Aplanetary system consists of a set of non-stellarbodies which aregravitationally bound to and inorbit of astar orstar system. Generally speaking, such systems will includeplanets, and may include other objects such asdwarf planets,asteroids,natural satellites,meteoroids,comets,planetesimals,[1][2] andcircumstellar disks. TheSolar System is an example of a planetary system, in whichEarth, seven other planets, and other celestial objects are bound to and revolve around theSun.[3][4] The termexoplanetary 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).
As of 30 October 2025, there are 6,128 confirmedexoplanets in 4,584 planetary systems, with 1,017 systemshaving more than one planet.[5]Debris disks are known to be common while other objects are more difficult to observe.
Of particular interest toastrobiology is thehabitable zone of planetary systems where planets could have surface liquid water, and thus, the capacity to support Earth-like life.
Heliocentrism is a planetary model that places the Sun is at the center of the universe, as opposed togeocentrism (placing Earth at the center of the universe).
De revolutionibus orbium coelestium byNicolaus Copernicus, published in 1543, presented the first mathematically predictive heliocentric model of a planetary system. 17th-century successorsGalileo Galilei,Johannes Kepler, andSir Isaac Newton developed an understanding ofphysics 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.
In the 16th century the Italian philosopherGiordano Bruno, an early supporter of theCopernican 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.[11]
In the 18th century, the same possibility was mentioned bySir Isaac Newton in the "General Scholium" that concludes hisPrincipia. 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 ofOne."[12]
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 thesearch for extraterrestrial intelligence andhas been a prevalent theme in fiction, particularly science fiction.
The first confirmed detection of anexoplanet was in 1992, with the discovery of several terrestrial-mass planets orbiting thepulsarPSR B1257+12. The first confirmed detection of exoplanets of amain-sequence star was made in 1995, when a giant planet,51 Pegasi b, was found in a four-day orbit around the nearbyG-type star51 Pegasi. The frequency of detections has increased since then, particularly through advancements inmethods of detecting extrasolar planets and dedicated planet-finding programs such as theKepler mission.
During formation of a system, much material is gravitationally-scattered into distant orbits, and some planets are ejected completely from the system, becomingrogue planets.
Planets orbitingpulsars have been discovered. Pulsars are the remnants of thesupernova 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 wouldkick 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 anaccretion disk of fallback matter surrounding a pulsar.[13] Fallback disks of matter that failed to escape orbit during a supernova may also form planets aroundblack holes.[14]
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.[15]
As stars evolve and turn intored giants,asymptotic giant branch stars, and eventuallyplanetary nebulae, they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are.[17][18] 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.
Free-floating planets 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[clarification needed] 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 aroundneutron stars because these are likely to be ejected from the cluster by apulsar 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 thegalactic tide and likely become free-floating again through encounters with other field stars or giantmolecular clouds.[19]
The Solar System consists of an inner region of smallrocky planets and outer region of largegiant planets. However, other planetary systems can have quite different architectures. At present,[when?] 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.[20] More commonly, systems consisting of multipleSuper-Earths have been detected.[21][22] These super-Earths are usually very close to their star, with orbits smaller than that ofMercury. Other systems have been found to have ahot Jupiter gas giant very close to the star. Theories such asplanetary migration or scattering have been proposed to explain the formation of large planets close to their parent stars.[23][24] Overall, studies suggest that architectures of planetary systems are dependent on the conditions of their initial formation.[25]
Planetary system architectures may be partitioned into four classes based on how the mass of the planets is distributed around thehost star:[26][27]
InSimilar 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 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.[28]
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 andKepler-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. TheSolar System, with smallrocky planets in the inner part andgiant planets in the outer part, is a type of Ordered system.
Most known exoplanets orbit stars roughly similar to theSun: that is,main-sequence stars ofspectral categories 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 dwarfs, ofspectral category M) are less likely to have planets massive enough to be detected by theradial-velocity method.[29][30] Nevertheless, several tens of planets around red dwarfs have been discovered by theKepler space telescope by thetransit 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.[31] 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.[32] 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.[33]
Debris disks detected inHST archival images of young stars,HD 141943 andHD 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 (theasteroid belt,Kuiper belt,scattered disc, andOort cloud) and clearly-observable disks have been detected around nearby solar analogs includingEpsilon Eridani andTau Ceti. Based on observations of numerous similar disks, they are assumed to be quite common attributes of stars on themain sequence.
