Say hello to MK 2, our solar system’s newest moon

When it comes to planets and moons these days, all the attention goes exoplanets, or those orbiting alien stars. Some in the scientific community are still trying to do an exhaustive search of our own solar system, however, especially in the little-known area beyond Neptune. This week, NASA announced a new discovery in that effort to catalog local wanderers: a new moon they’ve nicknamed MK 2.
The “MK” in the name refers to the moon’s parent, Makemake, a dwarf planet in the area of our solar system beyond Neptune, called the Kuiper belt. Makemake was the king of the so-called “trans-Neptunian objects” until scientists downgraded Pluto and made it the biggest and brightest body in the outer solar system. Makemake is about two-thirds Pluto’s size, but otherwise quite similar. Both are covered in frozen methane, both orbit far from the Sun, and, apparently, both have one or multiple moons.
Oh, and Makemake is pronounced either “maKI-maKI” or “ma-KAY-ma-KAY,” depending on who you ask.

The moon was discovered in a search of the area carried out by Hubble telescope, revealing the orbital and surface characteristics of MK 2, but leaving its density and internal structure unknown. By illuminating the circular path of MK 2 around Makemake, Hubble has shown that it was most likely formed by a collision with between Makemake and another Kuiper belt object, in the early history of the solar system. This is as opposed to the model in which it simply fell into orbit around Makemake as it passed.

A Hubble image of MK 2. Credit: NASA

Previous studies of Makemake had puzzled over the odd pattern of luminosity on its surface, but the presence of a moon, with its transits and moving shadow, could easily explain that.

Team leader Marc Buie said that this discovery, and the inevitable later investigation into the moon’s physical characteristics, will allow a new era of “comparative planetology in the outer solar system.” Right now, understanding of the Kuiper belt comes from detailed study of just a few major objects, which doesn’t do much to illuminate the overall population, so every new object has an inflated level of importance. Even with just this discovery, scientists can say that moons are likely quite common in the Kuiper belt, and that has implications for the likely level of crowding in that area a couple of billion years ago.

These six successive shots of Makemake showed a previously unknown object in its vicinity.

MK 2 itself differs from Makemake in that it is “charcoal black,” where its parent is snow white. Scientists believe this may simply be due to its size, and that it may be too small to gravitationally hold onto such a light surface material. This makes it far more typical for Kuiper belt objects, and seems to invite questioning about exactly what mass threshold could hold a surface powder — likely as a function of surface temperature and rotational speed.
Being so dark, MK 2 was previously impossible to detect, as the moon’s weak signal was easily washed out by the reflection from its surface. It took Hubble’s specialized equipment to look into such a bright spot and notice an all-new, jet black moon nearby.
NASA’s New Horizons is currently on its way to perform a number of Kuiper belt flybys — so hopefully we’ll get to learn more about MK 2 over the next couple of years.