|‡ Trans-Neptunian dwarf planets are
Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbital path. Nine Neptune trojans are currently known, of which six orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune and three orbit near Neptune's L5 region 60° behind Neptune. The Neptune trojans are termed 'trojans' by analogy with the Jupiter trojans.
The discovery of 2005 TN53 in a high-inclination (>25°) orbit was significant, because it suggested a “thick” cloud of trojans (Jupiter trojans have inclinations up to 40°), which is indicative of freeze-in capture instead of in situ or collisional formation. It is suspected that large (radius ≈ 100 km) Neptune trojans could outnumber Jupiter trojans by an order of magnitude.
In 2010, the discovery of the first known L5 Neptune trojan, 2008 LC18, was announced. Neptune's trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars.
Discovery and exploration
In 2001, the first Neptune trojan was discovered, 2001 QR322, near Neptune's L4 region, and with it the fifth[note 1] known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, which has constrained their possible origins (see below).
On August 12, 2010, the first L5 trojan, 2008 LC18, was announced. It was discovered by a dedicated survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds. This suggests that large L5 trojans are as common as large L4 trojans, to within uncertainty, further constraining models about their origins (see below).
It may be possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passes through this region of space en route to Pluto. Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizons's flight path, allowing detection of objects that the spacecraft could image. 2011 HM102, the highest-inclination Neptune trojan known, may be just bright enough for New Horizons to observe in end-2013 at a distance of 1.2 AU.
Dynamics and origin
The orbits of Neptune trojans are highly stable; Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System. Neptune's L5 can host stable trojans equally well as its L4.
It is possible for Neptune trojans to librate up to 30° from their associated Lagrangian points with a 10,000-year period.
The unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole. The existence of high-inclination Neptune trojans points to 'freeze-in' capture or variations on this process, or during a slow, smooth migration, instead of in situ or collisional formation, as the origin of Neptune trojans. The captured population already had to be dynamically excited for high-inclination trojans to exist. Although resonant trans-Neptunian objects are thought to have been captured by sweeping resonances during planet migration, this process would cause the escape of Neptune trojans. Irregular planetary migration would result in the depletion of the associated trojan reservoir. The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture and points to a common capture mechanism for both L4 and L5 trojans. The original population of trojans probably contained many objects on dynamically unstable orbits, and the current trojan population continues to contribute centaurs. On the other hand, a trojan on a stable orbit need not be primordial.
Although Neptune currently cannot efficiently capture trojans even for short periods, capture of centaurs into unstable Neptune-trojan orbits is expected to occur to some extent. A simulation study concluded that at any given time, 2.8% of the centaurs in the scattered population within 34 AU would be Neptune co-orbitals; of these, it was predicted that 54% would be in horseshoe orbits, 10% would be quasi-satellites, and 36% would be trojans (evenly split between the L4 and L5 groups).
The first four discovered Neptune trojans have similar colors. They are modestly red, slightly redder than the gray Kuiper belt objects, but not as extremely red as the high-perihelion cold classical Kuiper belt objects. This is similar to the colors of the blue lobe of the centaur color distribution, the Jupiter trojans, the irregular satellites of the gas giants, and possibly the comets, which is consistent with a similar origin of these populations of small Solar System bodies.
The Neptune trojans are too faint to efficiently observe spectroscopically with current technology, which means that a large variety of surface compositions are compatible with the observed colors.
The amount of high-inclination objects in such a small sample, in which relatively fewer high-inclination Neptune trojans are known due to observational biases, implies that high-inclination trojans may significantly outnumber low-inclination trojans. The ratio of high- to low-inclination Neptune trojans is estimated to be about 4:1. Assuming albedos of 0.05, there are an expected 400+250
−200 Neptune trojans with radii above 40 km in Neptune's L4. This would indicate that large Neptune trojans are 5 to 20 times more abundant than Jupiter trojans, depending on their albedos. There may be relatively fewer smaller Neptune trojans, which could be because these fragment more readily. Large L5 trojans are estimated to be as common as large L4 trojans.
