Another dwarf planet at the edge of the Solar System has been found to have a small companion moon. The world, named 2007 OR10, is the third largest dwarf planet that we know about — around 955 miles wide. The discovery was made possible by three NASA space telescopes, and it means we have found moons around practically every large dwarf planet more than 600 miles across in our cosmic neighborhood.
Astronomers started to think 2007 OR10 might have a moon after NASA’s Kepler Space Telescope showed that the dwarf planet was rotating more slowly than usual. Typically, objects in the Kuiper Belt — the large cloud of icy bodies beyond Neptune where this dwarf planet resides — take less than 24 hours to rotate. But Kepler found that 2007 OR10 took 45 hours to complete one rotation, indicating that a moon might be tugging on it and slowing down its spin.
The astronomers then turned to archive images of the Hubble Space Telescope. Sure enough, the telescope’s Wide Field Camera 3 had picked up the moon twice during two different observation runs in 2009 and 2010. “The initial investigator missed the moon in the Hubble images because it is very faint,” Csaba Kiss, an astronomer at Konkoly Observatory in Budapest, Hungary said in a statement. Kiss is the lead author on a paper detailing the moon’s discovery in The Astrophysical Journal Letters. The team then used past heat measurements of 2007 OR10, taken by the Herschel Space Observatory, to estimate how big the moon might be — pinning it down to somewhere between 150 miles to 250 miles in diameter.
The Hubble images definitely show that the moon is gravitationally bound to 2007 OR10, but the astronomers weren’t able to figure out what kind of orbit the tiny satellite takes around the dwarf planet. So the explanation for the slow spin of 2007 OR10 is still unresolved. “Ironically, because we don’t know the orbit, the link between the satellite and the slow rotation rate is unclear,” study co-author John Stansberry, an astronomer at the Space Telescope Science Institute in Baltimore, said in a statement.
Still it means that moons are seemingly abundant around dwarf planets at the Solar System’s edge. Last year, a moon was found around the distant dwarf planet Makemake. So now, practically all of the largest dwarf planets we know about, except for one named Sedna, have moons. And that gives us a bit more clues about how these space rocks formed. It’s possible that these moons are the results of distant bodies colliding at just the right speeds when the Solar System was being born 4.6 billion years ago. “If there were frequent collisions, then it was quite easy to form these satellites,” says Kiss.