Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a diameter of about 4880 km). The term minor planet is sometimes used as a synonym for asteroid though this is technically incorrect; asteroids are one group of minor planets, a category which also includes Trans-Neptunian objects and other small bodies in our solar system and elsewhere.
Sir William Herschel (discoverer of Uranus), coined the term asteroid for the first objects discovered in the 19th century, all of which orbit the sun between Mars and Jupiter, and generally in relatively low-eccentricity (i.e., not very elongated) orbits. But since then, minor planets have been found to cross the orbits of planets, from Mercury to Neptune -- with hundreds of transneptunian objects (TNOs) now known to exist well past Neptune's orbit. Though the main distinction between a minor planet and a comet lies in the fact that comets show a coma (or atmosphere) and/or a tail, due primarily to sublimation of ices by solar radiation, one can justifiably consider comets to be a subset of the large group known as minor planets; indeed, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as "stellar" objects, or minor planets.
Minor planets are divided into groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered (generally the largest). 'Groups' are relatively loose dynamical associations, whereas 'families' are much "tighter" and result most probably from the catastrophic breakup of a large parent asteroid sometime in the past. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
Families within the main asteroid belt
The only prominent families are:
- Eos asteroids have a semi-major axis between 2.99 AU and 3.03 AU, an eccentricity between 0.01 and 0.13, and an inclination between 8° and 12°. Named after 221 Eos, ~480 members known.
- Themis asteroids have a semi-major axis between 3.08 AU and 3.24 AU, an eccentricity between 0.09 and 0.22, and an inclination less than 3°. Named after 24 Themis, ~535 members known.
- Koronis asteroids have a semi-major axis between 2.83 AU and 2.91 AU, an eccentricity less than 0.11, and an inclination less than 3.5°. Named after 158 Koronis, ~310 members known.
- Eunomia asteroids (semi-major axis = 2.5 < a < 2.75 AU, eccentricity = 0.1 < e < 0.2, inclination = 13° < i < 20°), named after 15 Eunomia, ~370 members known.
- Flora asteroids have a semi-major axis between 2.1 AU and 2.3 AU with an inclination of less than 11°. Named after 8 Flora, ~590 members known. Also called the Ariadne asteroids after 43 Ariadne.
- Nysa asteroids have a semi-major axis between 2.41 AU and 2.5 AU, an eccentricity between 0.12 and 0.21, and an inclination between 1.5° and 4.3°. Named after 44 Nysa, ~375 members known. Also called the Hertha asteroids after 135 Hertha.
- Maria asteroids have a semi-major axis between 2.5 AU and 2.706 AU and an inclination between 12° and 17°. Named after 170 Maria, ~80 members known.
Other families have been identified using a variety of techniques, most prominently the Hierarchical Clustering Method (HCM) and the Wavelet Analysis Method (WAM):
- Vesta asteroids are fragments of 4 Vesta's parent body, ~235 members.
- Erigone asteroid, named after 163 Erigone, ~45 members.
- Ceres asteroids, also known as Minerva asteroids, named after 1 Ceres or 93 Minerva, ~88 members.
- Dora asteroids, named after 668 Dora, ~78 members.
- Adeona asteroids, named after 145 Adeona, ~65 members.
- Merxia asteroids, named after 808 Merxia, ~25-30 members.
- Misa asteroids, named after 569 Misa, ~26 members.
- Chloris asteroids, named after 410 Chloris, ~24 members.
- Rafita asteroids, named after 1644 Rafita, ~22 members.
- Astrid asteroids, named after 1128 Astrid, ~10 members.
- Meliboea asteroids, named after 137 Meliboea, ~15 members.
- Naëma asteroids, named after 845 Naëma, 6 or 7 members.
- Massalia asteroids, named after 20 Massalia, ~47 members.
- Lydia asteroids, named after 110 Lydia, ~38 members.
- Nemesis asteroids or Concordia asteroids, named after 128 Nemesis or 58 Concordia, ~29 members.
- Bower asteroids or Endymion asteroids, named after 1639 Bower or 342 Endymion, ~10-15 members.
- Hygiea asteroids, named after 10 Hygiea, ~150 members.
- Veritas asteroids or Undina asteroids, named after 490 Veritas or 92 Undina, ~29 members.
