(225088) 2007 OR10 is a very large trans-Neptunian object. 2007 OR10 is possibly the 5th largest TNO and 2nd largest SDO currently known.
2007 OR10's mass has never been measured, and its diameter is not well known. Some astronomers consider it a dwarf planet and others consider it likely to be one, but it has not been officially classified as a dwarf planet by the IAU.
Discovery
2007 OR10 was discovered by California Institute of Technology astronomers as part of the PhD thesis of Meg Schwamb, who was at the time a graduate student of Michael E. Brown. The survey that found 2007 OR10 was specifically looking for quite distant objects like Sedna.
Naming
The newly discovered TNO was provisionally designated 2007 OR10. Currently, it is the largest body in the Solar System without a name.
Michael E. Brown nicknamed the object "Snow White" for its presumed white color, as it would have to be very large or very bright to be detected by their survey.
Stats
Estimated Diameter: 1420 km (1000 - 1500 km)
Aphelion: 100.79 AU
Perihelion: 33.62 AU
Semi-major axis: 67.21 AU
Orbital Period: 550.98 years
Rotation period: ?
Date discovered: 2007.7.17
Satellite: ?
Classification: TNO, Scattered Disc Object
Orbit
2007 OR10 is on an orbit similar to that of the dwarf planet Eris, making it a scattered disc object.
The preliminary motion of 2007 OR10 librating in a 10:3 resonance with Neptune. This resonance is not confirmed and may be merely a near resonance.
2007 OR10 is currently 86 AU from the Sun. This makes it the 3rd farthest known large body currently known in the Solar System, after Eris (97 AU) and Sedna (88 AU). 2007 OR10 will be further from the Sun than Sedna in 2013 and will be further than both Sedna and Eris by 2045. 2007 OR10 will come to aphelion (furthest distance from the Sun) in 2130.
Physical characteristics
Despite the nickname "Snow White", 2007 OR10 is among the reddest Kuiper belt objects known, comparable only to Quaoar, so the nickname turned out to not be very appropriate. This is probably in part due to methane frosts, which turn red when bombarded by sunlight and cosmic rays.
2007 OR10's spectrum shows signatures for both water ice and methane. The presence of red methane frost on the surfaces of 2007 OR10 implies the existence of a tenuous methane atmosphere, slowly evaporating into space.
Although 2007 OR10 comes closer to the Sun than Quaoar, and is thus warm enough that a methane atmosphere should evaporate, its larger mass makes retention of an atmosphere just possible. The presence of water ice on the surface of 2007 OR10 implies a brief period of cryovolcanism in its distant past.
Dwarf-planet status
If a trans-Neptunian dwarf-planet candidate does not have a known natural satellite with a well determined orbit, astronomers can not directly calculate the objects mass to determine if it might be in hydrostatic equilibrium.
Most objects at that distance are too small and too far away to directly resolve their diameters. Thus candidates' diameters can generally only be estimated from their absolute magnitude and best-fit albedo estimates.
2007 OR10 has no known satellite, so its mass is unknown. The diameter range of 2007 OR10 is based off of an assumed albedo that is a best-fit from Mike Brown's model. It is accepted as a dwarf planet by Mike Brown, and is listed on his website as "nearly certainly" a dwarf planet, saying it "must be a dwarf planet even if predominantly rocky".
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