# Planetary system
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{{Short description|Set of non-stellar objects in orbit around a star}}
{{redirect|Solar systems|the Sun and its planetary system|Solar System|a system of stars|Star system|the solar power company|Solar Systems (company)}}
{{Use mdy dates|date=April 2019}}
thumb|250px|An artist's concept of a planetary system
A '''planetary system''' consists of a set of non-stellar [bodies](/source/Astronomical_object) which are [gravitationally](/source/gravity) bound to and in [orbit](/source/orbit) of a [star](/source/star) or [star system](/source/star_system). Generally speaking, such systems will include [planet](/source/planet)s, and may include other objects such as [dwarf planet](/source/dwarf_planet)s, [asteroid](/source/asteroid)s, [natural satellite](/source/natural_satellite)s, [meteoroid](/source/meteoroid)s, [comet](/source/comet)s, [planetesimal](/source/planetesimal)s,[{{Cite book |last=Darling |first=David J. |title=The universal book of astronomy: from the Andromeda Galaxy to the zone of avoidance |date=2004 |publisher=Wiley |isbn=978-0-471-26569-6 |location=Hoboken (NJ) |page=394}}][{{Cite book |title=Collins dictionary astronomy |date=2000 |publisher=HarperCollins |isbn=978-0-00-710297-6 |editor-last=Illingworth |editor-first=Valerie |edition=2 |location=Glasgow |page=314}}] and [circumstellar disk](/source/circumstellar_disk)s. The [Solar System](/source/Solar_System) is an example of a planetary system, in which [Earth](/source/Earth), seven other planets, and other celestial objects are bound to and revolve around the [Sun](/source/Sun).[p. 382, ''Collins Dictionary of Astronomy''.][{{Cite book |title=A dictionary of astronomy |date=2003 |publisher=Oxford University Press |isbn=978-0-19-860513-3 |editor-last=Ridpath |editor-first=Ian |edition=Rev. |series=Oxford paperback reference |location=Oxford; New York |page=420}}] The term '''exoplanetary system''' is sometimes used in reference to planetary systems other than the Solar System. By convention planetary systems are named after their host, or parent, star, as is the case with the Solar System being named after "Sol" (Latin for sun).
{{Extrasolar planet counts|full}} [Debris disk](/source/Debris_disk)s are known to be common while other objects are more difficult to observe.
Of particular interest to [astrobiology](/source/astrobiology) is the [habitable zone](/source/habitable_zone) of planetary systems where planets could have surface liquid water, and thus, the capacity to support Earth-like life.
==Definition==
The [International Astronomical Union](/source/International_Astronomical_Union) (IAU) has described a planetary system as the system of planets orbiting one or more stars, [brown dwarf](/source/brown_dwarf)s or [stellar remnant](/source/stellar_remnant)s. The IAU and [NASA](/source/NASA) consider the [Solar System](/source/Solar_System) a planetary system.[{{cite web | title=IAU Office of Astronomy for Education | website=IAU Office of Astronomy for Education | url=https://astro4edu.org/resources/glossary/term/468/ | access-date=July 10, 2025}}][{{cite web | title=Solar System: Facts | website=NASA Science | date=November 13, 2017 | url=https://science.nasa.gov/solar-system/solar-system-facts/ | access-date=July 10, 2025}}]
Other definitions of planetary system explicitly include all bodies gravitationally bound to one or more stars.[{{cite book | last=Pierrehumbert | quote=The term 'planetary system' has begun to gain currency to describe such objects, and it is the term we adopt to refer to a star and all the bodies gravitationally bound to it—the planets whether rocky, gassy, or icy, their moons, the asteroids, comets, and the far flung icy bodies that make up Kuiper Belts. Our own planetary system contains only one star, but other planetary systems commonly contain two or even three stars.|first=Raymond T. | title=Planetary Systems: A Very Short Introduction | chapter=1. Beginnings | publisher=Oxford University Press | date=December 9, 2021 | isbn=978-0-19-884112-8 | doi=10.1093/actrade/9780198841128.003.0001 | url=https://academic.oup.com/book/37015/chapter/322383653 | access-date=July 10, 2025 | pages=1–13}}]
==History==
===Heliocentrism===
[Heliocentrism](/source/Heliocentrism) is a planetary model that places the Sun is at the center of the universe, as opposed to [geocentrism](/source/geocentrism) (placing Earth at the center of the universe).
The idea was first proposed in [Western philosophy](/source/Western_philosophy) and [Greek astronomy](/source/Greek_astronomy) as early as the 3rd century BC by [Aristarchus of Samos](/source/Aristarchus_of_Samos),[Dreyer (1953), [https://archive.org/stream/historyofplaneta00dreyuoft#page/n148/mode/2up pp.135–48]; Linton (2004), [https://books.google.com/books?id=B4br4XJFj0MC&pg=PA38 pp.38–9)]. The work of Aristarchus's in which he proposed his heliocentric system has not survived. We only know of it now from a brief passage in [Archimedes](/source/Archimedes)'s ''[The Sand Reckoner](/source/The_Sand_Reckoner)''.] but received no support from most other ancient astronomers.
Some also interpret [Aryabhatta](/source/Aryabhatta)'s writings in [Āryabhaṭīya](/source/%C4%80ryabha%E1%B9%AD%C4%ABya) as implicitly heliocentric, although this has also been rebutted.[Noel Swerdlow, "Review: A Lost Monument of Indian Astronomy," ''Isis'', 64 (1973): 239–243.]
===Discovery of the Solar System===
{{Main|Discovery and exploration of the Solar System}}
[[File:De Revolutionibus manuscript p9b.jpg|thumb|190px|[Heliocentric model](/source/Heliocentric_model) of the [Solar System](/source/Solar_System) in [Copernicus](/source/Copernicus)' [manuscript](/source/manuscript)]]
''[De revolutionibus orbium coelestium](/source/De_revolutionibus_orbium_coelestium)'' by [Nicolaus Copernicus](/source/Nicolaus_Copernicus), published in 1543, presented the first mathematically predictive heliocentric model of a planetary system. 17th-century successors [Galileo Galilei](/source/Galileo_Galilei), [Johannes Kepler](/source/Johannes_Kepler), and [Sir Isaac Newton](/source/Sir_Isaac_Newton) developed an understanding of [physics](/source/physics) which led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth.
===Speculation on extrasolar planetary systems===
In the 16th century the Italian philosopher [Giordano Bruno](/source/Giordano_Bruno), an early supporter of the [Copernican](/source/Nicolaus_Copernicus) theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.["Cosmos" in
''The New Encyclopædia Britannica'' (15th edition, Chicago, 1991) '''16''':787:2a. "For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600."]
