Makemake is a dwarf planet about 1,400 kilometers in diameter (or about 2/3 the size of Pluto). It is about 45 AU from the Sun, making it part of the Kuiper belt. We now know it also has at least one moon.
The moon, known as MK 2, is about 160 kilometers in diameter. While Makemake is the second brightest Kuiper belt object (after Pluto), its moon is quite dark. This is similar to Pluto’s moon Charon, which is also much darker than its planet. It’s possible that MK 2 simply lacks the mass necessary to keep a gravitational hold on icy volatiles as they are warmed by the Sun.
One big advantage of this new discovery is that it will allow us to better understand the composition of Makemake. By watching the moon’s orbit we can accurately determine the mass of both MK 2 and Makemake. This will give us a good measure of their densities, which will allow us to determine if they are more rocky or icy.