S/2015 (136472) 1

S/2015 (136472) 1

Dwarf planet Makemake and its moon
Discovery
Discovered by	HST/Wide Field Camera 3 Alex Parker Marc Buie Will Grundy Keith Noll
Discovery date	April 2015
Designations
Pronunciation	//
Alternative names	MK 2 (unofficial)
Orbital characteristics
Semi-major axis	21,000 km to 300,000 km
Orbital period	12.4 days to 660 days
Satellite of	Makemake
Physical characteristics
Dimensions	~175 to 250 km
Geometric albedo	~0.1
Apparent magnitude	25.1

S/2015 (136472) 1, nicknamed MK 2 by the discovery team, is the only known moon of the dwarf planet Makemake. S/2015 (136472) 1 is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake. Observations leading to the discovery of the moon occurred in April 2015, using the Hubble Space Telescope's Wide Field Camera 3 and its discovery was announced on 26 April 2016.

A preliminary examination of the imagery suggests that S/2015 (136472) 1 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 will be needed in order to determine S/2015 (136472) 1's orbit. If it is circular, it would suggest that S/2015 (136472) 1 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 S/2015 (136472) 1'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 and, along with its dark surface, would contribute to previous surveys failing to observe it.