S/2015 (136472) 1

S/2015 (136472) 1, nicknamed MK2 by the discovery team, is the only known moon of the trans-Neptunian 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 and follow-up observations in 2018 will enable Makemake's mass to be determined, which will allow an estimate of its density and other characteristics.