In the 18th century, the same possibility was mentioned by [Sir Isaac Newton](/source/Sir_Isaac_Newton) in the "[General Scholium](/source/General_Scholium)" that concludes his ''[Principia](/source/Philosophiae_Naturalis_Principia_Mathematica)''. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of ''One''."[
{{Cite book |last1 = Newton
|first1 = Isaac
|first2 = I. Bernard
|last2 = Cohen
|first3 = Anne
|last3 = Whitman
|title = The Principia: A New Translation and Guide
|publisher = University of California Press
|date = 1999
|orig-date = First published 1713
|page = 940
|isbn = 0-520-20217-1
|url = https://archive.org/details/keytonewtonsdyna0000brac
|url-access = registration
}}
]
His theories gained popularity through the 19th and 20th centuries despite a lack of supporting evidence. Long before their confirmation by astronomers, conjecture on the nature of planetary systems had been a focus of the [search for extraterrestrial intelligence](/source/search_for_extraterrestrial_intelligence) and [has been a prevalent theme in fiction](/source/Extrasolar_planets_in_fiction), particularly science fiction.
===Detection of exoplanets===
The first confirmed detection of an [exoplanet](/source/exoplanet) was in 1992, with the discovery of several terrestrial-mass planets orbiting the [pulsar](/source/pulsar) [PSR B1257+12](/source/PSR_B1257%2B12). The first confirmed detection of exoplanets of a [main-sequence](/source/main-sequence) star was made in 1995, when a giant planet, [51 Pegasi b](/source/51_Pegasi_b), was found in a four-day orbit around the nearby [G-type star](/source/G-type_star) [51 Pegasi](/source/51_Pegasi). The frequency of detections has increased since then, particularly through advancements in [methods of detecting extrasolar planets](/source/methods_of_detecting_extrasolar_planets) and dedicated planet-finding programs such as the [Kepler mission](/source/Kepler_(spacecraft)).
==Origin and evolution==
{{See also|Nebular hypothesis|Planetary migration|Formation and evolution of the Solar System}}
[[File:Opo0113i.jpg|thumb|upright=1.5|Illustration of the dynamics of a [proplyd](/source/proplyd)]]
Planetary systems come from [protoplanetary disk](/source/protoplanetary_disk)s that form around stars as part of the process of [star formation](/source/star_formation).
During formation of a system, much material is gravitationally-scattered into distant orbits, and some planets are ejected completely from the system, becoming [rogue planet](/source/rogue_planet)s.
===Evolved systems===
====High-mass stars====
Planets orbiting [pulsar](/source/pulsar)s have been discovered. Pulsars are the remnants of the [supernova](/source/supernova) explosions of high-mass stars, but a planetary system that existed before the supernova would likely be mostly destroyed. Planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star, or in some cases the supernova would [kick](/source/pulsar_kick) the pulsar itself out of the system at high velocity so any planets that had survived the explosion would be left behind as free-floating objects. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may form in an [accretion disk](/source/accretion_disk) of fallback matter surrounding a pulsar.[{{cite journal|bibcode=1993ASPC...36..149P|title=Planet formation scenarios|last1=Podsiadlowski|first1=Philipp|volume=36|date=1993|page=149|journal=In: Planets Around Pulsars; Proceedings of the Conference}}] Fallback disks of matter that failed to escape orbit during a supernova may also form planets around [black hole](/source/black_hole)s.[{{cite journal |last1=Perna |first1=Rosalba |title=The Fate of Fallback Matter Around Newly Born Compact Objects |date=2013-12-17 |last2=Duffell |first2=Paul |last3=Cantiello |first3=Matteo |last4=MacFadyen |first4=Andrew |journal=The Astrophysical Journal |volume=781 |issue=2 |page=119 |doi=10.1088/0004-637X/781/2/119 |arxiv=1312.4981 |author-link1=Rosalba Perna}}]
====Lower-mass stars====
Many low-mass stars are expected to have rocky planets, with their planetary systems primarily consisting of rock- and ice-based bodies. This is because low-mass stars have less material in their planetary disks, making it unlikely that the planetesimals within will reach the critical mass necessary to form gas giants. The planetary systems of low-mass stars also tend to be compact, as such stars tend to have lower temperatures, resulting in the formation of protoplanets closer to the star.[{{Cite journal |last1=Miguel |first1=Y |last2=Cridland |first2=A |last3=Ormel |first3=C W |last4=Fortney |first4=J J |last5=Ida |first5=S |date=2019-10-30 |title=Diverse outcomes of planet formation and composition around low-mass stars and brown dwarfs |journal=Monthly Notices of the Royal Astronomical Society |article-number=stz3007 |language=en |doi=10.1093/mnras/stz3007 |doi-access=free |issn=0035-8711|arxiv=1909.12320 }}] [[File:Protoplanetary discs observed with SPHERE.jpg|thumb|Protoplanetary discs observed with the [Very Large Telescope](/source/Very_Large_Telescope).[{{cite web|title=Sculpting Solar Systems - ESO's SPHERE instrument reveals protoplanetary discs being shaped by newborn planets|url=https://www.eso.org/public/news/eso1640/|website=www.eso.org|access-date=December 7, 2016}}]]]
As stars evolve and turn into [red giant](/source/red_giant)s, [asymptotic giant branch](/source/asymptotic_giant_branch) stars, and eventually [planetary nebula](/source/planetary_nebula)e, they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are.[{{cite news |last=Ferreira |first=Becky |title=It's the End of a World as We Know It - Astronomers spotted a dying star swallowing a large planet, a discovery that fills in a "missing link" in understanding the fates of Earth and many other planets. |url=https://www.nytimes.com/2023/05/03/science/star-eating-planet.html |date=3 May 2023 |work=[The New York Times](/source/The_New_York_Times) |url-status=live |archive-url=https://archive.today/20230503155540/https://www.nytimes.com/2023/05/03/science/star-eating-planet.html |archive-date=3 May 2023 |access-date=3 May 2023 }}][{{cite news |last=Ferreira |first=Becky |title=The Juicy Secrets of Stars That Eat Their Planets - As scientists study thousands of planets around the galaxy, they are learning more about worlds that get swallowed up by their stars. |url=https://www.nytimes.com/2022/08/19/science/stars-planets-engulfment.html |date=19 August 2022 |work=[The New York Times](/source/The_New_York_Times) |access-date=19 August 2022 }}] As the star loses mass, planets that are not engulfed move further out from the star.
If an evolved star is in a binary or multiple system, then the mass it loses can transfer to another star, forming new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets, which may also be affected by the mass transfer.
