2007 OR10 has stood out as the largest known trans-Neptunian object lacking a Moon. Most of the other big worlds have at least one: Pluto has Charon, Styx, Nix, Hydra and Kerberos; Eris has Dysnomia; Makemake has one discovered this year; Haumea has Hi'iaka and Namaka; Orcus has Vanth; Quaoar has Weywot. Now Sedna is the largest known object out there for which we have not -- yet -- discovered a moon. (Hopefully somebody will look!)

With a slow rotation period of 45 hours, astronomers have suspected for a while that 2007 OR10 had a moon, whose tidal interactions with the primary would have been responsible for slowing its rotation. Marton et al report the discovery in a set of older Hubble WFC3 images taken in September 2010. The new moon likely has a diameter of roughly 300 kilometers (roughly Hyperion-sized), as compared to 2007 OR10's roughly 1575 kilometers (roughly Rhea-sized). Although 300 kilometers is fully 20% of the diameter of the primary, Marton and coworkers state that, given a likely very dark surface, it doesn't contribute enough reflected light to 2007 OR10's signal to affect diameter estimates for the larger body. It is also too far from the parent body for the two to be in mutually synchronous rotation.

Both Makemake and 2007 OR10's newly discovered moons need followup to better determine their properties and orbits. Alex Parker proposed to follow up Makemake with Hubble, but his proposal was declined; Marton and Kiss told me they have proposed to follow up OR10 with Hubble, but have not heard yet.

One thing we do know: when OR10 finally gets a name, it needs to be a name that allows for the easy generation of a companion name for its moon!

The original images were taken as a part of a large survey of trans-Neptunian objects by Wesley Fraser; the image I processed to produce the view above was IBJG01FMQ.

Many thanks to Alex Parker for pointing out this poster to me!