2001 QR322 and 2008 LC18 display significant dynamical instability. This means they could have been captured after planetary migration, but may as well be a long-term member that happens not to be perfectly dynamically stable.
As of October 2012, nine Neptune trojans are known, of which six orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune and three orbit near Neptune's L5 region 60° behind Neptune. These are listed in the following table. It is constructed from information obtained from the List Of Neptune Trojans maintained by the IAU Minor Planet Center and with diameters from Sheppard and Trujillo's paper on 2008 LC18, unless otherwise noted.
2005 TN74 and (309239) 2007 RW10 were believed to be Neptune trojans at the time of their discovery, but further observations have disconfirmed their membership. 2005 TN74 is currently thought to be in a 3:5 resonance with Neptune. (309239) 2007 RW10 is currently following a quasi-satellite loop around Neptune.
- "List Of Neptune Trojans". Minor Planet Center. Retrieved 2012-08-09.
- Sheppard, Scott S.; Trujillo, Chadwick A. (June 2006). "A Thick Cloud of Neptune Trojans and Their Colors" (PDF). Science 313 (5786): 511–514. Bibcode:2006Sci...313..511S. doi:10.1126/science.1127173. PMID 16778021. Retrieved 2008-02-26.
- Jewitt, David C.; Trujillo, Chadwick A.; Luu, Jane X. (2000). "Population and size distribution of small Jovian Trojan asteroids". The Astronomical journal 120 (2): 1140–7. arXiv:astro-ph/0004117. Bibcode:2000AJ....120.1140J. doi:10.1086/301453.
- E. I. Chiang and Y. Lithwick Neptune Trojans as a Testbed for Planet Formation, The Astrophysical Journal, 628, pp. 520–532 Preprint
- David Powell (30 January 2007). "Neptune May Have Thousands of Escorts". Space.com. Retrieved 2007-03-08.
- Scott S. Sheppard (2010-08-12). "Trojan Asteroid Found in Neptune's Trailing Gravitational Stability Zone". Carnegie Institution of Washington. Retrieved 2007-12-28.
- Sheppard, Scott S.; Trujillo, Chadwick A. (2010-08-12). "Detection of a Trailing (L5) Neptune Trojan". Science (AAAS) 329 (5997): 1304. Bibcode:2010Sci...329.1304S. doi:10.1126/science.1189666. PMID 20705814. Retrieved 2010-08-13.
- Parker, Alex (2012-10-09). "Citizen "Ice Hunters" help find a Neptune Trojan target for New Horizons". Planetary Society blogs. The Planetary Society. Retrieved 2012-10-09.
- Horner, J., Lykawka, P. S., Bannister, M. T., & Francis, P. 2008 LC18: a potentially unstable Neptune Trojan Accepted to appear in Monthly Notices of the Royal Astronomical Society
- Alexandersen, M.; Gladman, B.; Greenstreet, S.; Kavelaars, J. J.; Petit, J. -M.; Gwyn, S. (2013). "A Uranian Trojan and the Frequency of Temporary Giant-Planet Co-Orbitals". Science 341 (6149): 994–997. arXiv:1303.5774. doi:10.1126/science.1238072. PMID 23990557.
- The Tracking News
- "JPL Small-Body Database Browser: 2004 KV18". Retrieved 2012-03-03.
- MPEC 2005-U97 : 2005 TN74, 2005 TO74 Minor Planet Center
- "Distant EKOs, 55". Retrieved 2012-07-24.
- Orbit Fit and Astrometric record for 05TN74
- de la Fuente Marcos & de la Fuente Marcos (2012). "(309239) 2007 RW10: a large temporary quasi-satellite of Neptune". Astronomy and Astrophysics Letters 545: L9. arXiv:1209.1577. Bibcode:2012A%26A...545L...9D. doi:10.1051/0004-6361/201219931.
- Planetary Trojans – the main source of short period comets? (arXiv:1007.2541 : 15 Jul 2010)
- "New Trojan asteroid hints at huge Neptunian cloud". New Scientist.