WILLIAM & DEBORAH HILLYARD

Solar System -

Neptune

Cis-Neptunian Objects

There are two distinct populations of cis-Neptunian Object. 

Centaurs
Centaurs are a collection of minor planets that orbit the Sun between the orbits of Jupiter and Neptune, and generally cross the orbit of at least one giant planet.  Current thinking is that they are probably disrupted Kuiper Belt or Scattered Disk Objects, that fell inwards rather than having been disrupted outwards.  Some exhibit cometary tails so would likely become comets if disrupted.  The first object identified as a Centaur was Chiron, also classified as a comet, which is somewhere around 125 miles across, while the largest found to date, Chariklo, is approximately 160 miles in diameter.  It is likely that that Saturn's moon Phoebe (about 67 miles across) is a captured Centaur. 

The eccentricity of the orbits of Centaurs varies from, for example, Chariklo, which has a near circular orbit, to Pholus, Asbolus and Nessus that have very eccentric orbits.  Some Centaurs also have retrograde orbits, and/or orbits inclined substantially from the plane of the ecliptic.  Because they cross the orbits of the giant planets and are not contained in any orbital resonances, they tend to be unstable, and survive as Centaurs for only up to a few million years.  Having moved inwards from the Kuiper Belt area and beyond, these object spend time as Centaurs before they are disrupted so much they fall further inwards.  Depending on their composition, some may become comets in the Jupiter family and ultimately either fall into the sun, collide with another planet, or interact with a planet and get thrown out of the Solar System completely. 


Neptune's Trojans

Neptune's Trojans follow stable orbits in the Sun-Neptune L4 and L5 Lagrangian points, 60 ahead of and behind Neptune, respectively.  To date, December 2011, eight objects have been discovered since 2001.  Six have been identified in the L4 position, and two have been found in the trailing L5 Lagrangian point.  Those found have diameters, very approximately, in the range 56 to 180 km (35 to 112 miles).  Following the discovery of 2005 TN53 orbiting at an inclination of around 25º and 2007 VL305 at an inclination of 28º, it is likely that the zones around the Lagrangian points are relatively thick with many objects.  Statistical analysis indicates that it is likely that Neptune has at least ten times as many Trojans as Jupiter, and that there may even be far more than there are asteroids in the Asteroid Belt.  Neptune, being so far away makes it very difficult to find these small objects. 
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