=== Planet capture ===
[Free-floating planet](/source/Rogue_planet)s in open clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster size, and for a given cluster size it increases with the host/primary{{Clarify|reason=Please reword to avoid the use of a slash; this is ambiguous.|date=August 2021}} mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Some planet–host metallicity correlation may still exist due to the common origin of the stars from the same cluster. Planets would be unlikely to be captured around [neutron star](/source/neutron_star)s because these are likely to be ejected from the cluster by a [pulsar kick](/source/pulsar_kick) when they form. Planets could even be captured around other planets to form free-floating planet binaries. After the cluster has dispersed some of the captured planets with orbits larger than 106 AU would be slowly disrupted by the [galactic tide](/source/galactic_tide) and likely become free-floating again through encounters with other field stars or giant [molecular cloud](/source/molecular_cloud)s.[{{cite journal |last1=Perets |first1=Hagai B. |last2=Kouwenhoven |first2=M. B. N. |date=2012-03-13 |title=On the Origin of Planets at Very Wide Orbits from the Recapture of Free Floating Planets |journal=The Astrophysical Journal |volume=750 |issue=1 |page=83 |arxiv=1202.2362 |bibcode=2012ApJ...750...83P |doi=10.1088/0004-637X/750/1/83}}]
==System architectures==
The Solar System consists of an inner region of small [rocky planets](/source/rocky_planets) and outer region of large [giant planet](/source/giant_planet)s. However, other planetary systems can have quite different architectures. At present,{{When|date=August 2021}} few systems have been found to be analogous to the Solar System with small terrestrial planets in the inner region, as well as a gas giant with a relatively circular orbit, which suggests that this configuration is uncommon.[{{Cite web |date=2021-02-08 |title=We're in the cosmic 1% – planetplanet |url=https://planetplanet.net/2021/02/08/the-cosmic-one-percent/ |access-date=2025-01-25 |language=en-US}}] More commonly, systems consisting of multiple [Super-Earth](/source/Super-Earth)s have been detected.[[http://www.astro.washington.edu/users/rory/publications/br04.pdf Types and Attributes] at Astro Washington.com.][{{Cite journal |last1=Borucki |first1=William J. |last2=Koch |first2=David G. |last3=Basri |first3=Gibor |last4=Batalha |first4=Natalie |last5=Brown |first5=Timothy M. |last6=Bryson |first6=Stephen T. |last7=Caldwell |first7=Douglas |last8=Christensen-Dalsgaard |first8=Jørgen |last9=Cochran |first9=William D. |last10=DeVore |first10=Edna |last11=Dunham |first11=Edward W. |last12=Gautier |first12=Thomas N., III |last13=Geary |first13=John C. |last14=Gilliland |first14=Ronald |last15=Gould |first15=Alan |date=2011-07-01 |title=Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data |url=https://ui.adsabs.harvard.edu/abs/2011ApJ...736...19B/abstract |journal=The Astrophysical Journal |volume=736 |issue=1 |page=19 |doi=10.1088/0004-637X/736/1/19 |issn=0004-637X|arxiv=1102.0541 |bibcode=2011ApJ...736...19B }}] These super-Earths are usually very close to their star, with orbits smaller than that of [Mercury](/source/Mercury_(planet)). Other systems have been found to have a [hot Jupiter](/source/hot_Jupiter) gas giant very close to the star. Theories such as [planetary migration](/source/planetary_migration) or scattering have been proposed to explain the formation of large planets close to their parent stars.[{{cite journal|author=Stuart J. Weidenschilling|author2=Francesco Marzari|name-list-style=amp|title=Gravitational scattering as a possible origin for giant planets at small stellar distances|journal=Nature |volume=384|pages=619–621|date=1996|doi=10.1038/384619a0|pmid=8967949|issue=6610 |bibcode = 1996Natur.384..619W |s2cid=4304777|hdl=11577/123794|hdl-access=free}}][{{Cite web |date=2014-03-03 |title=Hot Super-Earths (and mini-Neptunes)! – planetplanet |url=https://planetplanet.net/2014/03/03/hot-super-earths-and-mini-neptunes/ |access-date=2025-01-25 |language=en-US}}] Overall, studies suggest that architectures of planetary systems are dependent on the conditions of their initial formation.[{{cite journal|last1=Hasegawa|first1=Yasuhiro|last2=Pudritz|first2=Ralph E.|title=The origin of planetary system architectures - I. Multiple planet traps in gaseous discs|journal=Monthly Notices of the Royal Astronomical Society|volume=417|issue=2|year=2011|pages=1236–1259|issn=0035-8711|doi=10.1111/j.1365-2966.2011.19338.x|doi-access=free |arxiv = 1105.4015 |bibcode = 2011MNRAS.417.1236H |s2cid=118843952}}]
===Classification===
Planetary system architectures may be partitioned into four classes based on how the mass of the planets is distributed around the [host star](/source/host_star):[{{Cite journal |last1=Mishra |first1=Lokesh |last2=Alibert |first2=Yann |last3=Udry |first3=Stéphane |last4=Mordasini |first4=Christoph |date=2023-02-01 |title=Framework for the architecture of exoplanetary systems - I. Four classes of planetary system architecture |url=https://www.aanda.org/articles/aa/abs/2023/02/aa43751-22/aa43751-22.html |journal=Astronomy & Astrophysics |language=en |volume=670 |pages=A68 |doi=10.1051/0004-6361/202243751 |issn=0004-6361|doi-access=free |arxiv=2301.02374 |bibcode=2023A&A...670A..68M }}][{{Cite journal |last1=Mishra |first1=Lokesh |last2=Alibert |first2=Yann |last3=Udry |first3=Stéphane |last4=Mordasini |first4=Christoph |date=2023-02-01 |title=Framework for the architecture of exoplanetary systems - II. Nature versus nurture: Emergent formation pathways of architecture classes |url=https://www.aanda.org/articles/aa/abs/2023/02/aa44705-22/aa44705-22.html |journal=Astronomy & Astrophysics |language=en |volume=670 |pages=A69 |doi=10.1051/0004-6361/202244705 |issn=0004-6361|doi-access=free |arxiv=2301.02373 |bibcode=2023A&A...670A..69M }}]
In '''Similar''' systems, the masses of all the planets are similar to each other. This architecture class is the most commonly-observed in our galaxy. [TRAPPIST-1](/source/TRAPPIST-1) is an example of a Similar system. Planets in Similar systems are said to be like 'peas in a pod', and the phrase now refers to a set of specific configuration characteristics.[{{Cite journal |last1=Mishra |first1=Lokesh |last2=Alibert |first2=Yann |last3=Leleu |first3=Adrien |last4=Emsenhuber |first4=Alexandre |last5=Mordasini |first5=Christoph |last6=Burn |first6=Remo |last7=Udry |first7=Stéphane |last8=Benz |first8=Willy |date=2021-12-01 |title=The New Generation Planetary Population Synthesis (NGPPS) VI. Introducing KOBE: Kepler Observes Bern Exoplanets - Theoretical perspectives on the architecture of planetary systems: Peas in a pod |url=https://www.aanda.org/articles/aa/abs/2021/12/aa40761-21/aa40761-21.html |journal=Astronomy & Astrophysics |language=en |volume=656 |pages=A74 |doi=10.1051/0004-6361/202140761 |issn=0004-6361|doi-access=free |arxiv=2105.12745 |bibcode=2021A&A...656A..74M }}]
A 'peas in a pod' system will have planets that are similar or ordered in size, similar and ordered in mass, and tend to display "packing". Packing refers to the tendency of smaller planets to be closer together, and of larger planets to have larger orbital spacing. Lastly, 'peas in a pod' systems tend to display similar spacing between a pair of adjacent planets and the next pair of adjacent planets.
