Astronomers have found a moon orbiting the third largest dwarf planet of our Solar System – 2007 OR10 – thereby suggesting that most of the objects in the Kuiper Belt that are larger than 965 kilometres across have at least one natural satellite.

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.

The discovery was made using the three powerful observatories including NASA’s Hubble Space Telescope. The distant dwarf planet 2007 OR10 orbits the Sun in the Kuiper Belt. The team uncovered the moon in archival images of 2007 OR10 taken by the Hubble Telescope. Astronomers got the hint of possibility of a moon orbiting 2007 OR10 when they were observing it using NASA’s Kepler Space Telescope.

One of the unique things about the 2007 OR10 is that it has a particularly slow rotation period of 45 hours when it comes to such objects as typical rotation periods of objects in Kuiper Belt is under 24 hours. This provided them with a clue that there could be something that influencing the rotation of the dwarf planet.

To investigate, astronomers turned to Hubble archive. The archive was analysed previously as well by different astronomers and scientists, but they likely missed the moon because it is very faint. 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.