Neptune trojan

Discovery and exploration

In 2001, the first Neptune trojan was discovered, , near Neptune's region, and with it the fifth known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan has indicated that the Neptune trojans populate thick clouds, which has constrained their possible origins (see below).

On August 12, 2010, the first trojan, , 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 trojans are as common as large trojans, to within uncertainty, further constraining models about their origins (see below).

It would have been possible for the New Horizons spacecraft to investigate Neptune trojans discovered by 2014, when it passed 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 Horizonss flight path, allowing detection of objects that the spacecraft could image. , the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU.{{cite web |access-date = 2012-10-09 |url-status = live |archive-url = https://web.archive.org/web/20121101020405/http://www.planetary.org/blogs/guest-blogs/20121009-parker-neptune-trojan-ice-hunters.html |archive-date = 2012-11-01

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.

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 a capture during planetary migration instead of in situ or collisional formation. This process is reversible allowing new trojans to be captured when the planetary migration continues. For high-inclination trojans to be captured the migration must have been slow, or their inclinations must have been acquired previously.

Colors

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.

Several Neptunian trojans have been observed to have very-red colors similar to cold classical Kuiper belt objects.{{cite journal |display-authors = etal |doi-access = free

Naming

In 2015, the IAU adopted a new naming scheme for Neptune trojans, which are to be named after Amazons, with no differentiation between objects in and . The Amazons were an all-female warrior tribe that fought in the Trojan War on the side of the Trojans against the Greeks. As of 2025, the named Neptune trojans are 385571 Otrera (after Otrera, the first Amazonian queen in Greek mythology) and 385695 Clete (after Clete, an Amazon and the attendant to the Amazons' queen Penthesilea, who led the Amazons in the Trojan war).

Members

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 Neptune trojans with radii above 40 km in Neptune's . 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 trojans are estimated to be as common as large trojans.

and 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 September 2023, 31 Neptune trojans are known, of which 27 orbit near the Sun–Neptune Lagrangian point 60° ahead of Neptune, 4 orbit near Neptune's region 60° behind Neptune, and one orbits on the opposite side of Neptune () but frequently changes location relative to Neptune to and . These are listed in the following table. It is constructed from the list of Neptune trojans maintained by the IAU Minor Planet Center{{cite web |access-date = 2012-08-09 |url-status = live |archive-url = https://archive.today/20120525133119/http://www.minorplanetcenter.org/iau/lists/NeptuneTrojans.html |archive-date = 2012-05-25 |author-link = Scott S. Sheppard |doi-access= free

{{L3|nolink=yes}} Members

{{L4|nolink=yes}} Members

{{L5|nolink=yes}} Members

and were thought to be Neptune trojans at the time of their discovery, but further observations have disconfirmed their membership. is currently thought to be in a 3:5 resonance with Neptune. is currently following a quasi-satellite loop around Neptune.

Notes

References

References

1. Stern, Alan. (May 1, 2006). "Where Is the Centaur Rocket?". Johns Hopkins APL.
2. Parker, Alex. (April 30, 2013). "2011 HM102: A new companion for Neptune". The Planetary Society.
3. (2013). "A Uranian Trojan and the Frequency of Temporary Giant-Planet Co-Orbitals". Science.
4. Horner, J., Lykawka, P. S., Bannister, M. T., & Francis, P. [https://arxiv.org/ftp/arxiv/papers/1202/1202.3279.pdf 2008 LC18: a potentially unstable Neptune Trojan] Accepted to appear in [[Monthly Notices of the Royal Astronomical Society]]
5. (2009). "Chaotic Capture of Neptune Trojans". The Astronomical Journal.
6. (2016). "Neptune trojan formation during planetary instability and migration". Astronomy & Astrophysics.
7. (2015). "The intrinsic Neptune Trojan orbit distribution: Implications for the primordial disk and planet migration". Icarus.
8. (10 April 2018). "DIVISION F / Working Group for Small Body Nomenclature Working Group for Small Body Nomenclature. THE TRIENNIAL REPORT (2015 Sept 1 – 2018 Feb 15)". IAU.
9. (30 November 2015). "385571 Otrera (2004 UP10)". Minor Planet Center.
10. (18 May 2019). "385695 Clete (2005 TO74)". Minor Planet Center.
11. "Conversion of Absolute Magnitude to Diameter".
12. (28 January 2016). "Observation of Two New L4 Neptune Trojans in the Dark Energy Survey Supernova Fields". The Astronomical Journal.
13. [http://www.minorplanetcenter.net/iau/mpec/K05/K05U97.html MPEC 2005-U97 : 2005 TN74, 2005 TO74] Minor Planet Center
14. "Distant EKOs, 55".
15. "Orbit and Astrometry for 05TN74".
16. de la Fuente Marcos. (2012). "(309239) 2007 RW10: a large temporary quasi-satellite of Neptune". Astronomy and Astrophysics Letters.