'''Mixed''' systems are planetary systems in which the masses of the planets show larger increasing or decreasing variations. [Gliese 876](/source/Gliese_876) and [Kepler-89](/source/Kepler-89) are examples of mixed systems.
'''Anti-Ordered''' systems have their massive planets close to the host star and the smaller planets further away. There are currently no known examples of this architecture class.
'''Ordered''' systems have their planets ordered such that the less massive ones are closer to the star and the more massive planets are further from the star, with the mass of each planet increasing with distance from the star. The [Solar System](/source/Solar_System), with small [rocky planets](/source/Terrestrial_planet) in the inner part and [giant planet](/source/giant_planet)s in the outer part, is a type of Ordered system.
===Components===
====Planets and stars====
{{main|Planet-hosting stars}}
300px|thumb|right|The Morgan-Keenan spectral classification
Most known exoplanets orbit stars roughly similar to the [Sun](/source/Sun): that is, [main-sequence stars](/source/main_sequence) of [spectral categories](/source/stellar_classification) F, G, or K. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that lower-mass stars ([red dwarf](/source/red_dwarf)s, of [spectral category](/source/stellar_classification) M) are less likely to have planets massive enough to be detected by the [radial-velocity method](/source/radial-velocity_method).[{{Cite journal| author=Andrew Cumming|date=2008|title=The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets|journal=Publications of the Astronomical Society of the Pacific|volume=120| issue=867|pages=531–554| arxiv=0803.3357|doi=10.1086/588487| bibcode=2008PASP..120..531C| author2=R. Paul Butler| author3=Geoffrey W. Marcy| last4=Vogt| first4=Steven S.| last5=Wright| first5=Jason T.| last6=Fischer| first6=Debra A.|s2cid=10979195| author-link3=Debra Fischer | display-authors=3}}][{{Cite journal |doi=10.1051/0004-6361:200500193 |title=The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581 |journal=Astronomy and Astrophysics |volume=443 |issue=3 |pages=L15–L18 |year=2005 |last1=Bonfils |first1=Xavier |last2=Forveille |first2=Thierry |last3=Delfosse |first3=Xavier |last4=Udry |first4=Stéphane |last5=Mayor |first5=Michel |last6=Perrier |first6=Christian |last7=Bouchy |first7=François |last8=Pepe |first8=Francesco |last9=Queloz |first9=Didier |last10=Bertaux |first10=Jean-Loup |bibcode=2005A&A...443L..15B |arxiv=astro-ph/0509211 |s2cid=59569803 }}] Nevertheless, several tens of planets around red dwarfs have been discovered by the [Kepler space telescope](/source/Kepler_space_telescope) by the [transit method](/source/transit_method), which can detect smaller planets.
Exoplanetary systems may also feature planets extremely different from those in the Solar System, such as Hot Jupiters, Hot Neptunes, and Super-Earths.[{{Cite web |title=The Different Kinds of Exoplanets You Meet in the Milky Way |url=https://www.planetary.org/articles/the-different-kinds-of-exoplanets-you-meet-in-the-milky-way |access-date=2025-10-25 |website=The Planetary Society |language=en}}] Hot Jupiters and Hot Neptunes are gas giants, like their namesakes, but orbit close to their stars and have orbital periods on the order of a few days.[{{Cite journal |last1=Fortney |first1=Jonathan J. |last2=Dawson |first2=Rebekah I. |last3=Komacek |first3=Thaddeus D. |date=2021 |title=Hot Jupiters: Origins, Structure, Atmospheres |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2020JE006629 |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=3 |article-number=e2020JE006629 |doi=10.1029/2020JE006629 |arxiv=2102.05064 |bibcode=2021JGRE..12606629F |issn=2169-9100}}] Super-Earths are planets that have a mass between that of Earth and planets like Neptune and Uranus, and can be made of rock and gas. There is a lot of variety among Super-Earths, with planets ranging from water worlds to mini-Neptunes.[{{Cite web |date=2020-10-22 |title=What Is a Super-Earth? - NASA Science |url=https://science.nasa.gov/exoplanets/super-earth/ |access-date=2025-10-25 |language=en-US}}]
====Circumstellar disks and dust structures====
{{main|Circumstellar disk}}
[[File:NASA-14114-HubbleSpaceTelescope-DebrisDisks-20140424.jpg|thumb|250px|right|[Debris disks](/source/Debris_disks) detected in [HST](/source/Hubble_Space_Telescope) archival images of young stars, ''HD 141943'' and ''HD 191089'', using improved imaging processes (April 24, 2014).]]
After planets, circumstellar disks are one of the most commonly-observed properties of planetary systems, particularly of young stars. The Solar System possesses at least four major circumstellar disks (the [asteroid belt](/source/asteroid_belt), [Kuiper belt](/source/Kuiper_belt), [scattered disc](/source/scattered_disc), and [Oort cloud](/source/Oort_cloud)) and clearly-observable disks have been detected around nearby solar analogs including [Epsilon Eridani](/source/Epsilon_Eridani) and [Tau Ceti](/source/Tau_Ceti). Based on observations of numerous similar disks, they are assumed to be quite common attributes of stars on the [main sequence](/source/main_sequence).
[Interplanetary dust cloud](/source/Interplanetary_dust_cloud)s have been studied in the Solar System and analogs are believed to be present in other planetary systems. Exozodiacal dust, an [exoplanet](/source/exoplanet)ary analog of [zodiacal dust](/source/zodiacal_dust), the 1–100 micrometre-sized grains of [amorphous carbon](/source/amorphous_carbon) and [silicate](/source/silicate) dust that fill the plane of the Solar System[{{cite journal
|author1=Stark, C. |author2=Kuchner, M. | title=The Detectability of Exo-Earths and Super-Earths Via Resonant Signatures in Exozodiacal Clouds
| journal=The Astrophysical Journal | year=2008 | volume=686
| issue = 1
| pages=637–648
| bibcode=2008ApJ...686..637S
| doi = 10.1086/591442 |arxiv = 0810.2702 |s2cid=52233547 }}] has been detected around the [51 Ophiuchi](/source/51_Ophiuchi), [Fomalhaut](/source/Fomalhaut),[{{cite journal
|author1=Lebreton, J. |author2=van Lieshout, R. |author3=Augereau, J.-C. |author4=Absil, O. |author5=Mennesson, B. |author6=Kama, M. |author7=Dominik, C. |author8=Bonsor, A. |author9=Vandeportal, J. |author10=Beust, H. |author11=Defrère, D. |author12=Ertel, S. |author13=Faramaz, V. |author14=Hinz, P. |author15=Kral, Q. |author16=Lagrange, A.-M. |author17=Liu, W. |author18=Thébault, P. | title= An interferometric study of the Fomalhaut inner debris disk. III. Detailed models of the exozodiacal disk and its origin
| journal=Astronomy and Astrophysics | year=2013 | volume=555
| pages=A146
| bibcode=2013A&A...555A.146L
| doi = 10.1051/0004-6361/201321415 |arxiv = 1306.0956 |s2cid=12112032 }}][{{cite journal
|author1=Absil, O. |author2=Le Bouquin, J.-B. |author3=Berger, J.-P. |author4=Lagrange, A.-M. |author5=Chauvin, G. |author6=Lazareff, B. |author7=Zins, G. |author8=Haguenauer, P. |author9=Jocou, L. |author10=Kern, P. |author11=Millan-Gabet, R. |author12=Rochat, S. |author13=Traub, W. | title= Searching for faint companions with VLTI/PIONIER. I. Method and first results
| journal=Astronomy and Astrophysics | year=2011 | volume=535
| pages=A68
| bibcode=2011A&A...535A..68A
| doi = 10.1051/0004-6361/201117719 |arxiv = 1110.1178 |s2cid=13144157 }}] [Tau Ceti](/source/Tau_Ceti),[{{cite journal
|author1=di Folco, E. |author2=Absil, O. |author3=Augereau, J.-C. |author4=Mérand, A. |author5=Coudé du Foresto, V. |author6=Thévenin, F. |author7=Defrère, D. |author8=Kervella, P. |author9=ten Brummelaar, T. A. |author10=McAlister, H. A. |author11=Ridgway, S. T. |author12=Sturmann, J. |author13=Sturmann, L. |author14=Turner, N. H. | title=A near-infrared interferometric survey of debris disk stars
| journal=Astronomy and Astrophysics | year=2007 | volume=475
| issue = 1
| pages=243–250
| bibcode=2007A&A...475..243D
| doi = 10.1051/0004-6361:20077625 |arxiv = 0710.1731 |s2cid=18317389 }}] and [Vega](/source/Vega) systems.
