Hubble Spots Moon Around Third Largest Dwarf Planet
Astronomers have uncovered a moon around another dwarf planet by using the combined power of three space observatories, including archival images from the Hubble Space Telescope. Called 2007 OR10, it is the third-largest dwarf planet in the Kuiper Belt. With this moon's discovery, most of the known dwarf planets in the Kuiper Belt larger than 600 miles across have companions. These bodies provide insight into how moons formed in the young solar system. In fact, there is an emerging view that collisions between planetary bodies can result in the formation of moons. Based on moon rock samples from NASA's Apollo mission, astronomers believe that Earth's only natural satellite was born out of a collision with a Mars-sized object 4.4 billion years ago. Read more
Title: Discovery of a satellite of the large trans-Neptunian object (225088) 2007OR10 Author: Csaba Kiss, Gábor Marton, Anikó Farkas-Takács, John Stansberry, Thomas Müller, József Vinkó, Zoltán Balog, Jose-Luis Ortiz, András Pál
2007OR10 is currently the third largest known dwarf planet in the transneptunian region, with an effective radiometric diameter of ~1535 km. It has a slow rotation period of ~45 h that was suspected to be caused by tidal interactions with a satellite undetected at that time. Here we report on the discovery of a likely moon of 2007OR10, identified on archival Hubble Space Telescope WFC3/UVIS system images. Although the satellite is detected at two epochs, this does not allow an unambiguous determination of the orbit and the orbital period. A feasible 1.5-5.8x10^21 kg estimate for the system mass leads to a likely 35 to 100 d orbital period. The moon is about 4.2m fainter than 2007OR10 in HST images that corresponds to a diameter of 237 km assuming equal albedos with the primary. Due to the relatively small size of the moon the previous size and albedo estimates for the primary remains unchanged. With this discovery all trans-Neptunian objects larger than 1000 km are now known to harbour satellites, an important constraint for moon formation theories in the young Solar system.
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. Read more