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[[File:InnerSolarSystem-en.png|thumb]]
A '''minor-planet group''' is a population of [[minor planet]]s that share broadly similar orbits. Members are generally unrelated to each other, unlike in an [[asteroid family]], which often results from the break-up of a single asteroid. It is customary to name a group of asteroids after the first member of that group to be discovered, which is often the largest.

== Groups out to the orbit of Earth ==

There are relatively few asteroids that orbit close to the Sun. Several of these groups are hypothetical at this point in time, with no members having yet been discovered; as such, the names they have been given are provisional.
* [[Vulcanoid asteroid]]s are hypothetical asteroids that orbit entirely within the orbit of [[Mercury (planet)|Mercury]] (have an [[aphelion]] of less than 0.3874&nbsp;AU). A few searches for vulcanoids have been conducted but none have been discovered so far.
* [[ꞌAylóꞌchaxnim asteroid]]s (previously named Vatira) are asteroids that orbit entirely within the orbit of [[Venus]] (have an aphelion of less than 0.718&nbsp;AU). {{As of|2022}}, one such asteroid is known: [[594913 ꞌAylóꞌchaxnim]].
* [[Atira asteroid]]s (Apohele; Interior-Earth Objects) are a small group of known asteroids whose aphelion is less than 0.983&nbsp;AU, meaning they orbit entirely within Earth's orbit. The group is named after its first confirmed member, [[163693&nbsp;Atira]]. {{As of|July 2025}}, the group consists of 36 members, two of which are named and nine of which are numbered.<ref>{{cite web |title=Small-Body Database Query |url=https://ssd.jpl.nasa.gov/tools/sbdb_query.html#!#results |website=Solar System Dynamics – Jet Propulsion Laboratory |publisher=NASA – California Institute of Technology |access-date=2025-07-12}}</ref>
* [[List of Mercury-crossing minor planets|Mercury-crosser asteroid]]s having a [[perihelion]] smaller than Mercury's 0.3075&nbsp;AU.
* [[List of Venus-crossing minor planets|Venus-crosser asteroid]]s having a perihelion smaller than [[Venus]]'s 0.7184&nbsp;AU. This group includes the above Mercury-crossers (if their aphelion is greater than Venus's perihelion. All known Mercury crossers satisfy this condition except ꞌAylóꞌchaxnim, which has an aphelion smaller than Venus's perihelion and a perihelion slightly smaller than Mercury's aphelion).
* [[List of Earth-crossing minor planets|Earth-crosser asteroid]]s having a perihelion smaller than [[Earth]]'s 0.9833&nbsp;AU. This group includes the above Mercury- and Venus-crossers, apart from the Apoheles. They are also divided into the
** [[Aten asteroid]]s having a [[semi-major axis]] less than 1&nbsp;AU, named after [[2062&nbsp;Aten]].
** [[Apollo asteroid]]s having a [[semi-major axis]] greater than 1&nbsp;AU, named after [[1862&nbsp;Apollo]].
* [[Arjuna asteroid]]s are somewhat vaguely defined as having orbits similar to Earth's; i.e. with an average orbital radius of around 1&nbsp;AU and with low eccentricity and inclination.<ref name=geo /> Due to the vagueness of this definition some asteroids belonging to the [[Atira asteroid|Atira]], [[Amor asteroid|Amor]], [[Apollo asteroid|Apollo]] or [[Aten asteroid|Aten]] groups can also be classified as Arjunas. The term was introduced by [[Spacewatch]] and does not refer to an existing asteroid; examples of Arjunas include [[1991&nbsp;VG]].
* [[Earth trojan]]s are asteroids located in the Earth–Sun [[Lagrangian point]]s {{L4}} and {{L5}}. Their location in the sky as observed from Earth's surface would be fixed at about 60 degrees east and west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations. The only known Earth trojans are {{mpl|2010 TK|7}} and {{mpl|2020 XL|5}}.
* [[Near-Earth asteroid]]s is a catch-all term for asteroids whose orbit closely approaches that of Earth. It includes almost all of the above groups, as well as the [[Amor asteroid]]s.

== Groups out to the orbit of Mars ==
* The [[Amor asteroid|Amor]] asteroids, named after [[1221 Amor]], are [[near-Earth asteroid]]s that are not [[List of Earth-crossing minor planets|Earth-crossers]], having a perihelion just outside the Earth's orbit.
* [[List of Mars-crossing minor planets|Mars-crosser asteroid]]s have orbits that cross that of Mars, but do not necessarily closely approach the Earth's.
* [[Mars trojan]]s follow or lead Mars on its orbit, at either of the two [[Lagrangian point]]s 60° ahead ({{L4}}) or behind ({{L5}}). As of November 2020, nine are known. The largest appears to be [[5261&nbsp;Eureka]].
* Many of the Earth-, Venus-, and Mercury-crosser asteroids have aphelia greater than 1&nbsp;AU.