====Comets====
{{main|Comet}}
{{As of|2014|11}} there are 5,253 known Solar System comets[{{cite web |last=Johnston |first=Robert |title=Known populations of solar system objects |url=http://www.johnstonsarchive.net/astro/sslist.html |date=August 2, 2014 |access-date=January 19, 2015 |archive-date=June 9, 2019 |archive-url=https://web.archive.org/web/20190609054832/http://www.johnstonsarchive.net/astro/sslist.html }}] and they are thought to be common components of planetary systems. The first exocomets were detected in 1987[{{cite journal | bibcode=1987A&A...185..267F |title=The Beta Pictoris circumstellar disk. V - Time variations of the CA II-K line|author1=Ferlet, R. |author2=Vidal-Madjar, A. |author3=Hobbs, L. M.|year=1987|journal=[Astronomy and Astrophysics](/source/Astronomy_and_Astrophysics)|volume=185|pages=267–270}}][{{cite journal |bibcode=1990A&A...236..202B |title=The Beta Pictoris circumstellar disk. X - Numerical simulations of infalling evaporating bodies |last1=Beust |first1=H. |last2=Lagrange-Henri |first2=A.M. |last3=Vidal-Madjar |first3=A. |last4=Ferlet |first4=R. |volume=236 |year=1990 |pages=202–216 |journal=[Astronomy and Astrophysics](/source/Astronomy_and_Astrophysics)}}] around [Beta Pictoris](/source/Beta_Pictoris), a very young [A-type main-sequence star](/source/A-type_main-sequence_star). There are now a total of 11 stars around which the presence of exocomets have been observed or suspected.[{{cite journal| bibcode=1990A&A...227L..13L |title=HR 10 - A new Beta Pictoris-like star?|author1=Lagrange-Henri, A. M. |author2=Beust, H. |author3=Ferlet, R. |author4=Vidal-Madjar, A. |author5=Hobbs, L. M.|year=1990|journal=[Astronomy and Astrophysics](/source/Astronomy_and_Astrophysics)|volume=227|pages=L13–L16}}][{{cite journal| bibcode=1997A&A...325..228L |title=HST-GHRS observations of candidate β Pictoris-like circumstellar gaseous disks.|author=Lecavelier Des Etangs, A.|display-authors=etal|year=1997|journal=[Astronomy and Astrophysics](/source/Astronomy_and_Astrophysics)|volume=325|pages=228–236}}][{{cite journal| bibcode=2013PASP..125..759W |title=Circumstellar Gas-Disk Variability Around A-Type Stars: The Detection of Exocomets?|author1=Welsh, B. Y. |author2=Montgomery, S. |name-list-style=amp |year=2013|journal=[Publications of the Astronomical Society of the Pacific](/source/Publications_of_the_Astronomical_Society_of_the_Pacific)|volume=125|issue=929|pages=759–774|doi=10.1086/671757|doi-access=free}}][{{cite journal| bibcode=2014A&A...561L..10K |title=Exocomets in the circumstellar gas disk of HD 172555|author1=Kiefer, F. |author2=Lecavelier Des Etangs, A.|display-authors=etal|year=2014|journal=[Astronomy and Astrophysics](/source/Astronomy_and_Astrophysics)|volume=561|pages=L10|doi=10.1051/0004-6361/201323128|arxiv = 1401.1365 |s2cid=118533377}}] All discovered exocometary systems ([Beta Pictoris](/source/Beta_Pictoris), [HR 10](/source/HR_10),[{{cite web |title='Exocomets' Common Across Milky Way Galaxy |url=http://www.space.com/19156-exocomets-alien-solar-systems.html |date=January 7, 2013 |publisher=Space.com |access-date=January 8, 2013 }}] and more recently [HD 172555](/source/HD_172555)[{{Cite web |title=NASA's Spitzer Telescope Witnesses Asteroid Smashup |url=https://www.jpl.nasa.gov/news/nasas-spitzer-telescope-witnesses-asteroid-smashup/ |access-date=2025-05-17 |website=NASA Jet Propulsion Laboratory (JPL) |language=en-US}}]
Based on observations of the Solar System's large collection of natural satellites, they are believed common components of planetary systems; however, the existence of [exomoons](/source/exomoons) has not yet been confirmed. The star [1SWASP J140747.93-394542.6](/source/1SWASP_J140747.93-394542.6), in the constellation [Centaurus](/source/Centaurus), is a strong candidate for a natural satellite.[{{Cite web |title=Saturn-like ring system eclipses Sun-like star |url=https://www.sciencedaily.com/releases/2012/01/120109115830.htm |access-date=2025-05-17 |website=ScienceDaily |language=en |quote=Mamajek thinks his team could be either observing the late stages of planet formation if the transiting object is a star or brown dwarf, or possibly moon formation if the transiting object is a giant planet.}}] Indications suggest that the confirmed extrasolar planet [WASP-12b](/source/WASP-12b) also has at least one satellite.[[http://www.ria.ru/science/20120206/558647431.html Российские астрономы впервые открыли луну возле экзопланеты] (in Russian) – "Studying of a curve of change of shine of WASP-12b has brought to the Russian astronomers unusual result: regular splashes were found out.<...> Though stains on a star surface also can cause similar changes of shine, observable splashes are very similar on duration, a profile and amplitude that testifies for benefit of exomoon existence."]
