A Ghost in the Outer Solar System
Beyond the orbit of Neptune lies the Kuiper Belt, a vast, frigid disc of icy bodies left over from the formation of the solar system. Here, the dwarf planet Makemake orbits the sun, taking about 305 Earth years to complete one circuit. For a decade after its own discovery in 2005, Makemake appeared to be alone. That changed in April 2015, when astronomers using the Hubble Space Telescope's Wide Field Camera 3 found a faint point of light moving with the dwarf planet.
The discovery, announced on April 26, 2016, revealed Makemake's only known moon. It has the provisional designation S/2015 (136472) 1 but is more commonly known by its nickname, MK 2. The satellite is incredibly faint, appearing more than 1,300 times dimmer than Makemake itself. Initial estimates place its diameter at around 175 kilometers (about 110 miles), a small fraction of Makemake's 1,430-kilometer diameter. MK 2 orbits its parent body at a distance of at least 21,000 kilometers (13,000 miles), with an orbital period of 12 days or more.
The detection of MK 2 allows scientists to calculate Makemake's mass and density for the first time by studying the moon's orbit. This data shows the composition and evolution of one of the largest objects in the Kuiper Belt.
The Challenge of Seeing in the Dark
The discovery of MK 2 solved a long-standing puzzle about Makemake. Previous infrared observations showed that the dwarf planet was emitting more heat than expected, suggesting the presence of dark, heat-absorbing patches on its surface. Makemake's brightness is remarkably consistent as it rotates, which contradicts the idea of a mottled surface.
The dark moon explains this. MK 2 is dark, with a geometric albedo (a measure of reflectivity) of only 2-4%. Makemake is the second-brightest object in the Kuiper Belt after Pluto, covered in bright, frozen methane that reflects over 80% of the light that hits it. It seems likely that the dark material thought to be on Makemake is actually the surface of its orbiting moon.
This difference, combined with MK 2's small size, made it very difficult to spot in the overwhelming glare of its parent body. Its orbit also appears to be oriented edge-on from our perspective on Earth, meaning it spends long periods hidden directly in front of or behind Makemake. The successful detection used the same techniques that Hubble employed to find Pluto's smallest moons, carefully analyzing the light at the very edge of the brighter object. This suggests that other large Kuiper Belt Objects may also host dark, undiscovered moons.