MK2 (S/2015 (136472) 1)

S/2015 (136472) 1, nicknamed MK2 by its discoverers, is the only known moon of the dwarf planet Makemake, as of August 2026. It was found in images taken by the Hubble Space Telescope's Wide Field Camera 3 on 27 April 2015 and announced on 26 April 2016 by a team led by Alex Parker and Marc Buie of the Southwest Research Institute, with Will Grundy and Keith Noll. The moon remains formally unnamed; MK2 is a nickname, not an official title from the International Astronomical Union.

MK2 is tiny and extremely dark. NASA gives its radius as about 80 km, so a diameter of roughly 160 km, while later analysis suggests about 175 km if its surface reflects around 4 per cent of sunlight. Either way, it is more than 1,300 times fainter than Makemake, a difference of 7.8 magnitudes. Its reflectivity, or albedo, is estimated at just 2 to 4 per cent, about as dark as charcoal. That contrasts sharply with Makemake, whose icy surface is one of the brightest in the Kuiper Belt. One possibility is that the moon is too small for its gravity to hold on to bright, volatile ices such as methane, leaving a darker residue behind.

The moon orbits at about 21,000 km from Makemake (NASA quotes approximately 13,000 miles) on a nearly circular path. NASA's discovery announcement could only say the orbit takes 12 days or longer; later tracking of the moon gives a period of about 18 days. Its orbit appears close to edge-on from Earth, which is why earlier searches missed it: for much of each orbit it sits inside Makemake's glare. Following the orbit lets astronomers measure Makemake's mass and density, which in turn reveals what the dwarf planet is made of.

No spacecraft has visited the Makemake system, and none is currently planned. Everything known about MK2 comes from Hubble and ground-based telescopes. Makemake itself was discovered in March 2005 by Mike Brown, Chad Trujillo and David Rabinowitz at Palomar Observatory, orbits the Sun at about 45.8 AU (6,847 million km) and takes about 305 years to complete one orbit. A common error is to give MK2 a proper name, or to describe it as bright and icy like its parent; it is neither.