===Orbital configurations===
Unlike the Solar System, which has orbits that are nearly circular, many of the known planetary systems display much higher [orbital eccentricity](/source/orbital_eccentricity).[Dvorak, R.; Pilat-Lohinger, E.; Bois, E.; Schwarz, R.; Funk, B.; Beichman, C.; Danchi, W.; Eiroa, C.; Fridlund, M.; Henning, T.; Herbst, T.; Kaltenegger, L.; Lammer, H.; Léger, A.; Liseau, R.; Lunine, Jonathan I.; Paresce F, Penny, A.; Quirrenbach, A.; Röttgering, H.; Selsis, F.; Schneider, J.; Stam, D.; Tinetti, G.; White, G.; "Dynamical habitability of planetary systems", Institute for Astronomy, University of Vienna, Vienna, Austria, January 2010] An example of such a system is [16 Cygni](/source/16_Cygni).
====Mutual inclination====
The mutual [inclination](/source/Exoplanetology) between two planets is the angle between their [orbital plane](/source/orbital_plane)s. Many compact systems with multiple close-in planets interior to the equivalent orbit of [Venus](/source/Venus) are expected to have very low mutual inclinations, so the system (at least the close-in part) would be even flatter than the Solar System. Captured planets could be captured into any arbitrary angle to the rest of the system. {{As of|2016}} there are only a few systems where mutual inclinations have actually been measured[{{cite journal | arxiv=1606.04485 | doi=10.3847/1538-3881/153/1/45 | title=Kepler-108: A Mutually Inclined Giant Planet System | year=2017 | last1=Mills | first1=Sean M. | last2=Fabrycky | first2=Daniel C. | journal=The Astronomical Journal | volume=153 | issue=1 | page=45 | bibcode=2017AJ....153...45M | s2cid=119295498 | doi-access=free }}] One example is the [Upsilon Andromedae](/source/Upsilon_Andromedae) system: the planets c and d have a mutual inclination of about 30 degrees.[{{cite journal | arxiv=1411.1059 | doi=10.1088/0004-637X/798/1/46 | title=The 3-dimensional architecture of the Upsilon Andromedae planetary system | year=2014 | last1=Deitrick | first1=Russell | last2=Barnes | first2=Rory | last3=McArthur | first3=Barbara | last4=Quinn | first4=Thomas R. | last5=Luger | first5=Rodrigo | last6=Antonsen | first6=Adrienne | last7=Fritz Benedict | first7=G. | journal=The Astrophysical Journal | volume=798 | page=46 | s2cid=118409453 }}][{{cite web|url=http://www.nasa.gov/mission_pages/hubble/science/outofwack.html |title=NASA – Out of Whack Planetary System Offers Clues to a Disturbed Past |publisher=Nasa.gov |date=2010-05-25 |access-date=2012-08-17}}]
====Orbital dynamics====
Planetary systems can be categorized according to their orbital dynamics as resonant, non-resonant-interacting, hierarchical, or some combination of these. In resonant systems the orbital periods of the planets are in integer ratios. The [Kepler-223](/source/Kepler-223) system contains four planets in an 8:6:4:3 [orbital resonance](/source/orbital_resonance).[{{cite web|last=Emspak|first=Jesse|title=Kepler Finds Bizarre Systems|url=http://www.ibtimes.com/articles/117984/20110302/kepler-finds-strange-worlds-fastest-planet.htm|work=International Business Times|date=March 2, 2011|publisher=International Business Times Inc.|access-date=March 2, 2011}}]
Giant planets are found in mean-motion resonances more often than smaller planets.[{{cite journal | arxiv=1410.4199 | doi=10.1146/annurev-astro-082214-122246 | title=The Occurrence and Architecture of Exoplanetary Systems | year=2015 | last1=Winn | first1=Joshua N. | last2=Fabrycky | first2=Daniel C. | journal=Annual Review of Astronomy and Astrophysics | volume=53 | pages=409–447 | bibcode=2015ARA&A..53..409W | s2cid=6867394 }}]
In interacting systems, the planets' orbits are close enough together that they perturb the orbital parameters. The Solar System could be described as weakly interacting, as opposed to strongly interacting systems, in which [Kepler's laws](/source/Kepler's_laws) do not hold.[{{cite arXiv |eprint=1006.3834 |last1=Fabrycky |first1=Daniel C. |title=Non-Keplerian Dynamics |class=astro-ph.EP |date=2010}}]
In hierarchical systems the planets are arranged so that the system can be gravitationally considered as a nested system of two-bodies, e.g. in a star with a close-in hot Jupiter with another gas giant much further out, the star and hot Jupiter form a pair that appears as a single object to another planet that is far enough out.
Other, as yet unobserved, orbital possibilities include: [double planet](/source/double_planet)s, various [co-orbital planets](/source/co-orbital_configuration) such as quasi-satellites, trojans, and exchange orbits, and interlocking orbits maintained by [precessing orbital planes](/source/nodal_precession).[{{cite journal | arxiv=0812.2949 | doi=10.1111/j.1365-2966.2009.14552.x | title=Equilibria in the secular, non-co-planar two-planet problem | year=2009 | last1=Migaszewski | first1=Cezary | last2=Goździewski | first2=Krzysztof | journal=Monthly Notices of the Royal Astronomical Society | volume=395 | issue=4 | pages=1777–1794 | doi-access=free | bibcode=2009MNRAS.395.1777M | s2cid=14922361 }}]
====Number of planets, relative parameters and spacings====
[[File:Orbits of some Kepler Planetary Systems.jpg|thumb|350px|The spacings between orbits vary widely amongst the different systems discovered by the [Kepler space telescope](/source/Kepler_space_telescope).]]
*[https://arxiv.org/abs/1212.1859 On The Relative Sizes of Planets Within Kepler Multiple Candidate Systems], [David R. Ciardi](/source/David_Ciardi) et al. December 9, 2012
*[https://arxiv.org/abs/1410.4192 The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets], Sarah Ballard, John Asher Johnson, October 15, 2014
*[https://arxiv.org/abs/1304.3341 Exoplanet Predictions Based on the Generalised Titius-Bode Relation], Timothy Bovaird, Charles H. Lineweaver, August 1, 2013
*[https://arxiv.org/abs/1404.2552 The Solar System and the Exoplanet Orbital Eccentricity - Multiplicity Relation], Mary Anne Limbach, Edwin L. Turner, April 9, 2014
*[https://arxiv.org/abs/1409.3320 The period ratio distribution of Kepler's candidate multiplanet systems], [Jason H. Steffen](/source/Jason_Steffen), Jason A. Hwang, September 11, 2014
*[https://arxiv.org/abs/1302.7190 Are Planetary Systems Filled to Capacity? A Study Based on Kepler Results], Julia Fang, Jean-Luc Margot, February 28, 2013
==Zones==
===Habitable zone===
{{Main|Habitable zone}}
thumb|Location of habitable zones around different types of stars
The habitable zone around a star is the region where the temperature range allows for liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star; this means the habitable zone will also vary accordingly. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet.
