Makemake and its moon (arrow)
|Discovery date||April 2015|
|~ 87.5 km (for an assumed albedo of 4%)|
S/2015 (136472) 1, nicknamed MK2 by the discovery team, is the only known moon of the dwarf planet Makemake. It is estimated to be 175 km (110 mi) in diameter (for an assumed albedo of 4%) and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake. Its orbital period is ≥ 12 days (the minimum values are those for a circular orbit; the actual orbital eccentricity is unknown). Observations leading to its discovery occurred in April 2015, using the Hubble Space Telescope's Wide Field Camera 3, and its discovery was announced on 26 April 2016.
Makemake and Sedna are the least-well-known of the large trans-Neptunian objects. The discovery of MK2 enables Makemake's mass to be determined, which will allow an estimate of its density.
A preliminary examination of the imagery suggests that MK2 has a reflectivity similar to charcoal, making it an extremely dark object. This is somewhat surprising because Makemake is the second-brightest-known object in the Kuiper belt. One hypothesis to explain this is that its gravity is not strong enough to prevent bright but volatile ices from being lost to space when it is heated by the distant Sun.
Further observations are needed in order to determine MK2's orbit. If it is circular, it would suggest that MK2 was formed by an ancient impact event, but if it is elliptical, it suggests that it may have been captured.
Alex Parker, the leader of the team that performed the analysis of the images at the Southwest Research Institute, said that MK2's orbit appears to be aligned edge-on to Earth-based observatories. This would make it much more difficult to detect because it would be lost in Makemake's glare much of the time, which, along with its dark surface, would contribute to previous surveys failing to observe it.
The nickname 'MK2' simply means object 2 in the Makemake system. A permanent name may be chosen from an associated figure in the mythology of Easter Island.