Computer modelling of an impact in 2013 detected around the starNGC 2547-ID8 by theSpitzer Space Telescope, and confirmed by ground observations, suggests the involvement of large asteroids orprotoplanets similar to the events believed to have led to the formation of terrestrial planets like the Earth.[46]
Based on observations of the Solar System's large collection of natural satellites, they are believed common components of planetary systems; however, the existence ofexomoons has not yet been confirmed. The star1SWASP J140747.93-394542.6, in the constellationCentaurus, is a strong candidate for a natural satellite.[47] Indications suggest that the confirmed extrasolar planetWASP-12b also has at least one satellite.[48]
Unlike the Solar System, which has orbits that are nearly circular, many of the known planetary systems display much higherorbital eccentricity.[49] An example of such a system is16 Cygni.
The mutualinclination between two planets is the angle between theirorbital planes. Many compact systems with multiple close-in planets interior to the equivalent orbit ofVenus 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[update] there are only a few systems where mutual inclinations have actually been measured[50] One example is theUpsilon Andromedae system: the planets c and d have a mutual inclination of about 30 degrees.[51][52]
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. TheKepler-223 system contains four planets in an 8:6:4:3orbital resonance.[53]Giant planets are found in mean-motion resonances more often than smaller planets.[54]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 whichKepler's laws do not hold.[55]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.
Location of habitable zone 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,[57] 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.[58]
Studies in 2013 indicate that an estimated 22±8% of Sun-like[a] stars have an Earth-sized[b] planet in the habitable[c] zone.[59][60]
TheVenus zone is the region around a star where aterrestrial planet would haverunaway greenhouse conditions likeVenus, 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% ofred dwarfs have potentially Venus-like planets based on planet size and distance from star, increasing to 45% forK-type andG-type stars.[d] Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus.[61][62]
90% of planets with known distances are within about 2000light years of Earth, as of July 2014.
TheMilky Way is 100,000 light-years across, but 90% of planets with known distances are within about 2000light years of Earth, as of July 2014. One method that can detect planets much further away ismicrolensing. The upcomingNancy Grace Roman Space Telescope could use microlensing to measure the relative frequency of planets in thegalactic bulge versus thegalactic disk.[63] So far, the indications are that planets are more common in the disk than the bulge.[64] Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge isMOA-2011-BLG-293Lb at a distance of 7.7 kiloparsecs (about 25,000 light years).[65]
Population I, ormetal-rich stars, are those young stars whosemetallicity is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by theaccretion of metals.[citation needed] The Sun is an example of a metal-rich star. These are common in the disks of galaxies.[66] 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 regularelliptical orbits around theGalactic Center, with a lowrelative velocity.[67]
Population II, ormetal-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.[68] Intermediate population II stars are common in thebulge near the center of theMilky Way,[citation needed] whereas Population II stars found in thegalactic halo are older and thus more metal-poor.[citation needed]Globular clusters also contain high numbers of population II stars.[69]In 2014, the first planets around a halo star were announced aroundKapteyn's star, the nearest halo star to Earth, around 13 light years away. However, later research suggests thatKapteyn b is just an artefact of stellar activity and that Kapteyn c needs more study to be confirmed.[70] The metallicity of Kapteyn's star is estimated to be about 8[e] times less than the Sun.[71]
Differenttypes of galaxies have different histories ofstar formation and henceplanet 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.[72]Stars inelliptical galaxies are much older than stars inspiral galaxies. Most elliptical galaxies contain mainlylow-mass stars, with minimalstar-formation activity.[73] The distribution of the different types of galaxies in theuniverse depends on their location withingalaxy clusters, with elliptical galaxies found mostly close to their centers.[74]
^For the purpose of this 1 in 5 statistic, "Sun-like" meansG-type star. Data for Sun-like stars were not available so this statistic is an extrapolation from data aboutK-type stars
^For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
^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).
^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.
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^Pierrehumbert, Raymond T. (December 9, 2021). "1. Beginnings".Planetary Systems: A Very Short Introduction. Oxford University Press. pp. 1–13.doi:10.1093/actrade/9780198841128.003.0001.ISBN978-0-19-884112-8. RetrievedJuly 10, 2025.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.
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^"Saturn-like ring system eclipses Sun-like star".ScienceDaily. RetrievedMay 17, 2025.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.
^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."
^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
^Deitrick, Russell; Barnes, Rory; McArthur, Barbara; Quinn, Thomas R.; Luger, Rodrigo; Antonsen, Adrienne; Fritz Benedict, G. (2014). "The 3-dimensional architecture of the Upsilon Andromedae planetary system".The Astrophysical Journal.798: 46.arXiv:1411.1059.doi:10.1088/0004-637X/798/1/46.S2CID118409453.
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