- Brasilia asteroids, named after 293 Brasilia, ~14 members.
- Karin asteroids are a sub-family of the Koronis family; they number 39, the most prominent being 832 Karin. It is a young family which can be traced back to a break-up occurring 5.8 ± 0.2 million years ago.
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 asteroids are hypothetical asteroids with an aphelion less than 0.4 AU, ie, they orbit entirely within the orbit of Mercury. A few searches for Vulcanoids have been conducted but there have been none discovered so far.
- Apoheles are asteroids whose aphelion is less than 1 AU, meaning they orbit entirely within Earth's orbit. "Apohele" is Hawaiian for "orbit". Other proposed names for this group are Inner-Earth Objects (IEOs) and Anons (as in "Anonymous"). As of May 2004 there are only two known Apoheles: 2003 CP20 and 2004 JG6.
- Arjuna asteroids are somewhat vaguely defined as having orbits similar to Earth's; ie, with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, Apollo or 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 VG.
- Earth Trojans are asteroids located in the Earth-Sun Lagrangian points 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. No Earth trojans are currently known.
Groups out to the orbit of Mars
- The Aten, Apollo and Amor asteroids orbit between Earth and Mars; they are named after 2062 Aten, 1862 Apollo and 1221 Amor, respectively.
- Mars-crosser asteroids have orbits that cross that of Mars.
- Mars Trojans follow or lead Mars on its orbit, at either of the two Lagrangian points 60° ahead (L4) or behind (L5). The only one known is 5261 Eureka. The Minor Planet Center has not listed any Mars trojans with confirmed orbits , for controversial reasons.
Groups out to the orbit of Jupiter
A large number of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the Main belt. These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence, through orbital resonance, clears Kirkwood gaps in the asteroid belt, first recognised by Daniel Kirkwood in 1874. As a result of these gaps the asteroids in this region are divided into a large number of groups. They are:
- Hungaria asteroids, with a mean orbital radius between 1.78 AU and 2 AU, an eccentricity less than 0.18, and inclination between 16° and 34°. Named after 434 Hungaria, these are just outside Mars orbit, and are possibly attracted by the 2:9 resonance.
- Phocaea asteroids, with a mean orbital radius between 2.25 AU and 2.5 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 1:4 resonance with Jupiter. Named after 25 Phocaea.
- Main Belt I asteroids have a mean orbital radius between 2.3 AU and 2.5 AU and an inclination of less than 18°. This group appears to be a catch-all that includes everything in the inner main belt that doesn't belong to the Nysa or Flora families, with the division at 2.3 AU apparently an arbitrary one without physical significance.
- Alinda asteroids have a mean orbital radius of 2.5 AU and an eccentricity between 0.4 and 0.65 (approximately). These objects are held by the 1:3 resonance with Jupiter. Named after 887 Alinda.
- Pallas asteroids have a mean orbital radius between 2.5 AU and 2.82 AU and an inclination between 33° and 38°. Named after 2 Pallas.
- Main Belt II asteroids have a mean orbital radius between 2.5 AU and 2.706 AU and an inclination less than 33°.
- Main Belt IIb asteroids have a mean orbital radius between 2.706 AU and 2.82 AU and an inclination less than 33°.
- Main Belt IIIa asteroids have a mean orbital radius between 2.82 AU and 3.03 AU, an eccentricity less than .35, and an inclination less than 30°.
- Griqua asteroids have an orbital radius between 3.1 AU and 3.27 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 Griqua and 8373 Stephengould the most prominent.
- Main Belt IIIb asteroids have a mean orbital radius between 3.03 AU and 3.27 AU, an eccentricity less than .35, and an inclination less than 30°.
- Cybele asteroids have a mean orbital radius between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. This group appears to cluster around the 4:7 resonance with Jupiter. Named after 65 Cybele.
- Hilda asteroids have a mean orbital radius between 3.7 AU and 4.2 AU, an eccentricity greater than 0.07, and an inclination less than 20°. These asteroids are in a 2:3 resonance with Jupiter. Named after 153 Hilda.
- Thule asteroids appear to consist of only one object, 279 Thule, in a 3:4 resonance with Jupiter.
