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{{redirect|Solar systems|the Sun and the planetary system around it|Solar System|a system of stars|Star system|the solar power company|Solar Systems (company)}}▼
{{Short description|Set of non-stellar objects in orbit around a star}}
▲{{redirect|Solar systems|the Sun and
{{Use mdy dates|date=April 2019}}
[[File:Artist Concept Planetary System.jpg|thumb|250px|An artist's concept of a planetary system]]
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A '''planetary system''' is a set of [[gravity|gravitationally]] bound non-[[Star|stellar]] objects in or out of [[orbit]] around a [[star]] or [[star system]]. Generally speaking, systems with one or more [[planet]]s constitute a planetary system, although such systems may also consist of bodies such as [[dwarf planet]]s, [[asteroid]]s, [[natural satellite]]s, [[meteoroid]]s, [[comet]]s, [[planetesimal]]s<ref>p. 394, ''The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance'', David J. Dsrling, Hoboken, New Jersey: Wiley, 2004. {{ISBN|0-471-26569-1}}.</ref><ref>p. 314, ''Collins Dictionary of Astronomy'', Valerie Illingworth, London: Collins, 2000. {{ISBN|0-00-710297-6}}.</ref> and [[circumstellar disk]]s. The [[Sun]] together with the planetary system revolving around it, including [[Earth]], forms the [[Solar System]].<ref>p. 382, ''Collins Dictionary of Astronomy''.</ref><ref>p. 420, ''A Dictionary of Astronomy'', Ian Ridpath, Oxford, New York: Oxford University Press, 2003. {{ISBN|0-19-860513-7}}.</ref> The term '''exoplanetary system''' is sometimes used in reference to other planetary systems.
{{Extrasolar planet counts|full}} [[Debris disk|Debris disks]] are also known to be common, though other objects are more difficult to observe.
Of particular interest to [[astrobiology]] is the [[habitable zone]] of planetary systems where planets could have surface liquid water, and thus the capacity to support Earth-like life.
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{{Main|Discovery and exploration of the Solar System}}
[[File:De Revolutionibus manuscript p9b.jpg|thumb|190px|[[Heliocentric model]] of the [[Solar System]] in [[Copernicus]]' [[manuscript]]]]
''[[De revolutionibus orbium coelestium]]'' by [[Nicolaus Copernicus]], published in 1543, presented the first mathematically predictive heliocentric model of a planetary system.
===Speculation on extrasolar planetary systems===
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</ref>
His theories gained traction{{Colloquialism|date=August 2021}} through the 19th and 20th centuries despite a lack of supporting evidence.
===Detection of exoplanets===
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====Lower-mass stars====
[[File:Protoplanetary discs observed with SPHERE.jpg|thumb|Protoplanetary discs observed with the [[Very Large Telescope]].<ref>{{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}}</ref>]]
As stars evolve and turn into [[red giant]]s, [[asymptotic giant branch]] stars, and [[planetary nebula]]e they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are.<ref name="NYT-20230503">{{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]] |url-status=live |archiveurl=https://archive.today/20230503155540/https://www.nytimes.com/2023/05/03/science/star-eating-planet.html |archivedate=3 May 2023 |accessdate=3 May 2023 }}</ref><ref name="NYT-20220819">{{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]] |accessdate=19 August 2022 }}</ref> 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.
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==System architectures==
The Solar System consists of an inner region of small [[rocky planets]] and outer region of large [[
At present,{{When|date=August 2021}} few systems have been found to be analogous to the Solar System with terrestrial planets close to the parent star. More commonly, systems consisting of multiple [[Super-Earth]]s have been detected.<ref><span class="plainlinks">[http://www.astro.washington.edu/users/rory/publications/br04.pdf Types and Attributes]</span> at Astro Washington.com.</ref>
===Classification
* '''Similar:'''
* '''Mixed:'''
* '''Anti-Ordered:'''
* '''Ordered:'''
===Components===
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Computer modelling of an impact in 2013 detected around the star [[NGC 2547]]-ID8 by the [[Spitzer Space Telescope]], and confirmed by ground observations, suggests the involvement of large asteroids or [[protoplanet]]s similar to the events believed to have led to the formation of terrestrial planets like the Earth.<ref>[https://www.jpl.nasa.gov/news/news.php?feature=4273 NASA's Spitzer Telescope Witnesses Asteroid Smashup]</ref>
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]] has
===Orbital configurations===
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====Mutual inclination====
The mutual [[Exoplanetology#Inclination vs. spin–orbit angle|inclination]] between two planets is the angle between their [[orbital plane]]s. Many compact systems with multiple close-in planets interior to the equivalent orbit of [[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<ref>
====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]] system contains four planets in an 8:6:4:3 [[orbital resonance]].<ref>{{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}}</ref>
Giant planets are found in mean-motion resonances more often than smaller planets.<ref>
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. In strongly interacting systems [[Kepler's laws]] do not hold.<ref>{{cite arXiv |eprint=1006.3834 |last1=Fabrycky |first1=Daniel C. |title=Non-Keplerian Dynamics |class=astro-ph.EP |date=2010}}</ref>
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
Other, as yet unobserved, orbital possibilities include: [[double planet]]s; various [[co-orbital configuration|co-orbital planets]] such as quasi-satellites, trojans and exchange orbits; and interlocking orbits maintained by [[nodal precession|precessing orbital planes]].<ref>
====Number of planets, relative parameters and spacings====
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====Planet capture====
