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Minor planet

Minor planet

Overview

An asteroid group or minor-planet group is a population of minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a 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...

s that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family
Asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions...

, 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.

There are relatively few asteroids that orbit close to the Sun.
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An asteroid group or minor-planet group is a population of minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a 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...

s that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family
Asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions...

, 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
    Vulcanoid asteroid
    The vulcanoids are a hypothetical population of asteroids that may orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915...

    s are hypothetical asteroids with an aphelion less than 0.4 AU, i.e., they orbit entirely within the orbit of Mercury
    Mercury (planet)
    Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

    . A few searches for vulcanoids have been conducted but there have been none discovered so far.
  • Apohele
    Apohele asteroid
    Apohele asteroids, or alternatively Interior-Earth Objects or Atira asteroids, are a subclass of Aten asteroids. They have not only their perihelion within Earth's orbit, but also their aphelion; that is, their entire orbit is within Earth's .The first suspected Apophele was , the first confirmed...

    s are asteroids whose aphelion is less than 0.983 AU, meaning they orbit entirely within Earth's orbit. Other proposed names for this group are Inner-Earth Objects or Interior Earth Objects (IEOs) and Anons (as in "Anonymous"). As of March 2008 there are only five known Apoheles with an arc of observations greater than 20 days: (163693) Atira, , , and ; while there are other four possible candidates, but with a too short arc of observations: , , and .
  • Mercury-crosser asteroids having a perihelion smaller than Mercury's 0.3075 AU.
  • Venus-crosser asteroid
    Venus-crosser asteroid
    A Venus-crosser is an asteroid whose orbit crosses that of Venus. The known numbered Venus-crossers and outer-grazers are listed below .Venus also has a quasi-satellite, 2002 VE68...

    s having a perihelion smaller than Venus
    Venus
    Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

    's 0.7184 AU. This group includes the above Mercury-crossers (if their aphelion is greater than Venus's perihelion. All known Mercury crossers satisfy this condition).
  • Earth-crosser asteroid
    Earth-crosser asteroid
    An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. The known numbered Earth-crossers are listed here. Those Earth-crossers whose semi-major axes are smaller than Earth's are Aten asteroids; the remaining ones are Apollo...

    s having a perihelion smaller than Earth
    Earth
    Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

    's 0.9833 AU. This group includes the above Mercury- and Venus-crossers, apart from the Apoheles. They are also divided into the
    • Aten asteroid
      Aten asteroid
      The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...

      s having a semi-major axis
      Semi-major axis
      The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

       less than 1 AU, named after 2062 Aten
      2062 Aten
      2062 Aten is an asteroid that was discovered at the Palomar Mountain Observatory by Eleanor F. Helin, who is now the principal scientist for the NEAT project. It is named after Aten, the Egyptian god of the sun....

      .
    • Apollo asteroid
      Apollo asteroid
      The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...

      s having a semi-major axis
      Semi-major axis
      The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

       greater than 1 AU, named after 1862 Apollo
      1862 Apollo
      1862 Apollo is a Q-type asteroid, discovered by Karl Reinmuth in 1932, but lost and not recovered until 1973. It is named after the Greek god Apollo....

      .
  • Arjuna asteroid
    Arjuna asteroid
    The Arjuna asteroids are a class of near-Earth asteroids whose orbits are very Earth-like in character, having low inclination, orbital periods close to one Earth year, and low eccentricity. The class is named after Arjuna, a central hero in Hindu mythology. The definition is somewhat fuzzy and...

    s are somewhat vaguely defined as having orbits similar to Earth's; i.e., 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
    Apohele asteroid
    Apohele asteroids, or alternatively Interior-Earth Objects or Atira asteroids, are a subclass of Aten asteroids. They have not only their perihelion within Earth's orbit, but also their aphelion; that is, their entire orbit is within Earth's .The first suspected Apophele was , the first confirmed...

    , Amor
    Amor asteroid
    The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

    , Apollo
    Apollo asteroid
    The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...

     or Aten
    Aten asteroid
    The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...

     groups can also be classified as Arjunas. The term was introduced by Spacewatch
    Spacewatch
    Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets...

     and does not refer to an existing asteroid; examples of Arjunas include 1991 VG
    1991 VG
    1991 VG is a Near-Earth object discovered by James Scotti. Due to its unusual orbit and rapid variation in brightness, there was early speculation that it could be a man-made spacecraft or rocket. In recent years, however, it has been discovered that most small objects with diameters less than...

