Moon orbiting solar system's 3rd largest dwarf planet found

Scientists have discovered a new moon orbiting the third largest dwarf planet, that resides in the frigid outskirts in our solar system.

With this discovery, most of the known dwarf planets in the Kuiper Belt larger than 965 kilometres across have companions. These bodies provide insight into how moons formed in the young solar system.

The combined power of three space observatories, including NASA's Hubble Space Telescope, has helped uncover the moon orbiting the dwarf planet 2007 OR10 in the Kuiper Belt, a realm of icy debris left over from our solar system's formation 4.6 billion years ago. "The discovery of satellites around all of the known large dwarf planets - except for Sedna - means that at the time these bodies formed billions of years ago, collisions must have been more frequent, and that's a constraint on the formation models," said Csaba Kiss of the Konkoly Observatory in Hungary.

"If there were frequent collisions, then it was quite easy to form these satellites," said Kiss, lead author of the study published in The Astrophysical Journal Letters.

The objects most likely slammed into each other more often because they inhabited a crowded region. The team uncovered the moon in archival images of 2007 OR10 taken by the Hubble Telescope.

Observations taken of the dwarf planet by NASA's Kepler Space Telescope first tipped off the astronomers of the possibility of a moon circling it.

Kepler revealed that 2007 OR10 has a slow rotation period of 45 hours. "Typical rotation periods for Kuiper Belt Objects are under 24 hours," Kiss said. "We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon. The initial investigator missed the moon in the Hubble images because it is very faint," he said.

The astronomers spotted the moon in two separate Hubble observations spaced a year apart. The images show that the moon is gravitationally bound to 2007 OR10 because it moves with the dwarf planet, as seen against a background of stars. The astronomers calculated the diameters of both objects based on observations in far-infrared light by the Herschel Space Observatory, which measured the thermal emission of the distant worlds.

The dwarf planet is about 1,528 kilometres across, and the moon is estimated to be 240 kilometres to 400 kilometres in diameter. 2007 OR10, like Pluto, follows an eccentric orbit, but it is currently three times farther than Pluto is from the sun. 2007 OR10 is a member of an exclusive club of nine dwarf planets. Of those bodies, only Pluto and Eris are larger than 2007 OR10.

It was discovered in 2007 by astronomers Meg Schwamb, Mike Brown, and David Rabinowitz as part of a survey to search for distant solar system bodies using the Samuel Oschin Telescope at the Palomar Observatory in the US.