Comet P/2010 TO20 LINEAR-Grauer was discovered in October 2010 by the LINEAR survey and was initially classified as a Jupiter Trojan. A year later, Grauer found that LINEAR-Grauer was actually active and the object was reclassified as a comet. I first thought this could be the first active Trojan to be discovered.
This image shows comet LINEAR-Grauer as I saw it at the ESO NTT telescope in Chile on 2011/10/29. The anti-solar (yellow arrow) and anti-orbital (orange arrow) directions are indicated.
Is it a Trojan?
Jupiter Trojans are asteroids that lead and trail Jupiter along its orbit by about 60 degrees. This is a plan view of the solar system showing the positions of Trojans Hektor and Patroclus on 2012/10/26.
If Trojans formed at their current location or even further away from the Sun then they should contain water ice. If heated, that ice would sublimate and produce cometary activity. However, no active/cometary Trojan has ever been seen. Water ice has also never been detected (using spectroscopy) on Trojan surfaces. The only indirect evidence that Trojans contain ice comes from density estimates for three of them. Patroclus, 29314 and 17365 all have densities less or equal to 1 gram per cubic centimeter indicating an icy interior.
Probably a Centaur
It turns out LINEAR-Grauer is probably not a Trojan. Numerical simulations of its orbital evolution show that this object probably originated in the outer solar system and has recently arrived in the Jupiter region. The figures below show the increasing mean distance to the sun and perihelion distance as we look further into the past as given by the simulations. Objects in transit from the outer to the inner solar system are called Centaurs. They originate in the Kuiper belt beyond Neptune and the ones that survive all the way in without colliding with a planet or being scattered away become Jupiter family comets.
A mini-29P/Schwassmann-Wachmann 1
Comet LINEAR-Grauer has a nearly circular, low inclination orbit similar to that of Centaur/comet 29P/Schwassmann-Wachmann 1. The two orbits, plus those of Earth, Mars, Jupiter and Saturn are show below.
These objects are similar in more ways than one. They both seem to have recently arrived from the outer solar system, they have similar surface colours, and they are both active (probably due to ice sublimation) and lose mass at a similar rate per unit area. The main difference is that LINEAR-Grauer is ten times smaller in size.
| Comet | Orbit (a, e, i) | Colour (V-R) | Radius | Specific Mass Loss Rate |
| L-G | 5.6 AU, 0.09, 2.6 deg | 0.47±0.06 mag | < 3 km | 7.7e–7 kg/m^2/s |
| 29P | 6.0 AU, 0.04, 9.4 deg | 0.50±0.03 mag | 23±3 km | 9.7e–7 kg/m^2/s |
It will be interesting to see if, like 29P, LINEAR-Grauer will remain active for its whole orbit around the sun.
The results were published in Monthly Notices of the Royal Astronomical Society. You can find an open-source copy of the paper here.
Pedro Lacerda 