==The asteroid belt==
{{main|Asteroid belt}}
[[File:Kirkwood Gaps.svg|thumb|[[Histogram]] showing the four most prominent [[Kirkwood gap]]s and a possible division into inner, middle and outer [[main-belt]] asteroids:<br />{{legend2|#005aff|border=1px solid #333|inner main-belt ([[Semi-major axis|''a'']] &lt; 2.5 [[Astronomical unit|AU]])}}<br />{{legend2|#ffa500|border=1px solid #222|middle main-belt (2.5 AU &lt; ''a'' &lt; 2.82 AU)}}<br />{{legend2|#55d400|border=1px solid #333|outer main-belt (''a'' &gt; 2.82 AU}}]]
[[File:Main belt i vs a.png|thumb|Asteroid groups out to the orbit of Jupiter. The asteroid belt is shown in red]]

The overwhelming majority of known asteroids have orbits lying between the orbits of [[Mars]] and [[Jupiter]], roughly between 2 and 4&nbsp;[[astronomical unit|AU]]. These could not form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence, through [[orbital resonance]], clears [[Kirkwood gap]]s in the asteroid belt, first recognised by [[Daniel Kirkwood]] in 1874.

The region with the densest concentration (lying between the Kirkwood gaps at 2.06 and 3.27&nbsp;AU, with [[eccentricity (orbit)|eccentricities]] below about 0.3, and inclinations smaller than 30°) is called the [[asteroid belt]]. It can be further subdivided by the Kirkwood Gaps into the:
* '''Inner asteroid belt''', inside of the strong Kirkwood gap at 2.50&nbsp;AU due to the 3:1 Jupiter [[orbital resonance]]. The largest member is [[4&nbsp;Vesta]].
** It apparently also includes a group called the main-belt I asteroids which have a semi-major axis between 2.3&nbsp;AU and 2.5&nbsp;AU and an inclination of less than 18°.
* '''Middle''' (or intermediate) '''asteroid belt''', between the 3:1 and 5:2 Jupiter orbital resonances, the latter at 2.82&nbsp;AU. The largest member is [[Ceres (dwarf planet)|Ceres]]. This group is apparently split into the:
** Main-belt IIa asteroids which have a semi-major axis between 2.5&nbsp;AU and 2.706&nbsp;AU and an inclination less than 33°.
** Main-belt IIb asteroids which have a semi-major axis between 2.706&nbsp;AU and 2.82&nbsp;AU and an inclination less than 33°.
* '''Outer asteroid belt''' between the 5:2 and 2:1 Jupiter orbital resonances. The largest member is [[10&nbsp;Hygiea]]. This group is apparently split into the:
** Main-belt IIIa asteroids which have a semi-major axis between 2.82&nbsp;AU and 3.03&nbsp;AU, an eccentricity less than .35, and an inclination less than 30°.
** Main-belt IIIb asteroids which have a semi-major axis between 3.03&nbsp;AU and 3.27&nbsp;AU, an eccentricity less than .35, and an inclination less than 30°.

