Monday 14 May 2012

Neptune's Trailing Trojans.

Trojan bodies are objects that orbit in, or close to, the L₄ and L₅ Legrange Points 60° ahead of and behind a larger body, where the gravity of that body and the object it is orbiting cancel each-other out. In our Solar system this means asteroids and (recently) artificial satellites in the Legrange Points of the planets, though in theory in other stellar systems planets could occupy the Legrange Points of larger planets, and moons could occupy the Legrange Points of larger moons.

The largest number of Trojan Asteroids yet discovered in our system share the orbit of Jupiter, though they have been found for several other planets (including Earth). In theory the largest number could share the orbit of Neptune (bigger orbit, more Trojans), though as yet only eight Neptune Trojans have been discovered, six at the leading (L₄) Legrange Point, and two at the trailing (L₅) Point. This is largely due to the great distance to the orbit of Neptune, but also due to the trailing Legrange Point being in line with the (bright) Galactic Center.

Diagrammatic representation of the Solar System showing the position of Jupiter's Trojans and Neptune's leading Trojans. Scott Sheppard/SpaceRef.

The orbits of the Neptune's Proceeding Trojans have previously been calculated, but those of the Trailing Trojans have proved harder, due to a lower number of confident sightings. The orbits of the Trojans are interesting because they shed light on the origin of the Solar System. If there are a large population of Trojan Asteroids in stable orbits, then it is likely that they may have formed in the same orbit as the planet, implying that the forming Neptune did not clear its orbit of all other debris, but if the majority of the Trojans are in unstable orbits then they are likely to have originated elsewhere and been captured recently, and are probably short term residents at the Legrange Points. The orbit's of the Trailing Trojans are of additional interest since NASA's New Horizons probe will pass through Neptune's L₅ Legrange Point within the next few years on its journey to Pluto.

The course of the New Horizons Probe. NASA/JPL.

In a paper published on the arXiv database at Cornell University Library on 10 May 2012, Pu Guan, Li-Yong Zhou and Jian Li of the Astronomy Department at Nanjing University and the Key Laboratory of Modern Astronomy and Astrophysics publish the results of a new study in which they calculate the orbits of the two known Trailing Trojans of Neptune, 2008 LC18 and 2004 KV18.

2004 KV18 was found to be in an unstable 'Tadpole Orbit'; an orbit which revolves around the Legrange Point, trailing further behind when it is furthest from Neptune, giving the orbit a 'tadpole' shape. This orbit is highly eccentric, and inclined to the plane of the Solar System by about 13.5°. It has probably been in this orbit for over 34 000 years, but no longer than 203 000 years. It will probably survive in this orbit for another 46 000-165 000 years. It is highly unlikely that 2004 KV18 originated near the orbit of Neptune, it most likely originated as a Trans-Neptune Object, and that it will one day go back to being one.

2008 LC18 in contrast is in a more stable orbit, though this is also tadpole shaped, and set at an angle of roughly 27.5° to the plane of the Solar System. Guan et al. calculate that there is a roughly 25% chance that it has been in roughly this orbit since the origin of the Solar System, but that it more likely originated as a Trans-Neptune Object, being captured in a Trojan orbit approximately 1.678 billion years ago, and it will probably eventually escape from this orbit in another 3.5 billion years, probably moving inwards to become one of the asteroids known as Centaurs; bodies that orbit between the orbits of Jupiter and Neptune, crossing the orbits of one or more of the giant planets.

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