Today, NASA announced that images it has been sitting on for a year show a moon orbiting a dwarf planet in the Kuiper belt. The moon, formally known as S/2015 (136472) and going informally as MK 2, is in orbit around Makemake, a dwarf planet two-thirds the size of Pluto that spends most of its time more than 40 Astronomical Units away from the Sun (1 AU is the typical Earth-Sun distance).

MK 2 is more than 1,300 times fainter than the planet it orbits, largely because it has a very dark surface compared to Makemake's icy white color. It also appears to orbit within the plane of the Solar System, which means it's indistinguishable from Makemake for much of its orbit—Hubble managed to catch it when it was more than 20,000km from the dwarf planet. Estimates are that MK 2 is about 160km across compared to Makemake's 1,400km.

Early observations show that the orbit takes at least 12 days, and the shape of the orbit is roughly circular. This data suggests that MK 2 formed from debris liberated from Makemake by an impact; passing objects that are captured by planets typically have eccentric orbits. Detailed observations of MK 2's orbit will allow us to determine the density of Makemake, which will then tell us something about its composition, so NASA will continue observing the new body.

NASA says that MK 2 may solve an apparent Makemake mystery. Observations have indicated that the dwarf planet may have some relatively warm regions on its surface, something that would be hard to explain given its small size and highly reflective cover. But what we may have been seeing was the dark moon passing between us and Makemake and radiating out some of the sunlight it absorbed.