- Trojan asteroids have a mean orbital radius between 5.05 AU and 5.4 AU, and lie in elongated, curved regions around the two Lagrangian points 60° ahead and behind of Jupiter. The leading point, L4, is called the 'Greek' node and the trailing L5 point is called the 'Trojan' node, 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 Patroclus in the Trojan node and 624 Hektor in the Greek node are "misplaced" in the enemy camps.
Between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU), there's a 'forbidden zone'. Aside from 279 Thule and five objects in 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 Jupiter are believed to be composed of ices and other volatiles. Many are similar to comets, differing only in that the perihelia of their orbits are too distant from the Sun to produce a significant tail.
- Damocloid asteroids, also known as the "Oort cloud group," are named after 5335 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 orbits. The definition of this group is somewhat fuzzy, and may overlap significantly with comets.
- Centaurs have a mean orbital radius roughly between 5.4 AU and 30 AU. They are currently believed to be Trans-Neptunian Objects that "fell in" after encounters with gas giants. The first of these to be discovered was 2060 Chiron.
- The Neptune Trojans currently consist of only two objects, 2001 QR322 and 2004 UP₁₀.
- Trans-Neptunian Objects (TNOs) are anything with a mean orbital radius greater than 30 AU. This classification includes the Kuiper Belt Objects (KBOs) and the Oort cloud.
- Kuiper Belt Objects extend from roughly 30 AU to 50 AU and are broken into the following subcategories:
- Plutinos are KBOs 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 AU and 40.5 AU to be a Plutino.
- Cubewanos, also known as "classical KBOs". They are named after (15760) 1992 QB₁ and have a mean orbital radius between approximately 40.5 AU and 47 AU. Cubewanos are objects in the Kuiper belt that didn't get scattered and didn't get locked into a resonance with Neptune.
- Additional groups exist for other orbital resonances with Neptune than the 2:3 resonance of the Plutinos and the 1:1 resonance of the Neptune Trojans (such as 2001 QR322), but they have not yet been officially named. There are several known objects in the 2:1 resonance, unofficially dubbed "Twotinos," with a mean orbital radius of roughly 48 AU and an eccentricity of 0.37. There are several objects in the 2:5 resonance (mean orbital radius of 55 AU), and objects in the 4:5, 4:7, 3:5, and 3:4 resonances.
- Scattered Disk Objects (SDOs) generally have very large orbits of up to a few hundred AU. They are assumed to be objects that encountered Neptune and were "scattered" into long-period, very elliptical orbits with perihelia that are still not too far from Neptune's orbit.
- The Oort cloud is a hypothetical cloud of comets with a mean orbital radius between approximately 50,000 AU and 100,000 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 with the Oort cloud.
- Kuiper Belt Objects extend from roughly 30 AU to 50 AU and are broken into the following subcategories:
Quasi-satellites and "horseshoe objects"
Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA₂₉. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.
Sometimes these "horseshoe objects" temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their prior status. Both Earth and Venus are known to have quasi-satellites.
- Committee on Small Body Nomenclature
- List of minor planet orbital groupings and families
- James L. Hilton: When Did the Asteroids Become Minor Planets?
- Minor planet groups/families
- Hirayama, Kiyotsugu; Groups of asteroids probably of common origin (1918)
- Hirayama, Kiyotsugu; Groups of asteroids probably of common origin, Astronomical Journal, Vol. 31, No. 743, pp. 185-188 (October 1918)
- Kirkwood, Daniel; Relations between the Motions of some of the Minor Planets (1874)
- Bendjoya, Philippe; and Zappalà, Vincenzo; Asteroid Family Identification, in Asteroids III, pp. 613-618, University of Arizona Press (2002), ISBN 0816522812
- Zappalà, Vincenzo; Cellino, Alberto; Farinella, Paolo; and Knežević, Zoran; Asteroid families I - Identification by hierarchical clustering and reliability assessment, Astronomical Journal, Vol. 100, p. 2030 (December 1990)
- Zappalà, Vincenzo; Cellino, Alberto; Farinella, Paolo; and Milani, Andrea; Asteroid families II - Extension to unnumbered multiopposition asteroids, Astronomical Journal, Vol. 107, pp. 772-801 (February 1994)
- Nesvorný, David; Bottke Jr., William F.; Dones, Luke; and Levison, Harold F.; The recent breakup of an asteroid in the main-belt region, Nature, Vol. 417, pp. 720-722 (June 2002)