Neptune trojans are small Solar System bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, similar to the trojans of other planets. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbital path. 35 Neptune trojans are currently known, of which 28 orbit near the Sun–Neptune Lagrangian point 60° ahead of Neptune and 7 orbit near Neptune's region 60° behind Neptune. The Neptune trojans are termed 'trojans' by analogy with the Jupiter trojans.
The discovery of 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 of that size by an order of magnitude.E. I. Chiang and Y. Lithwick Neptune Trojans as a Testbed for Planet Formation, The Astrophysical Journal, 628, pp. 520–532 Preprint
In 2010, the discovery of the first known Neptune trojan, , was announced. Neptune's trailing region is currently very difficult to observe because when viewed from Earth, 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, , 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. However, New Horizons may not have had sufficient downlink bandwidth, so it was eventually decided to give precedence to the preparations for the Pluto flyby.
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 can host stable trojans equally well as its . Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10,000-year period. Neptune trojans that escape enter orbits similar to centaurs. Although Neptune cannot currently capture stable trojans, roughly 2.8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, and 36% would be trojans (evenly split between the and groups).
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. The estimated equal number of large and trojans indicates that there was no gas drag during capture and points to a common capture mechanism for both and trojans. The capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the locations where Uranus passes Neptune can circulate with a period that is in resonance with the libration periods of Neptune trojans. This results in repeated perturbations that increase the libration of existing trojans causing their orbits to become unstable. 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.
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, 35 Neptune trojans are known, of which 28 orbit near the Sun–Neptune Lagrangian point 60° ahead of Neptune, 7 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 and with diameters from Sheppard and Trujillo's paper on , unless otherwise noted.
Members
Name Prov.designation Lagrangianpoint q () Q () e i (°) Abs. mag Diameter(km) Notes MPC 21.109 40.613 0.310 19.2 7.2 2010 Jumping trojan
Members
Name Prov.designation Lagrangianpoint q () Q () e i (°) Abs. mag Diameter(km) Notes MPC 385571 Otrera 29.318 30.942 0.031 1.4 8.8 2004 First Neptune trojan numbered and named 385695 Clete 28.469 31.771 0.052 5.3 8.3 2005 – 28.130 32.028 0.065 28.1 8.5 2007 – 29.622 30.503 0.009 9.6 7.8 2016 – 29.064 30.878 0.025 22.3 7.3 2016 – 29.404 31.011 0.031 1.3 8.1 2001 First Neptune trojan discovered, unstable Trojan 29.077 31.014 0.028 8.2 7.6 2006 – 28.092 32.162 0.067 25.0 9.0 2005 First high-inclination trojan discovered 28.608 31.253 0.048 6.6 8.1 2016 – 27.913 32.189 0.070 4.3 7.8 2016 – 27.806 32.259 0.072 20.8 9.3 2014 – 28.794 31.538 0.042 28.4 7.6 2019 – 29.366 30.783 0.028 10.1 8.8 2020 – 28.611 31.784 0.053 7.5 8.9 2021 – 28.092 32.135 0.066 13.1 8.2 2020 – 27.787 32.683 0.081 18.6 9.6 2021 – 27.563 32.525 0.087 31.3 8.3 2018 – 26.961 33.215 0.101 18.8 8.3 2014 Most eccentric stable Neptune trojan 28.137 31.971 0.067 19.4 9.3 2015 – 28.426 31.614 0.050 29.5 8.4 2020 – 27.038 33.060 0.096 33.7 8.2 2020 – 28.661 31.457 0.045 35.8 8.2 2018 – 27.309 33.243 0.098 30.8 8.6 2021 – 27.742 32.236 0.074 30.8 10.2 2018 – 27.513 32.497 0.086 16.9 9.0 2018 – 28.488 31.488 0.049 5.0 8.4 2018 – 27.612 32.327 0.073 17.2 9.2 2018 – 29.211 31.174 0.033 38.9 7.3 2021 Highest known inclination
members
Name Prov.designation Lagrangianpoint q () Q () e i (°) Abs. mag Diameter(km) Notes MPC 24.553 35.851 0.183 13.6 8.7 2011 Temporary Neptune trojan 27.365 32.479 0.079 27.6 8.2 2008 First trojan discovered 27.662 32.455 0.083 29.4 8.1 2012 – 26.624 34.084 0.124 6.6 6.8 2016 Stability uncertain 28.706 31.214 0.042 6.0 7.66 2025 – 28.471 31.406 0.049 28.8 8.51 2025 – 27.731 32.367 0.077 29.7 7.78 2025 –
MPEC 2005-U97 : 2005 TN74, 2005 TO74 Minor Planet Center 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.
See also Nice model Nice 2 model , a temporary quasi-satellite of Neptune.
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