Habitable zones have usually been defined in terms of surface temperature; however, over half of Earth's biomass is from subsurface microbes,[{{cite journal | last1 = Amend | first1 = J. P. | last2 = Teske | first2 = A. | year = 2005 | title = Expanding frontiers in deep subsurface microbiology | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 219 | issue = 1–2| pages = 131–155 | doi=10.1016/j.palaeo.2004.10.018| bibcode = 2005PPP...219..131A }}] and temperature increases as depth underground increases, so the subsurface can be conducive for life when the surface is frozen; if this is considered, the habitable zone extends much further from the star.[[https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-25639306 Further away planets 'can support life' say researchers], BBC, January 7, 2014 Last updated at 12:40]
Studies in 2013 indicate that an estimated 22±8% of Sun-like[For the purpose of this 1 in 5 statistic, "Sun-like" means [G-type star](/source/G-type_star). Data for Sun-like stars were not available so this statistic is an extrapolation from data about [K-type star](/source/K-type_star)s] stars have an Earth-sized[For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii] planet in the habitable[For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).] zone.[{{cite web|last=Sanders|first=R.|date=November 4, 2013|title=Astronomers answer key question: How common are habitable planets?|url=http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|work=newscenter.berkeley.edu|access-date=November 6, 2014|archive-url=https://web.archive.org/web/20141107081158/http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|archive-date=November 7, 2014}}][{{cite journal
|last1=Petigura |first1=E. A.|last2=Howard |first2=A. W.|last3=Marcy |first3=G. W.
|date=2013|title=Prevalence of Earth-size planets orbiting Sun-like stars|journal=[Proceedings of the National Academy of Sciences](/source/Proceedings_of_the_National_Academy_of_Sciences)|volume= 110|issue= 48|pages=19273–19278 |arxiv= 1311.6806|bibcode= 2013PNAS..11019273P|doi=10.1073/pnas.1319909110 |pmid=24191033 |pmc=3845182|doi-access=free}}]
===Venus zone===
The '''Venus zone''' is the region around a star where a [terrestrial planet](/source/terrestrial_planet) would have [runaway greenhouse](/source/runaway_greenhouse) conditions like [Venus](/source/Venus), but not so near the star that the atmosphere completely escapes. As with the habitable zone, the location of the Venus zone depends on several factors, including the type of star and properties of the planets such as mass, rotation rate, and atmospheric clouds. Studies of the Kepler spacecraft data indicate that 32% of [red dwarf](/source/red_dwarf)s have potentially Venus-like planets based on planet size and distance from star, increasing to 45% for [K-type](/source/K-type_main-sequence_star) and [G-type](/source/G-type_main-sequence_star) stars.[For the purpose of this, terrestrial-sized means 0.5–1.4 Earth radii, the "Venus zone" means the region with approximately 1 to 25 times Earth's stellar flux for M and K-type stars and approximately 1.1 to 25 times Earth's stellar flux for G-type stars.] Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus.[[http://hzgallery.org/venus.html Habitable Zone Gallery - Venus]][{{Cite journal |last1=Kane |first1=Stephen R. |last2=Kopparapu |first2=Ravi Kumar |last3=Domagal-Goldman |first3=Shawn D. |author-link3=Shawn Domagal-Goldman |year=2014 |title=On the Frequency of Potential Venus Analogs from Kepler Data |journal=The Astrophysical Journal |volume=794 |issue=1 |pages=L5 |arxiv=1409.2886 |bibcode=2014ApJ...794L...5K |doi=10.1088/2041-8205/794/1/L5 |s2cid=119178082}}]
==Galactic distribution of planets==
{{See also|Galactic habitable zone|Extragalactic planet|Globular cluster#Planets}}
[[File:Planet Discovery Neighbourhood in Milky Way Galaxy.jpeg|thumb|300px|90% of planets with known distances are within about 2000 [light years](/source/light_years) of Earth, as of July 2014.]]
The [Milky Way](/source/Milky_Way) is 100,000 light-years across, but 90% of planets with known distances are within about 2000 [light years](/source/light_years) of Earth, as of July 2014. One method that can detect planets much further away is [microlensing](/source/microlensing). The upcoming [Nancy Grace Roman Space Telescope](/source/Nancy_Grace_Roman_Space_Telescope) could use microlensing to measure the relative frequency of planets in the [galactic bulge](/source/galactic_bulge) versus the [galactic disk](/source/galactic_disk).[[http://exep.jpl.nasa.gov/exopag/exopag9/agenda/jyee_ExoPAG9.pdf SAG 11: Preparing for the WFIRST Microlensing Survey] {{Webarchive|url=https://web.archive.org/web/20140222134148/http://exep.jpl.nasa.gov/exopag/exopag9/agenda/jyee_ExoPAG9.pdf |date=February 22, 2014 }}, Jennifer Yee] So far, the indications are that planets are more common in the disk than the bulge.[[http://science.gsfc.nasa.gov/660/seminars/ASDcolloq/fall2010.html Toward a New Era in Planetary Microlensing] {{Webarchive|url=https://web.archive.org/web/20141103160816/http://science.gsfc.nasa.gov/660/seminars/ASDcolloq/fall2010.html |date=November 3, 2014 }}, Andy Gould, September 21, 2010] Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge is [MOA-2011-BLG-293Lb](/source/MOA-2011-BLG-293Lb) at a distance of 7.7 kiloparsecs (about 25,000 light years).[{{cite journal | arxiv=1310.3706 | doi=10.1088/0004-637X/780/1/54 | title=Moa-2011-BLG-293Lb: First Microlensing Planet Possibly in the Habitable Zone | date=2013 | last1=Batista | first1=V. | last2=Beaulieu | first2=J. -P. | last3=Gould | first3=A. | last4=Bennett | first4=D. P. | last5=Yee | first5=J. C. | last6=Fukui | first6=A. | last7=Gaudi | first7=B. S. | last8=Sumi | first8=T. | last9=Udalski | first9=A. | journal=The Astrophysical Journal | volume=780 | page=54 }}]
''Population I'', or ''metal-rich stars'', are those young stars whose [metallicity](/source/metallicity) is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by the [accretion](/source/accretion_(astrophysics)) of metals.{{citation needed|date=December 2014}} The Sun is an example of a metal-rich star. These are common in the disks of galaxies.[{{Cite web |title=Population I {{!}} COSMOS |url=https://astronomy.swin.edu.au/cosmos/*/Population+I |access-date=2025-10-25 |website=astronomy.swin.edu.au}}] Generally, the youngest stars, the extreme population I, are found farther in and intermediate population I stars are farther out, etc. The Sun is considered an intermediate population I star. Population I stars have regular [elliptical orbit](/source/elliptical_orbit)s around the [Galactic Center](/source/Galactic_Center), with a low [relative velocity](/source/relative_velocity).[{{cite journal| title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect|author=Charles H. Lineweaver |date=2000| doi=10.1006/icar.2001.6607| journal=Icarus| volume=151| issue=2| pages=307–313|arxiv=astro-ph/0012399|bibcode = 2001Icar..151..307L |s2cid=14077895 }}]