    .
  • Earth Trojan
    Earth Trojan asteroid
    Earth trojans are asteroids that orbit in the vicinity of the Earth-Sun Lagrangian points and . They are named after the Trojan asteroids that are associated with the analogous Lagrangian points of Jupiter....

    s are asteroids located in the Earth-Sun Lagrangian point
    Lagrangian point
    The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

    s and . 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 Trojan is .
  • Near-Earth asteroids is a catch-all group for asteroids whose orbit closely approaches that of Earth. It includes almost all of the above groups, as well as the Amor asteroid
    Amor asteroid
    The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

    s.

Groups out to the orbit of Mars

  • The Amor
    Amor asteroid
    The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

     asteroids, named after 1221 Amor
    1221 Amor
    1221 Amor is the namesake of the Amor asteroids, a group of near-Earth asteroids whose orbits range between those of Earth and Mars. Amors are often Mars-crossers but they are not Earth-crossers....

     are Near-Earth asteroids that are not Earth-crossers
    Earth-crosser asteroid
    An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. The known numbered Earth-crossers are listed here. Those Earth-crossers whose semi-major axes are smaller than Earth's are Aten asteroids; the remaining ones are Apollo...

    , having a perihelion just outside the Earth's orbit.
  • Mars-crosser asteroid
    Mars-crosser asteroid
    A Mars-crosser is an asteroid whose orbit crosses that of Mars. The known numbered Mars-crossers are listed here. They include the two numbered Mars trojans 5261 Eureka and ....

    s have orbits that cross that of Mars, but do not necessarily closely approach the Earth's.
  • Mars trojans follow or lead Mars on its orbit, at either of the two Lagrangian point
    Lagrangian point
    The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

    s 60° ahead or behind . As of March 2008, four are known. The largest appears to be 5261 Eureka
    5261 Eureka
    5261 Eureka is the first asteroid discovered that turned out to be a Mars trojan asteroid. It was discovered by David H. Levy and Henry Holt at Palomar Observatory on June 20, 1990. It trails Mars at a distance varying by only 0.3 AU during each revolution...

    .
  • Many of the Earth-, Venus-, and Mercury-crosser asteroids have aphelia greater than 1 AU.

The asteroid belt



The overwhelming majority of known asteroids have orbits lying between the orbits of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, roughly between 2 to 4 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

. These could not form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence, through orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

, clears Kirkwood gap
Kirkwood gap
A Kirkwood gap is a gap or dip in the distribution of main-belt asteroids with semi-major axis , as seen in the histogram below...

s in the asteroid belt, first recognised by Daniel Kirkwood
Daniel Kirkwood
Daniel Kirkwood was an American astronomer.Born in Harford County, Maryland, he was graduated in mathematics from the York County Academy in York, Pennsylvania in 1838...

 in 1874.

The region with the densest concentration (lying between the Kirkwood gaps at 2.06 and 3.27 AU, with eccentricities below about 0.3, and inclinations smaller than 30°) is often called the Main belt. It can be further subdivided by the Kirkwood Gaps into the:
  • Inner Asteroid Belt, inside of the strong Kirkwood gap at 2.50 AU due to the 3:1 Jupiter orbital resonance
    Orbital resonance
    In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

    . The largest member is 4 Vesta
    4 Vesta
    Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

    .
    • It apparently also includes a group called the Main Belt I asteroids which have a semi-major axis between 2.3 AU and 2.5 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 AU. The largest member is Ceres. This group is apparently split into the:
    • Main Belt IIa asteroids which have a semi-major axis between 2.5 AU and 2.706 AU and an inclination less than 33°.
    • Main Belt IIb asteroids which have a semi-major axis between 2.706 AU and 2.82 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 Hygiea
    10 Hygiea
    10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...

    . This group is apparently split into the:
    • Main Belt IIIa asteroids which have a semi-major axis between 2.82 AU and 3.03 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 AU and 3.27 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 of the Main Belt, distinguished either by mean distance from the Sun, or particular combinations of several orbital elements:
  • 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
    434 Hungaria
    434 Hungaria is a relatively small inner Main belt asteroid. It is classified as an E-type asteroid. It is the namesake for Hungaria asteroids which orbit the sun on the inside of the 1:4 Kirkwood gap, standing out of the core of the main belt....