===Other groups out to the orbit of Jupiter===

There are a number of more or less distinct asteroid groups outside the asteroid belt, distinguished either by mean distance from the Sun, or particular combinations of several orbital elements:
* The [[Hungaria family|Hungaria asteroid]]s, with a mean orbital radius between 1.78&nbsp;AU and 2&nbsp;AU, an eccentricity less than 0.18, and inclination between 16° and 34°. Named after [[434&nbsp;Hungaria]], these are just outside Mars's orbit, and are possibly attracted by the 9:2 Jupiter resonance or the 3:2 Mars resonance.
* The [[Phocaea family|Phocaea asteroid]]s, with a mean orbital radius between 2.25&nbsp;AU and 2.5&nbsp;AU, an eccentricity greater than 0.1, and inclination between 18° and 32°. Some sources group the Phocaeas asteroids with the Hungarias, but the division between the two groups is real and caused by the 4:1 resonance with Jupiter. Named after [[25&nbsp;Phocaea]].
* The [[Alinda asteroid]]s have a mean orbital radius of 2.5&nbsp;AU and an eccentricity between 0.4 and 0.65 (approximately). These objects are held by the 3:1 resonance with Jupiter and a 4:1 resonance with [[Earth]]. Many Alinda asteroids have perihelia very close to Earth's orbit and can be difficult to observe for this reason. Alinda asteroids are ''not'' in stable orbits and eventually will collide either with Jupiter or terrestrial planets. Named after [[887&nbsp;Alinda]].
* The [[Pallas family]] asteroids have a mean orbital radius between 2.7 and 2.8&nbsp;AU and an inclination between 30° and 38°. Named after [[2&nbsp;Pallas]].
* The [[Griqua group|Griqua asteroid]]s have an orbital radius between 3.1&nbsp;AU and 3.27&nbsp;AU and an eccentricity greater than 0.35. These asteroids are in stable 2:1 [[libration]] with Jupiter, in high-inclination orbits. There are about 5 to 10 of these known so far, with [[1362&nbsp;Griqua]] and [[8373&nbsp;Stephengould]] the most prominent.
* The [[Cybele asteroids]] have a mean orbital radius between 3.27&nbsp;AU and 3.7&nbsp;AU,<ref name="Carruba-2013" /> an eccentricity less than 0.3,<ref name=tanton /> and an inclination less than 30°.<ref name="Carruba-2013" /> Named after [[65&nbsp;Cybele]].<ref name=tanton />
* [[Hilda group|Hilda asteroid]]s have a mean orbital radius between 3.7&nbsp;AU and 4.2&nbsp;AU, an eccentricity greater than 0.07, and an inclination less than 20°. These asteroids are in a 3:2 resonance with Jupiter. Named after [[153&nbsp;Hilda]].
* The [[Thule asteroid]]s are in a 4:3 resonance with Jupiter and the group is known to consist of [[279&nbsp;Thule]], {{mpl|(186024) 2001 QG|207}}, and {{mpl|(185290) 2006 UB|219}}.<ref name="Broz-2008" />
* The [[Jupiter trojan]]s have a mean orbital radius between 5.05&nbsp;AU and 5.4&nbsp;AU, and lie in elongated, curved regions around the two [[Lagrangian point]]s 60° ahead and behind of Jupiter. The leading point, {{L4}}, is called the [[Greek camp]] and the trailing {{L5}} point is called the [[Trojan camp]], after the two opposing camps of the legendary [[Trojan War]]; with one exception apiece, objects in each node are named for members of that side of the conflict. [[617&nbsp;Patroclus]] in the Trojan camp and [[624&nbsp;Hektor]] in the Greek camp are "misplaced" in the enemy camps.

There is a forbidden zone between the Hildas and the Trojans (roughly 4.05&nbsp;AU to 4.94&nbsp;AU). Aside from [[279&nbsp;Thule]] and 228 objects in mostly unstable-looking orbits, Jupiter's gravity has swept everything out of this region.

== Groups beyond the orbit of Jupiter ==

Most of the minor planets beyond the orbit of Jupiter are believed to be composed of [[ice]]s and other [[Volatile (astrogeology)|volatiles]]. Many are similar to [[comet]]s, differing only in that the [[perihelion|perihelia]] of their orbits are too distant from the Sun to produce a significant tail.
* The [[Damocloid asteroid]]s, also known as the "Oort cloud group," are named after [[5335&nbsp;Damocles]]. They are defined to be objects that have "fallen in" from the [[Oort cloud]], so their aphelia are generally still out past [[Uranus]], but their perihelia are in the inner Solar System. They have high eccentricities and sometimes high inclinations, including [[retrograde orbit]]s. The definition of this group is somewhat fuzzy, and may overlap significantly with comets.
* The [[Centaur (minor planet)|Centaurs]] have a mean orbital radius roughly between 5.4&nbsp;AU and 30&nbsp;AU. They are currently believed to be [[trans-Neptunian object]]s that "fell in" after encounters with gas giants. The first of these to be identified was [[2060&nbsp;Chiron]] ([[944&nbsp;Hidalgo]] was discovered before, but not identified as a distinct orbital class).

