Hubble spots a tiny moon orbiting Pluto’s little sister, Makemake

Astronomers are discovering more about Makemake, a Pluto-like dwarf planet in the outer reaches of our solar system.

NASA's 26-year-old Hubble telescope has added another discovery to its illustrious career. It found and photographed a teeny moon, about 100 miles in diameter, orbiting a teeny planet called Makemake, which circles the sun at the outskirts of our solar system.

Now that astronomers have found Makemake's satellite, they can discover the dwarf planet's composition and evolution.  

Makemake, discovered in 2005, was named after the creation god in the mythology of Easter Island's Rapa Nui people. It is located on the edge of our solar system in a clump of baby-size planets that include Pluto and Eris, found beyond the outermost "real" planet Neptune. Makemake's neighborhood there is called the Kuiper Belt, a ribbon of comets, asteroids, and frozen debris circling the sun, left over from the birth of our solar system 4.5 billion years ago.

Most of the largest, officially recognized dwarf planets in the Kuiper Belt are known to have moons; Pluto, once considered a full-fledged planet, has five. But until recently, Makemake didn't appear to have a companion satellite.

"Makemake is in the class of rare Pluto-like objects, so finding a companion is important," explained Alex Parker, an astronomer at the Southwest Research Institute in Boulder, in a NASA statement. Dr. Parker led the analysis of Hubble's photo of Makemake's moon, nicknamed MK 2.

"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.

Astronomers can calculate the mass of a planet by observing the gravitational pull it exerts on its satellite. The more massive the planet, the more strongly it tugs on its moon, causing it to orbit faster.

In other words, observing how MK 2 orbits Makemake will let astronomers estimate the icy planet's mass, and thus its density and most of its composition – probably frozen methane, like its neighbor Pluto.

The bright dwarf planet's diameter (870 miles) is just a tenth of Earth's and two-thirds that of Pluto. Its tiny moon, merely a blip compared to the size of Earth's, is about 13,000 miles away from it. MK 2 was overlooked for so long because the luminous dwarf planet outshines its humble companion.

"So it’s not that it's faint, it's that it's shy," Parker told Wired. "It's always hiding behind Makemake." 

When Pluto's moon Charon was first spotted in 1978, it changed our understanding of the planet. Scientists were finally able to calculate Pluto's mass, which turned out to be hundreds of times smaller than originally estimated when it was found in 1930.

"That’s the kind of transformative measurement that having a satellite can enable," Parker said.