[[Rogue planet|Free-floating 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 <!--abstract seems to have a typo 10^6: rest of paper uses 10^5-->10<sup>5</sup> AU<!--see preceding comment-->. 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]]s because these are likely to be ejected from the cluster by a [[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 10<sup>6</sup> AU would be slowly disrupted by the [[galactic tide]] and likely become free-floating again through encounters with other field stars or giant [[molecular cloud]]s.<ref name="WideRecapture">[https://arxiv.org/abs/1202.2362 On the origin of planets at very wide orbits from the recapture of free-floating planets], Hagai B. Perets, M. B. N. Kouwenhoven, 2012</ref>
==Zones==
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Habitable zones have usually been defined in terms of surface temperature; however, over half of Earth's biomass is from subsurface microbes,<ref>{{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 }}</ref> 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.<ref>[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</ref>
Studies in 2013
|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]]|volume= 110|issue= 48|pages=19273–19278
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===Venus zone===
The '''Venus zone''' is the region around a star where a [[terrestrial planet]] would have [[runaway greenhouse]] conditions like [[Venus]], but not so near the star that the atmosphere completely evaporates. 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]]s have potentially Venus-like planets based on planet size and distance from star, increasing to 45% for [[K-type main-sequence star|K-type]] and [[G-type main-sequence star|G-type]] stars.<ref group=lower-alpha name=footnoteE>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.</ref> Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus.<ref>[http://hzgallery.org/venus.html Habitable Zone Gallery - Venus]</ref><ref>
==Galactic distribution of planets==
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''Population I'', or ''metal-rich stars'', are those young stars whose [[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 (astrophysics)|accretion]] of metals.{{citation needed|date=December 2014}} The Sun is an example of a metal-rich star. These are common in the [[spiral arm]]s of the [[Milky Way]].{{citation needed|date=December 2014}} 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]]s around the [[Galactic Center]], with a low [[relative velocity]].<ref>{{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 }}</ref>
''Population II'', or ''metal-poor stars'', are those with relatively low metallicity which can have hundreds (e.g. [[BD +17° 3248]]) or thousands (e.g. [[Sneden's Star]]) times less metallicity than the Sun. These objects formed during an earlier time of the universe.{{citation needed|date=December 2014}} Intermediate population II stars are common in the [[bulge (astronomy)|bulge]] near the center of the [[Milky Way]],{{citation needed|date=December 2014}} whereas Population II stars found in the [[Galactic spheroid#Galactic spheroid|galactic halo]] are older and thus more metal-poor.{{citation needed|date=December 2014}} [[Globular clusters]] also contain high numbers of population II stars.<ref>{{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 }}</ref>
In 2014, the first planets around a halo star were announced around [[Kapteyn's star]], the nearest halo star to Earth, around 13 light years away. However, later research suggests that [[Kapteyn b]] is just an artefact of stellar activity and that Kapteyn c needs more study to be confirmed.<ref>[https://arxiv.org/abs/1505.02778 Stellar activity mimics a habitable-zone planet around Kapteyn's star], Paul Robertson (1 and 2), Arpita Roy (1 and 2 and 3), [[Suvrath Mahadevan]] (1 and 2 and 3) ((1) Dept. of Astronomy and Astrophysics, Penn State University, (2) Center for Exoplanets & Habitable Worlds, Penn State University, (3) The Penn State Astrobiology Research Center), (Submitted on May 11, 2015 (v1), last revised June 1, 2015 (this version, v2))</ref> The metallicity of Kapteyn's star is estimated to be about 8<ref group=lower-alpha name="kepteynmetal">[[Metallicity]] of [[Kapteyn's star]] estimated at [Fe/H]= −0.89. 10<sup>−0.89</sup> ≈ 1/8</ref> times less than the Sun.<ref name="kapteyn">[https://arxiv.org/abs/1406.0818 Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf], Guillem Anglada-Escudé, Pamela Arriagada, Mikko Tuomi, Mathias Zechmeister, James S. Jenkins, Aviv Ofir, Stefan Dreizler, Enrico Gerlach, Chris J. Marvin, Ansgar Reiners, Sandra V. Jeffers, R. Paul Butler, Steven S. Vogt, Pedro J. Amado, Cristina Rodríguez-López, Zaira M. Berdiñas, Julian Morin, Jeff D. Crane, Stephen A. Shectman, Ian B. Thompson, Mateo Díaz, Eugenio Rivera, Luis F. Sarmiento, Hugh R.A. Jones, (Submitted on June 3, 2014)</ref>
Different [[Galaxy morphological classification|types of galaxies]] have different histories of [[star formation]] and hence [[Nebular hypothesis#Formation of planets|planet formation]]. 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.<!--"If all galaxies were just like the Milky Way, then the GHZ could just be applied to other galaxies. But, they
Stars in [[elliptical galaxy|elliptical galaxies]] are much older than stars in [[spiral galaxy|spiral galaxies]]. Most elliptical galaxies contain mainly [[stellar evolution#Low-mass stars|low-mass stars]], with minimal [[star formation|star-formation]] activity.<ref name="author">John, D, (2006), ''Astronomy'', {{ISBN|1-4054-6314-7}}, p. 224-225</ref> The distribution of the different types of galaxies in the [[universe]] depends on their location within [[galaxy cluster]]s, with elliptical galaxies found mostly close to their centers.<ref>{{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 }}</ref>
==See also==
*[[Solar System#Comparison with extrasolar systems|Comparison between the Solar System and extrasolar systems]]
*[[Protoplanetary disk]]
*[[List of exoplanets]]
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{{Exoplanet}}
{{Stellar system}}
{{Star}}
{{Portal bar|Astronomy|Spaceflight|Outer space|Solar System}}
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