small fraction of these are located in the vicinity of the Solar System.[3] Within 10 parsecs (32.6 light-years), there are 104 exoplanets listed as confirmed by the NASA Exoplanet Archive.[note 1][4] Among the over 500 known stars and brown dwarfs within 10 parsecs,[5][note 2] around 60 have been confirmed to have planetary systems; 51 stars in this range are visible to the naked eye,[note 3][7] eight of which have planetary systems.
The first report of an exoplanet within this range was in 1998 for a planet orbiting around Gliese 876 (15.3 light-years (ly) away), and the latest as of 2023 are two around Gliese 367 (30.7 ly). The closest exoplanets are those found orbiting the star closest to the Solar System, which is Proxima Centauri 4.25 light-years away. The first confirmed exoplanet discovered in the Proxima Centauri system was Proxima Centauri b, in 2016. HD 219134 (21.6 ly) has six exoplanets, the highest number discovered for any star within this range.
Most known nearby exoplanets orbit close to their stars. A majority are significantly larger than Earth, but a few have similar masses, including planets around YZ Ceti, Gliese 367, and Proxima Centauri which may be less massive than Earth. Several confirmed exoplanets are hypothesized to be potentially habitable, with Proxima Centauri b and GJ 1002 b (15.8 ly) considered among the most likely candidates.[8] The International Astronomical Union has assigned proper names to some known extrasolar bodies, including nearby exoplanets, through the NameExoWorlds project. Planets named in the 2015 event include the planets around Epsilon Eridani (10.5 ly) and Fomalhaut,[note 4][11] while planets named in the 2022 event include those around Gliese 436, Gliese 486, and Gliese 367.[12]
Unlike for bodies within the Solar System, there is no clearly established method for officially recognizing an exoplanet. According to the International Astronomical Union, an exoplanet should be considered confirmed if it has not been disputed for five years after its discovery.[90] There have been examples where the existence of exoplanets has been proposed, but even after follow-up studies their existence is still considered doubtful by some astronomers. Such cases include Wolf 359 (7.9 ly, in 2019),[23]LHS 288 (15.8 ly, in 2007),[91]Gliese 682 (16.3 ly, in 2014),[47]40 Eridani A (16.3 ly, in 2018),[92][72]
and GJ 1151 (26.2 ly, in 2021).[93][94][95]
There are also several instances where proposed exoplanets were later disproved by subsequent studies, including candidates around Alpha Centauri B (4.36 ly),[96]Barnard's Star (5.96 ly),[97][98]Kapteyn's Star (12.8 ly),[99]Van Maanen 2 (14.1 ly),[100]Groombridge 1618 (15.9 ly),[101]AD Leonis (16.2 ly),[102]VB 10 (19.3 ly),[103] and Fomalhaut (25.1 ly).[2]
In 2021, a candidate planet was detected around Vega, though it has yet to be confirmed.[104] Another candidate planet, Candidate 1, was directly imaged around Alpha Centauri A, though it may also be a clump of asteroids or an artifact of the discovery mechanism.[105]
The Working Group on Extrasolar Planets of the International Astronomical Union adopted in 2003 a working definition on the upper limit for what constitutes a planet: not being massive enough to sustain thermonuclear fusion of deuterium. Some studies have calculated this to be somewhere around 13 times the mass of Jupiter, and therefore objects more massive than this are usually classified as brown dwarfs.[106] Some proposed candidate exoplanets have been shown to be massive enough to fall above the threshold, and thus are likely brown dwarfs, as is the case for: SCR 1845-6357 B (13.1 ly),[107]SDSS J1416+1348 B (30.3 ly),[108] and WISE 1217+1626 B (30 ly).[109]
Excluded from the current list are known examples of potential free-floating sub-brown dwarfs, or "rogue planets", which are bodies that are too small to undergo fusion yet they do not revolve around a star. Known such examples include: WISE 0855–0714 (7.4 ly),[110] UGPS 0722-05, (13.4 ly)[111]WISE 1541−2250 (18.6 ly),[112] and SIMP J01365663+0933473 (20.0 ly).[113]
↑Listed values are primarily taken from NASA Exoplanet Archive,[4] but other databases include a few additional exoplanet entries tagged as "Confirmed" that have yet to be compiled into the NASA archive. Such databases include:
↑For reference, the 100th closest known star system in April 2021 was EQ Pegasi (20.4 ly).[5]
↑According to the Bortle scale, an astronomical object is visible to the naked eye under "typical" dark-sky conditions in a rural area if it has an apparent magnitude smaller than +6.5. To the unaided eye, the limiting magnitude is +7.6 to +8.0 under "excellent" dark-sky conditions (with effort).[6]
↑The star Epsilon Eridani was named Ran (after Rán, the Norse goddess of the sea), and the planet Epsilon Eridani b was named AEgir (after Ægir, Rán's husband),[9] while the planet Fomalhaut b was named Dagon (after Dagon, an ancient Syrian “fish god”[10]).[11]
↑Exoplanet naming convention assigns uncapitalized letters starting from b to each planet based on chronological order of their initial report, and in increasing order of distance from the parent star for planets reported at the same time. Omitted letters signify planets that have yet to be confirmed, or planets that have been retracted altogether.
↑Most reported exoplanet masses have very large error margins (typically, between 10% and 30%). The mass of an exoplanet has generally been inferred from measurements on changes in the radial velocity of the host star, but this kind of measurement only allows for an estimate on the exoplanet's orbital parameters, but not on their orbital inclination (i). As such, most exoplanets only have an estimated minimum mass (Mreal*sin(i)), where their true masses are statistically expected to come close to this minimum, with only about 13% chance for the mass of an exoplanet to be more than double its minimum mass.[13]
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