Exoplanet

Size-exaggerated artist's conception showing the ratio of planets to stars in our galaxy
Artist's impression of how commonly planets orbit the stars in the Milky Way[1]
Histogram of Discovered Exoplanets each year as of 26 November 2017
Discovered exoplanets each year as of 26 November 2017[2]
Size comparison of Jupiter and the exoplanet TrES-3b
Size comparison of Jupiter and the exoplanet TrES-3b. TrES-3b has an orbital period of only 31 hours[3] and is classified as a Hot Jupiter for being large and close to its star, making it one of the easiest planets to detect by the transit method.
Histogram Chart of Confirmed Exoplanets by distance
NASA histogram chart of confirmed exoplanets by distance

An exoplanet (UK: t/, US: t/)[4] or extrasolar planet is a planet outside our solar system. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such.[5] However, the planets in 2,853 systems, with 633 systems having more than one planet.[6]

The High Accuracy Radial Velocity Planet Searcher (HARPS), since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand[7][8] candidate planets,[9][10] of which about 11% may be false positives.[11]In several cases, multiple planets have been observed around a star.[12]About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c] Assuming there are 200 billion stars in the Milky Way,[d] one can hypothesize that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[13]

The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[14][15] about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets further away in galaxies beyond the local Milky Way galaxy, may exist.[16][17] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[18]

The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[19]

Besides exoplanets, there are also rogue planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants, in which case they are often counted, like WISE 0855−0714, as sub-brown dwarfs.[20] The rogue planets in the Milky Way possibly number in the billions (or more).[21][22]

Nomenclature

Exoplanet HIP 65426b is the first discovered planet around star HIP 65426.[23]

The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the designation is normally formed by taking the name or, more commonly, designation of its parent star and adding a lower case letter.[24] The first planet discovered in a system is given the designation "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

Other Languages
Afrikaans: Eksoplaneet
azərbaycanca: Ekzoplanet
беларуская: Экзапланета
беларуская (тарашкевіца)‎: Экзаплянэта
български: Екзопланета
brezhoneg: Ezplanedenn
Чӑвашла: Экзопланета
čeština: Exoplaneta
dansk: Exoplanet
Deutsch: Exoplanet
euskara: Exoplaneta
français: Exoplanète
한국어: 외계 행성
Հայերեն: Էկզոմոլորակ
हिन्दी: बहिर्ग्रह
Bahasa Indonesia: Planet luar surya
íslenska: Fjarreikistjarna
ქართული: ეგზოპლანეტა
қазақша: Экзопланета
Kiswahili: Sayari ya nje
latviešu: Citplanēta
Lëtzebuergesch: Exoplanéit
lietuvių: Egzoplaneta
Limburgs: Exoplaneet
magyar: Exobolygó
मराठी: परग्रह
Bahasa Melayu: Planet luar suria
Nederlands: Exoplaneet
norsk: Eksoplanet
norsk nynorsk: Ekstrasolar planet
occitan: Exoplaneta
oʻzbekcha/ўзбекча: Ekzosayyora
português: Exoplaneta
română: Exoplanetă
русский: Экзопланета
Scots: Exoplanet
Simple English: Extrasolar planet
slovenščina: Zunajosončni planet
srpskohrvatski / српскохрватски: Ekstrasolarni planet
svenska: Exoplanet
татарча/tatarça: Экзопланета
українська: Екзопланета
Lingua Franca Nova: Planetas estrasolal