(225088) 2007 OR10 is a trans-Neptunian object (TNO) orbiting the Sun in the scattered disc, approximately 1500 kilometers in diameter. It is the third-largest known body in the Solar System beyond Neptune, and the largest known body in the Solar System without a name. According to current estimates as of May 2016, it is slightly larger than Makemake or Haumea, and is hence almost certainly a dwarf planet. It has one known moon.
(225088) 2007 OR10 was discovered by California Institute of Technology astronomers as part of the PhD thesis of Megan E. Schwamb, who was at the time a graduate student of Michael E. Brown.
Brown nicknamed the object "Snow White" for its presumed white color, because it would have to be very large or very bright to be detected by their survey. It was also the "seventh dwarf" discovered by Brown's team, after Quaoar in 2002, Sedna in 2003, Haumea and Orcus in 2004, and Makemake and Eris in 2005. However, 2007 OR10 turned out to be one of the reddest objects in the Kuiper belt, comparable only to Quaoar, so the nickname was dropped.
2007 OR10 is currently the largest known object in the Solar System without an official name. In 2011 Brown decided he finally had enough information to justify giving it one, because the discovery of water ice and the possibility of methane makes it noteworthy enough to warrant further study. However, as of 2015, Brown had yet to propose a name, though he notes that in 2017 anyone will be able to make a proposal.
(225088) 2007 OR10 came to perihelion around 1857. As of February 2016 it is located 87.5 AU from the Sun and moving at 2.7 kilometers per second (6,000 miles per hour) with respect to the Sun. This makes it the third-farthest known large body in the Solar System, after V774104 (103 AU) and Eris (96.3 AU), and farther out than Sedna (85.7 AU). It has been farther from the Sun than Sedna since 2013. 2007 OR10 will be farther than both Sedna and Eris by 2045, and it will reach aphelion in 2130.
The size of an object can be calculated from its absolute magnitude (H) and the albedo (the amount of light it reflects). 2007 OR10 has an absolute magnitude (H) of 1.92, which makes it the fifth-brightest TNO known, a little less bright than Sedna (H=1.5; D≈1,000 km) and brighter than Orcus (H=2.2; D≈800 km).
2007 OR10 is among the reddest objects known. This is probably in part due to methane frosts, which turn red when bombarded by sunlight and cosmic rays.
The spectrum of 2007 OR10 shows signatures for both water ice and methane, which makes it similar in composition to Quaoar. The presence of red methane frost on the surfaces of both 2007 OR10 and Quaoar implies the existence of a tenuous methane atmosphere on both objects, 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. In particular, 2007 OR10's large size means that it is likely to retain even nitrogen, which almost all TNOs lose over the course of their existence. The presence of water ice on the surface of 2007 OR10 implies a brief period of cryovolcanism in its distant past.
The Deep Ecliptic Survey (DES) shows the orbit to be in a 3:10 resonance with Neptune. The MPC lists it as a scattered-disc object. 2007 OR10 was discovered when searching for objects in the region of Sedna.
2007 OR10 has been observed 46 times over seven oppositions with a precovery image from 1985.
It was formally announced on 7 January 2009.
Based on the most recent size estimates as of May 2016, 2007 OR10 would be the third-largest dwarf planet, after Pluto and Eris, and before Makemake, Haumea, and Ceres, though the error bars overlap with those of Makemake. The IAU has not addressed the possibility of accepting additional dwarf planets since before the discovery of 2007 OR10 was announced. Brown states that it "must be a dwarf planet even if predominantly rocky", and Scott Sheppard and colleagues think that it is "likely" to be a dwarf planet, based on its minimum possible diameter (552 km) and what is understood of the conditions for hydrostatic equilibrium in cold icy–rocky bodies. 2007 OR10 is too distant for its diameter to be resolved directly; Brown's estimate of 1,000–1,500 km is based on calculating the albedo that is the best fit in his model, which agrees with the 7006128000000000000♠1280±210 km determined from observations by the Herschel space observatory. If the orbit of OR10's small satellite (see below) can be well determined, its mass could be calculated directly; mass is also a factor in hydrostatic equilibrium.
In 2016, analysis of Hubble images of 2007 OR10 taken in 2010 revealed a satellite around 300 km in diameter and orbiting at a distance of at least 15,000 km. It was announced at the DPS48 meeting on 17 October 2016. The satellite is probably too small and dark to affect size estimates for the likely dwarf planet.