''Population II'', or ''metal-poor stars'', are those with relatively low metallicity which can have hundreds (e.g. [BD +17° 3248](/source/BD_%2B17%C2%B0_3248)) or thousands (e.g. [Sneden's Star](/source/Sneden's_Star)) times less metallicity than the Sun. These objects formed during an earlier time of the universe.[{{Cite web |title=Population II {{!}} COSMOS |url=https://astronomy.swin.edu.au/cosmos/*/Population+II |access-date=2025-10-25 |website=astronomy.swin.edu.au}}] Intermediate population II stars are common in the [bulge](/source/bulge_(astronomy)) near the center of the [Milky Way](/source/Milky_Way),{{citation needed|date=December 2014}} whereas Population II stars found in the [galactic halo](/source/Galactic_spheroid) are older and thus more metal-poor.{{citation needed|date=December 2014}} [Globular clusters](/source/Globular_clusters) also contain high numbers of population II stars.[{{cite journal | author=T. S. van Albada | author2=Norman Baker | title=On the Two Oosterhoff Groups of Globular Clusters | journal=Astrophysical Journal | volume=185 | date=1973 | pages=477–498 | doi=10.1086/152434 | bibcode=1973ApJ...185..477V| doi-access=free }}]
In 2014, the first planets around a halo star were announced around [Kapteyn's star](/source/Kapteyn's_star), the nearest halo star to Earth, around 13 light years away. However, later research suggests that [Kapteyn b](/source/Kapteyn_b) is just an artefact of stellar activity and that Kapteyn c needs more study to be confirmed.[{{cite journal | arxiv=1505.02778 | doi=10.1088/2041-8205/805/2/L22 | title=Stellar Activity Mimics a Habitable-Zone Planet Around Kapteyn's Star | date=2015 | last1=Robertson | first1=Paul | last2=Roy | first2=Arpita | last3=Mahadevan | first3=Suvrath | journal=The Astrophysical Journal | volume=805 | issue=2 | pages=L22 | bibcode=2015ApJ...805L..22R }}] The metallicity of Kapteyn's star is estimated to be about 8[[Metallicity](/source/Metallicity) of [Kapteyn's star](/source/Kapteyn's_star) estimated at [Fe/H]= −0.89. 10−0.89 ≈ 1/8] times less than the Sun.[{{cite journal | arxiv=1406.0818 | doi=10.1093/mnrasl/slu076 | doi-access=free | title=Two planets around Kapteyn's star: A cold and a temperate super-Earth orbiting the nearest halo red dwarf | date=2014 | last1=Anglada-Escudé | first1=G. | last2=Arriagada | first2=P. | last3=Tuomi | first3=M. | last4=Zechmeister | first4=M. | last5=Jenkins | first5=J. S. | last6=Ofir | first6=A. | last7=Dreizler | first7=S. | last8=Gerlach | first8=E. | last9=Marvin | first9=C. J. | last10=Reiners | first10=A. | last11=Jeffers | first11=S. V. | last12=Butler | first12=R. P. | last13=Vogt | first13=S. S. | last14=Amado | first14=P. J. | last15=Rodríguez-López | first15=C. | last16=Berdiñas | first16=Z. M. | last17=Morin | first17=J. | last18=Crane | first18=J. D. | last19=Shectman | first19=S. A. | last20=Thompson | first20=I. B. | last21=Díaz | first21=M. | last22=Rivera | first22=E. | last23=Sarmiento | first23=L. F. | last24=Jones | first24=H. R. A. | journal=Monthly Notices of the Royal Astronomical Society: Letters | volume=443 | pages=L89–L93 }}]
Different [types of galaxies](/source/Galaxy_morphological_classification) have different histories of [star formation](/source/star_formation) and hence [planet formation](/source/Nebular_hypothesis). Planet formation is affected by the ages, metallicities, and orbits of stellar populations within a galaxy. Distribution of stellar populations within a galaxy varies between the different types of galaxies.[{{cite journal | arxiv=astro-ph/0503298 | doi=10.1007/s11084-005-5010-8 | title=Habitable Zones in the Universe | date=2005 | last1=Gonzalez | first1=Guillermo | journal=Origins of Life and Evolution of Biospheres | volume=35 | issue=6 | pages=555–606 | pmid=16254692 | bibcode=2005OLEB...35..555G }}]
Stars in [elliptical galaxies](/source/elliptical_galaxy) are much older than stars in [spiral galaxies](/source/spiral_galaxy). Most elliptical galaxies contain mainly [low-mass stars](/source/stellar_evolution), with minimal [star-formation](/source/star_formation) activity.[John, D, (2006), ''Astronomy'', {{ISBN|1-4054-6314-7}}, p. 224-225] The distribution of the different types of galaxies in the [universe](/source/universe) depends on their location within [galaxy cluster](/source/galaxy_cluster)s, with elliptical galaxies found mostly close to their centers.[{{cite journal |author=Dressler, A. |date=March 1980 |title=Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies. |journal=The Astrophysical Journal |volume=236 |pages=351–365 |bibcode=1980ApJ...236..351D |doi=10.1086/157753|doi-access=free }}]
==See also==
*[Comparison between the Solar System and extrasolar systems](/source/Solar_System)
*[Protoplanetary disk](/source/Protoplanetary_disk)
*[List of exoplanets](/source/List_of_exoplanets)
*[List of multiplanetary systems](/source/List_of_multiplanetary_systems)
==Notes==
{{Notelist}}
==References==
{{Reflist}}
==Further reading==
{{Commons category|Planetary systems}}
* {{cite journal |bibcode=2000ApJ...537L.147O |doi=10.1088/0004-637X/700/2/L73 |title=On the Relationship Between Debris Disks and Planets |year=2009 |last1=Kóspál |first1=Ágnes |last2=Ardila |first2=David R. |last3=Moór |first3=Attila |last4=Ábrahám |first4=Péter |journal=The Astrophysical Journal |volume=700 |issue=2 |pages=L73–L77 |arxiv=astro-ph/0007014 |s2cid=16636256 |url=https://resolver.caltech.edu/CaltechAUTHORS:20090910-140315653 }}
* {{cite journal |bibcode=2009ApJ...700L..73K |doi=10.1086/312779 |title=Signatures of Exosolar Planets in Dust Debris Disks |year=2000 |last1=Ozernoy |first1=Leonid M. |last2=Gorkavyi |first2=Nick N. |last3=Mather |first3=John C. |last4=Taidakova |first4=Tanya A. |journal=The Astrophysical Journal |volume=537 |issue=2 |pages=L147–L151 |arxiv=0907.0028 |s2cid=1149097 }}
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Adapted from the Wikipedia article [Planetary system](https://en.wikipedia.org/wiki/Planetary_system) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Planetary_system?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.