A small, dark moon has been spotted orbiting the icy dwarf planet Makemake in the far reaches of the Solar System.
Makemake is the second brightest dwarf planet in the Kuiper Belt – a reservoir of leftover frozen material from the birth of our Solar System 4.5 billion years ago. (Pluto is the brightest.)
Last April, the Hubble Space Telescope saw Makemake has its own little satellite, which has been provisionally named S/2015 (136472) 1, but nicknamed MK 2. MK 2 is only around 160 kilometres wide, but more than 1,300 times fainter than Makemake, making it exceptionally difficult to see.
“Makemake is in the class of rare Pluto-like objects, so finding a companion is important,” says image analysis lead Alex Parker of the Southwest Research Institute, Boulder, Colorado.
“The discovery of this moon has given us an opportunity to study Makemake in far greater detail than we ever would have been able to without the companion.”
Despite being covered in bright ice, Makemake is a fairly recent discovery, first spotted in 2005. Like all trans-Neptunian objects – that is, bodies orbiting the Sun beyond Neptune – it's very cold, with an average temperature of -243 ºC.
Other trans-Neptunian objects have at least one satellite. Dwarf planets Eris, Haumea and Pluto have one, two and five respectively. But MK 2's discovery may solve one Makemake mystery.
Infrared images have, in the past, showed patches of the dwarf planet appearing warmer than the rest. Now, based on the new Hubble images, they have simply been MK2's dark surface.
Exactly why MK 2 is so dark compared to Makemake is yet to be determined. Perhaps it is so small, and its gravitational pull so weak, that is can't hang onto ices and they sublimate away.
But, the researchers say, more Hubble images will let them determine the shape of the MK 2's orbit. A tight, circular orbit means the little moon was probably formed when Makemake collided with another object. But if it's an elongated elliptical orbit, it was likely captured.
And more study of the satellite will reveal Makemake's density. This will let planetary scientists compare it to Pluto, and see if they're made of the same stuff.
"This new discovery," Marc Buie also of the Southwest Research Institute says, "opens a new chapter in comparative planetology in the outer Solar System."