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Makemake and its newly discovered moon, known for now as MK2, are shown in this illustration. (NASA/ESA/Alex Parker)

Scientists Discover a Dark Moon in the Distant Solar System

Dwarf planet Makemake, which orbits the sun once every 310 Earth-years, has a dark little moon. Just 100 miles across, the moon evaded detection for more than a decade, hiding in the glare of its parent planet.

But it couldn’t escape the stare of the sharpest eye in the sky forever: When scientists aimed the Hubble Space Telescope at Makemake for more than two hours in April 2015, they discovered a faint point of light moving through the sky along with the icy world.

Until now, Makemake was the only officially recognized distant dwarf planet without a moon, a dubious distinction that has now been lost.

Is That … a Moon?

At first, Alex Parker wasn’t sure he’d spotted a new moon in the Hubble observations.

“I was sure someone had seen it already,” says Parker, of the Southwest Research Institute. So, he approached collaborator Marc Buie and asked, “Has anyone seen the moon in the Makemake data?”

Buie’s reply — “There’s a moon in the Makemake data?”— convinced Parker he was onto something.

“It was at that point that everything got exciting and kicked into high gear,” says Parker, who along with Buie reported the discovery of the moon, known as MK2 for now (or S/2015 (136472) 1, more officially) on Tuesday.

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Hubble image of Makemake and its moon. (NASA/ESA/Alex Parker)

By carefully studying the orbit of MK2, scientists will not only be able to determine how the moon formed—whether Makemake’s gravity snatched it or it grew out of a collision—but also learn more about Makemake itself. Specifically, the moon’s orbit will reveal the mass of the small, icy world. From there, they’ll be able to calculate Makemake’s density and determine what it is likely made of, and compare it to other far-flung icy worlds such as Pluto, Eris, and egg-shaped Haumea.

“The wide range of densities of the dwarf planets is one of the most interesting mysteries out there. But we still have so few objects that each one adds a critical part of the story,” says Caltech’s Mike Brown, who, with his colleagues, discovered Makemake in 2005. “I’m accepting bets currently.”

Makemake’s Dark Mystery

Makemake is a strange world. Shaped like a flattened sphere about 870 miles across, it lives in the Kuiper Belt—the icy debris ring beyond the orbit of Neptune—and is reddish in color. Like some other Kuiper Belt objects, Makemake spins very quickly, pirouetting every 7.7 hours. Its slightly oval orbit takes it much farther from the sun than Pluto, which treks around the sun in a comparatively snappy 248 years.

Finding MK2 in orbit around Makemake could solve one of the abiding mysteries about the icy dwarf planet, Parker said.

When scientists first observed the whirling Makemake, they noted that it was continually bright, meaning that its surface is probably uniformly covered in bright, reflective ices. But heat signatures from the faraway planet were slightly varied, suggesting that at least one warm, dark patch might be present on Makemake’s surface. Years of observations failed to reconcile the two data sets, as a dark patch never showed up in observations.

“Well, imagine that the dark material isn’t on Makemake’s surface … it’s in orbit!” Parker said. “If the moon is very dark, it accounts for most previous thermal measurements!”

Indeed, MK2 is much darker than Makemake itself, which is about 1,300 times brighter than its companion.

What else is hiding in our solar system, waiting to be discovered?