Pluto is probably feeling pretty annoyed by his "little sister" right now.
First Makemake, the icy dwarf planet that hangs out in the Kuiper Belt, helped get Pluto bumped from the official planet list: In the mid-2000s, scientists argued that the just-discovered trans-Neptunian objects Makemake and Eris should join Pluto under the new designation "dwarf planet."
Now the distant dwarf planet, which is a mere 870 miles across, just notched another point in their space sibling rivalry: Researchers at the Hubble Space Telescope say they've found a moon for it, spinning through space some 13,000 miles from its owner.
The new moon, provisionally given the unwieldy name "S/2015 (136472) 1" (but called "MK 2" for short), was detected last April by some of the same scientists who discovered Eris and Makemake 10 years ago. It was hard to find amid the glare from Makemake, which is the second-brightest Kuiper Belt object after Pluto.
But despite Hubble's advanced age — a ripe 26 as of this month — the space telescope has apparently still got it: Its ability to pinpoint faint objects next to bright ones at a relatively high resolution let astronomers discern MK 2 amid the haze surrounding Makemake. The moon is 100 miles in diameter and charcoal black, possibly because it's so small it lacks the gravitational power necessary to hold onto a bright, icy crust. Lots of comets and other Kuiper Belt objects have the same problem.
The moon's discovery bolsters the infamous case made by "Pluto Killer" Mike Brown, who was on the original Makemake team (but is not an author on the latest study). It increases the parallels between Pluto and its fellow dwarf planets, suggesting that have enough in common to be their own class of space object.
The presence of a moon also gives astronomers a lot more information to work with when studying Makemake, which was named for the creation deity in the Rapa Nui culture of Easter Island.
Just as you learn more about a person by seeing them interact with others, scientists know more about planets (and dwarf planets) when they're observed with some company. Monitoring MK2 as it orbits Makemake will give researchers information about the the gravitational relationship between them, which will allow them to calculate the mass of the planet.
"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," Alex Parker, an astronomer at the Southwest Research Institute, Boulder, Colo., said in a statement. He pointed out that astronomers were only able to pin down the mass of Pluto after identifying its moon Charon in the 1970s. The former ninth planet turned out to be much, much smaller than scientists had thought it to be.
"That's the kind of transformative measurement that having a satellite can enable," Parker said.