Four temporary Neptune co-orbitals: (148975) 2001 XA255, (310071) 2010 KR59, (316179) 2010 EN65, and 2012 GX17

Numerical simulations suggest that Neptune primordial co-orbitals may outnumber the equivalent population hosted by Jupiter, yet the objects remain elusive. Since the first discovery in 2001 just 10 minor planets have been identified as Neptune co-orbitals. On the other hand, some simulations predict that a negligible fraction of passing bodies are captured into the 1:1 commensurability with Neptune today. Hundreds of objects have been discovered in the outer solar system during the various wide-field surveys carried out during the past decade, and many of them have been classified using cuts in the pericentre and other orbital elements. This leads to possible misclassifications of resonant objects. Here, we explore this possibility to uncover neglected Neptune co-orbitals. We confirm that 4 objects previously classified as Centaurs by the MPC currently are temporary Neptune co-orbitals. (148975) 2001 XA255 is the most dynamically unstable of the four. It appears to be a relatively recent (50 kyr) visitor from the scattered disk in its way to the inner solar system. (310071) 2010 KR59 is following a complex horseshoe orbit, (316179) 2010 EN65 is in the process of switching from leading to trailing Trojan, and 2012 GX17 is a promising trailing Trojan candidate in urgent need of follow-up. The four objects move in highly inclined orbits and have significant eccentricities. These dynamically hot objects are not primordial 1:1 librators, but are captured and likely originated from beyond Neptune.Comment: 4 pages, 3 figures + 3 additional figures for the on-line edition, abstract shortened, accepted for publication in Astronomy and Astrophysics Letter

2001 QR322 – an update on Neptune’s first unstable Trojan companion

The Neptune Trojans are the most recent addition to the panoply of Solar system small body populations. The orbit of the first discovered member, 2001 QR322, was investigated shortly after its discovery, based on early observations of the object, and it was found to be dynamically stable on timescales comparable to the age of the Solar system. As further observations were obtained of the object over the following years, the best-fit solution for its orbit changed. We therefore carried out a new study of 2001 QR322’s orbit in 2010, finding that it lay on the boundary between dynamically stable and unstable regions in Neptune’s Trojan cloud, and concluding that further observations were needed to determine the true stability of the object’s orbit. Here we follow up on that earlier work, and present the preliminary results of a dynamical study using an updated fit to 2001 QR322’s orbit. Despite the improved precision with which the orbit of 2001 QR322 is known, we find that the best-fit solution remains balanced on a knife-edge, lying between the same regions of stability and instability noted in our earlier work. In the future, we intend to carry out new observations that should hopefully refine the orbit to an extent that its true nature can finally be disentangled