    , these are just outside Mars's orbit, and are possibly attracted by the 9:2 Jupiter resonance or the 3:2 Mars 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 4:1 resonance with Jupiter. Named after 25 Phocaea
    25 Phocaea
    25 Phocaea is a main-belt asteroid.It was discovered by J. Chacornac at Marseille, on April 6. 1853. It was his first asteroid discovery out of a total of six. It is named after Phocaea, the ancient Greek name for Foça in Turkey, from where came the founders of Marseille.-References:...

    .
  • 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 3:1 resonance with Jupiter and a 4:1 resonance with Earth
    Earth
    Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

    . 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 Alinda
    887 Alinda
    887 Alinda is an asteroid orbiting the Sun. It was originally discovered by German astronomer Max Wolf in 1918. It lies within the main asteroid belt and is the namesake for the Alinda family group of asteroids, although it is also classified as an Amor III asteroid...

    .
  • Pallas family
    Pallas family
    The Pallas or Palladian family of asteroids is a grouping of B-type asteroids at very high inclinations in the intermediate asteroid belt . It was first noted by Kiyotsugu Hirayama in 1928....

     asteroids have a mean orbital radius between 2.7 and 2.8 AU and an inclination between 30° and 38°. Named after 2 Pallas
    2 Pallas
    Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

    .
  • Griqua asteroid
    Griqua family
    The Griqua asteroids are a group of asteroids in the Main Belt orbiting the sun between 3.1 and 3.27 AU. Asteroids in this group have eccentricities greater than 0.35. The group derives its name from the asteroid 1362 Griqua....

    s 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
    Libration
    In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

     with Jupiter, in high-inclination orbits. There are about 5 to 10 of these known so far, with 1362 Griqua
    1362 Griqua
    1362 Griqua is an outer main-belt asteroid discovered on July 31, 1935 by Cyril V. Jackson at Union Observatory. It is one of very few asteroids located in the 2 : 1 mean motion resonance with Jupiter.- References :...

     and 8373 Stephengould
    8373 Stephengould
    8373 Stephengould is an outer main-belt asteroid discovered on January 1, 1992 by Carolyn S. Shoemaker and Eugene Merle Shoemaker at Palomar. It is one of very few asteroids located in the 2 : 1 mean motion resonance with Jupiter. The asteroid was named after the Harvard paleontologist Stephen Jay...

     the most prominent.
  • Cybele asteroid
    Cybele asteroid
    Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele...

    s 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 7:4 resonance with Jupiter. Named after 65 Cybele
    65 Cybele
    65 Cybele is one of the largest asteroids in the Solar System and is located in the outer asteroid belt. It gives its name to the Cybele family of asteroids that orbit outward from the Sun from the 2:1 orbital resonance with Jupiter. Cybele is a C-type asteroid, meaning that it is dark in color...

    .
  • 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 3:2 resonance with Jupiter. Named after 153 Hilda
    153 Hilda
    153 Hilda is a large asteroid in the outer main belt, with a diameter of 170 km. Because it is composed of primitive carbonaceous materials, it has a very dark surface. It was discovered by Johann Palisa on November 2, 1875 from the Austrian Naval Observatory at Pula...

    .
  • Thule asteroids are in a 4:3 resonance with Jupiter and the group is known to consist of 279 Thule
    279 Thule
    279 Thule is a large asteroid from the asteroid belt. It is classified as a D-type asteroid and is probably composed of organic-rich silicates, carbon and anhydrous silicates....

    , , and .
  • Trojan asteroid
    Trojan asteroid
    The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...

    s have a mean orbital radius between 5.05 AU and 5.4 AU, and lie in elongated, curved regions around the two Lagrangian point
    Lagrangian point
    The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

    s 60° ahead and behind of Jupiter. The leading point, , is called the 'Greek' node and the trailing point is called the 'Trojan' node, after the two opposing camps of the legendary Trojan War
    Trojan War
    In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans after Paris of Troy took Helen from her husband Menelaus, the king of Sparta. The war is among the most important events in Greek mythology and was narrated in many works of Greek literature, including the Iliad...

