Dwarf planet Makemake has its own moon!

NASA’s Hubble Space Telescope has spotted a small, dark moon orbiting Makemake, the second brightest icy dwarf planet in the Kuiper Belt.

The moon - provisionally designated S/2015 (136472) 1 and nicknamed MK 2 - is more than 1,300 times fainter than Makemake.

MK 2 was seen about 20,921 km from the dwarf planet, and its diameter is estimated to be 160 km across. Makemake is 1,400 km wide. The dwarf planet, discovered in 2005, is named for a creation deity of the Rapa Nui people of Easter Island.

The Kuiper Belt is a vast reservoir of leftover frozen material from the construction of our solar system 4.5 billion years ago and home to several dwarf planets.

Some of these worlds have known satellites, but this is the first discovery of a companion object to Makemake.  Makemake is one of five dwarf planets recognised by the International Astronomical Union. The observations were made in April 2015 with Hubble’s Wide Field Camera 3.

The observing team used the same Hubble technique to observe the moon as they did for finding the small satellites of Pluto in 2005, 2011, and 2012. Several previous searches around Makemake had turned up empty.

“Our preliminary estimates show that the moon’s orbit seems to be edge-on, and that means that often when you look at the system you are going to miss the moon because it gets lost in the bright glare of Makemake,” said Alex Parker of Southwest Research Institute in US, who led the image analysis for the observations.

A moon’s discovery can provide valuable information on the dwarf-planet system. By measuring its orbit, astronomers can calculate a mass for the system and gain insight into its evolution.

Uncovering the moon also reinforces the idea that most dwarf planets have satellites. “Makemake is in the class of rare Pluto-like objects, so finding a companion is important,” Parker said.

“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,” he said.

Finding this moon only increases the parallels between Pluto and Makemake. Both objects are already known to be covered in frozen methane.

As was done with Pluto, further study of the satellite will easily show the density of Makemake, a key result that will indicate if the bulk compositions of Pluto and Makemake are also similar.

The researchers will need more Hubble observations to make accurate measurements to determine if the moon’s orbit is elliptical or circular.

Preliminary estimates indicate that if the moon is in a circular orbit, it completes a circuit around Makemake in 12 days or longer.