== Groups at or beyond the orbit of Neptune ==
* The [[Neptune trojan]]s as of February 2020 consist of 29 objects. The first one to be discovered was {{mpl|2001 QR|322}}.
* [[Trans-Neptunian object]]s (TNOs) are anything with a mean orbital radius greater than 30&nbsp;AU. This classification includes the Kuiper-belt objects (KBOs), the scattered disc, and the Oort cloud.
** [[Kuiper belt|Kuiper-belt]] objects extend from roughly 30&nbsp;AU to 50&nbsp;AU and are broken into the following subcategories:
*** [[Resonant trans-Neptunian object|Resonant objects]] occupy orbital resonances with Neptune, excluding the 1:1 resonance of the Neptune trojans. 
****[[Plutino]]s are by far the most common resonant KBOs and are in a 2:3 resonance with Neptune, just like [[Pluto]]. The perihelion of such an object tends to be close to Neptune's orbit (much as happens with Pluto), but when the object comes to perihelion, Neptune alternates between being 90 degrees ahead of and 90 degrees behind of the object, so there's no chance of a collision. The MPC defines any object with a mean orbital radius between 39&nbsp;AU and 40.5&nbsp;AU to be a plutino. [[90482&nbsp;Orcus]] and [[28978&nbsp;Ixion]] are among the brightest known.
**** Other resonances. There are several known objects in the 1:2 resonance, dubbed ''twotinos'', with a mean orbital radius of 47.7&nbsp;AU and an eccentricity of 0.37. There are several objects in the 2:5 (mean orbital radius of 55&nbsp;AU), 4:7, 4:5, 3:10, 3:5, and 3:4 resonances, among others. The largest in the 2:5 resonance is {{mpl|(84522) 2002 TC|302}}, and the largest in the 3:10 resonance is [[225088 Gonggong]].
*** [[Classical Kuiper belt object|Classical Kuiper-belt objects]], also known as cubewanos (after [[15760 Albion]], which had the provisional designation {{mp|1992 QB|1}} between its discovery in 1992 and its formal naming in 2018), have a mean orbital radius between approximately 40.5&nbsp;AU and 47&nbsp;AU. Cubewanos are objects in the Kuiper belt that didn't get scattered and didn't get locked into a resonance with Neptune. The largest is {{dp|Makemake}}.
** [[Scattered disc]] objects (SDOs) typically have, unlike cubewanos and resonant objects, high-inclination, high-eccentricity orbits with perihelia that are still not too far from Neptune's orbit. They are assumed to be objects that encountered Neptune and were "scattered" out of their originally more circular orbits close to the ecliptic. The most massive known dwarf planet, [[Eris (dwarf planet)|Eris]], belongs to this category.
*** [[Detached object]]s (extended scattered disk) with generally highly elliptical, very large orbits of up to a few hundred AU and a perihelion too far from [[Neptune]]'s orbit for any significant interaction to occur. A more typical member of the extended disk is {{mpl|(148209) 2000 CR|105}}.
****[[Sednoid]]s have perihelia very far removed from the orbit of Neptune. This group is named after the best-known member, [[90377&nbsp;Sedna]]. As of 2020, only 4 objects in this category have been identified, but it is suspected that there are many more.
**The [[Oort cloud]] is a hypothetical cloud of comets with a mean orbital radius between approximately 50,000&nbsp;AU and 100,000&nbsp;AU. No Oort-cloud objects have been detected; the existence of this classification is only inferred from indirect evidence. Some astronomers have tentatively associated [[90377 Sedna|90377&nbsp;Sedna]] with the inner Oort cloud.

== See also ==
* [[Dwarf planet]]
* {{Section link|List of minor planets|Category}}
* [[Minor Planet Center]]
* [[Mesoplanet]]

==References==
<references>
<ref name=tanton>{{cite book| url = https://books.google.com/books?id=K-0xBsMsursC&pg=PA96| title = Linda T. Elkins-Tanton – '''Asteroids, Meteorites, and Comets''' (2010) – Page 96 (Google Books)| isbn = 978-1-4381-3186-3| last1 = Elkins-Tanton| first1 = Linda T.| date = 2010| publisher = Infobase}}</ref>

<ref name="Carruba-2013">{{Cite journal
  |first1     = V. |last1 = Carruba
  |first2     = R. C. |last2 = Domingos
  |first3     = D. |last3 = Nesvorný
  |first4     = F. |last4 = Roig
  |first5     = M. E. |last5 = Huaman
  |first6     = D. |last6 = Souami
  |date       = August 2013
  |title      = A multidomain approach to asteroid families' identification
  |journal    = Monthly Notices of the Royal Astronomical Society
  |volume     = 433
  |issue      = 3
  |pages      = 2075–2096
  |bibcode    = 2013MNRAS.433.2075C 
  |doi        = 10.1093/mnras/stt884
  |doi-access = free
 |arxiv      = 1305.4847
  |s2cid      = 118511004 }}</ref>

<ref name=geo>{{Cite journal
  |title=Geometric characterization of the Arjuna orbital domain
  |last1=de la Fuente Marcos | first1=C.
  |last2=de la Fuente Marcos |first2= R.
  |date=February 12, 2015
  |journal=[[Astronomische Nachrichten]]
  |volume=336
  |issue=1
  |pages=5–22
  |doi=10.1002/asna.201412133
  |arxiv=1410.4104
  |bibcode=2015AN....336....5D }}</ref>

<ref name="Broz-2008">{{cite journal
 |last=Brož |first=M.
 |author2=Vokrouhlický, D.
 |title=Asteroid families in the first-order resonances with Jupiter
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=390 |issue=2 |pages=715–732 |date=2008
 |doi=10.1111/j.1365-2966.2008.13764.x
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</references> <!-- end of reflist -->

== External links ==
* [https://web.archive.org/web/20110418072433/https://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ Asteroid Classification I – Dynamics], ''[[Minor Planet Center]]'', (archived; 18 Apr 2011)

{{Wikidata property|P196}}

{{Asteroids}}
{{Small Solar System bodies}}
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[[Category:Minor planet groups and families| ]]
[[Category:Lists of Solar System objects|Minor-planet groups]]
[[Category:Types of groupings]]