    ; with one exception apiece, objects in each node are named for members of that side of the conflict. 617 Patroclus
    617 Patroclus
    617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...

     in the Trojan node and 624 Hektor
    624 Hektor
    624 Hektor is the largest Jupiter Trojan. It was discovered in 1907 by August Kopff.Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiter's leading Lagrangian point, , called the 'Greek' node after one of the two sides in the legendary Trojan War...

     in the Greek node are "misplaced" in the enemy camps.


There is a forbidden zone between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU). Aside from 279 Thule
279 Thule
279 Thule is a large asteroid from the asteroid belt. It is classified as a D-type asteroid and is probably composed of organic-rich silicates, carbon and anhydrous silicates....

 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 of Jupiter are believed to be composed of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

s and other volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

. Many are similar to comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, differing only in that the perihelia of their orbits are too distant from the Sun to produce a significant tail.
  • Damocloid asteroid
    Damocloid asteroid
    Damocloids are minor planets such as 5335 Damocles and 1996 PW that have Halley family or long-period highly eccentric orbits typical of periodic comets such as Comet Halley, but without showing a cometary coma or tail....

    s, also known as the "Oort cloud group," are named after 5335 Damocles
    5335 Damocles
    5335 Damocles is the archetype of the Damocloids, asteroids that are inactive nuclei of the Halley Family and long-period comets. It was discovered in 1991 and named after Damocles, a figure of Greek mythology....

    . They are defined to be objects that have "fallen in" from the Oort cloud
    Oort cloud
    The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

    , so their aphelia are generally still out past Uranus
    Uranus
    Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

    , 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
    Centaur (planetoid)
    Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human...

     have a mean orbital radius roughly between 5.4 AU and 30 AU. They are currently believed to be Trans-Neptunian Object
    Trans-Neptunian object
    A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

    s that "fell in" after encounters with gas giants. The first of these to be discovered was 2060 Chiron
    2060 Chiron
    2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

    .

Groups at or beyond the orbit of Neptune

  • The Neptune Trojans currently consist of six objects: , , , , and .
  • Trans-Neptunian Object
    Trans-Neptunian object
    A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

    s (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:
      • Plutino
        Plutino
        In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...

        s are KBOs in a 2:3 resonance with Neptune, just like Pluto
        Pluto
        Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

        . 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. 90482 Orcus
        90482 Orcus
        90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified...

         and 28978 Ixion
        28978 Ixion
        28978 Ixion is a Kuiper belt object discovered on May 22, 2001. Ixion is a plutino and a very likely dwarf planet; its diameter of 650 km estimated by Spitzer makes it about the fifth largest plutino. It is named after Ixion, a figure from Greek mythology...

         are among the brightest known.
      • Cubewano
        Cubewano
        A classical Kuiper belt object, also called a cubewano is a low-eccentricity Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune’s orbit...

        s, also known as "classical KBOs". They are named after 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. (with two satellites) and are among the brightest.
      • Additional groups of resonant objects
        Resonant trans-Neptunian object
        In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc...

         occupy other orbital resonances with Neptune than the 2:3 resonance of the plutinos and the 1:1 resonance of the Neptune Trojans (such as ), but they have not yet been officially named. There are several known objects in the 1:2 resonance, unofficially dubbed twotinos, with a mean orbital radius of 47.7 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), unlike cubewanos and resonant objects, typically have highly inclined, 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 initial more circular, close to the ecliptic orbits. The recently famous, Pluto-size Eris belongs to this category.
      • Extended Scattered Disk (detached) objects with generally highly elliptical, very large orbits of up to a few hundred AU. Their perihelion is too far away from Neptune
        Neptune
        Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

         for any significant interaction to occur. One typical member of the extended disk is , while some researchers include Sedna
        90377 Sedna
        90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

         in this class.
    • The Oort cloud
      Oort cloud
      The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

       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
      90377 Sedna
      90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

       with the Oort cloud.

See also

  • Dwarf planet
    Dwarf planet
    A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

  • List of minor planets
  • Minor Planet Center
    Minor Planet Center
    The Minor Planet Center operates at the Smithsonian Astrophysical Observatory , which is part of the Center for Astrophysics along with the Harvard College Observatory ....

  • Mesoplanet
    Mesoplanet
    Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than Ceres. Assuming "size" is defined by linear dimension , mesoplanets should be approximately 1,000 km to 5,000 km in diameter...