Jupiter is known to have over 5000 co-orbital asteroids, and Neptune is believed likely to have even more. However Neptune is far further away than Jupiter, and objects sharing its orbit are correspondingly dimmer and hard to observe, with the result that only ten such bodies are known. It is likely that many objects currently classified as Centaurs (minor planets which never come closer to the Sun than Jupiter, and whose average distance from the Sun is no closer than Neptune's) may in fact be co-orbital with Neptune.
In a paper published on the online arXiv database at Cornell University Library on 12 October 2012, and in the journal Astronomy and Astrophysics on 15 October 2012, Carlos and Raúl de la Fuente Marcos of the Universidad Complutense de Madrid examine eleven bodies with orbits known to be close to that of Neptune, and conclude that four of these are in fact co-orbital.
The first of these objects is (148975) 2001 XA₂₅₅, discovered in December 2001 by a team of astronomers at Mauna Kea Observatory led by David Jewitt. This body is currently classified as a Centaur, with an eccentric orbit that brings it just within the orbit of Saturn and outside the orbit of Neptune, completing one orbit every 2.86 Earth years. De la Fuente Marcos and de la Fuente Marcos conclude that this object is in a horseshoe orbit relative to Neptune; that is to say an object that appears to describe a horseshoe shape when seen from another, larger object. This is caused by the smaller body moving in an orbit slightly faster than the larger object, until it catches up with that object, when it looses some inertial energy to the larger body. This causes the larger body to speed up slightly (though probably not perceptibly), and the smaller body to slow down. The smaller body then slows down relative to the larger body, until the larger body catches up with it, at which point a second exchange of inertial energy takes place, causing the smaller body to accelerate and the larger body to slow down, a state of affairs that can potentially continue indefinitely.
(148975) 2001 XA₂₅₅ appears to be a former Scattered Disk object (an object permanently outside the orbit of Neptune) that was nocked inwards within the Solar System about 50 000 years ago. It may have originated between 100 and 200 AU from the Sun (i.e. between 100 and 200 times as far from the Sun as the Earth). It is thought that (148975) 2001 XA₂₅₅ entered its current orbit about 10 000 years ago, and is unlikely to survive there for more than another 2000 years, due to the unstable nature of the orbit, which is perturbed by Saturn, Uranus, and Neptune.
The second object considered a Neptune co-orbital by de la Fuente Marcos and de la Fuente Marcos is (310071) 2010 KR₅₉, discovered in May 2010 by NASA’s Wide-field Infrared Survey Explorer. Like (148975) 2001 XA₂₅₅, (310071) 2010 KR₅₉ is considered to be a Centaur, with an orbit that takes it outside that of Neptune, but never brings it within the orbit of Saturn. De la Fuente Marcos and de la Fuente Marcos consider this object to be in a horseshoe orbit with a 1:1 resonance with Neptune (i.e. having the same orbital time as Neptune, encountering it once every orbit). They think it has been in this orbit for about 100 000 years, and will remain in it for a few hundred thousand years.
The orbit of (310071) 2010 KR₅₉. Created using the JPL Small-Body Database Browser.
The third object considered is (316179) 2010 EN₆₅, discovered in March 2010 by David Rabinowitz and Suzanne Tourtellotte of Yale University using the 1.3 m reflector at the Cerro Tololo Inter-American Observatory. This has previously been described as a Centaur, a Scattered Disk Object and an object with a 'Neptune-like' orbit. De la Fuente Marcos and de la Fuente Marcos consider this object to be a 'Jumping Trojan' with a horseshoe objects that takes it between Neptune's L₄ and L₅ Lagrangian points (points 60° ahead of and behind Neptune in its orbit, where the gravity of Neptune and the Sun cancel one-another out).
The orbit of (316179) 2010 EN₆₅. Created using the JPL Small-Body Database Browser.
The final object considered is 2012 GX₁₇, discovered by the Pan-STARRS 1 telescope at, Haleakala Observatories in Hawaii in March this year (2012), and considered to be a Centaur. De la Fuente Marcos and de la Fuente Marcos consider this obtect to be a potential Trailing Trojan of Neptune, with a similar orbital period and occupying the planet's L₅ Lagrangian point.
The orbit of 2012 GX₁₇. Created using the JPL Small-Body Database Browser.
The orbits of 2001 XA₂₅₅, 2010 KR₅₉, 2010 EN₆₅ and 2012 GX₁₇ as calculated by Carlos and Raúl de la Fuente Marcos. De la Fuente Marcos and de la Fuente Marcos (2012).
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