Quite by accident, astronomers using the Hubble Space Telescope have discovered a fourth moon orbiting the (dwarf) planet Pluto.
While it will no doubt be given a suitably mythical name, it's been temporarily dubbed P4. It's the smallest of Pluto's moons, with an estimated diameter of eight to 21 miles.
P4 was discovered during a Hubble survey that was looking for rings around the dwarf planet.
"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than three billion miles," says Mark Showalter of the SETI Institute, who led the observations.
And it's particularly useful right now to know that P4 is there. NASA is currently planning its New Horizons mission, scheduled to fly through the Pluto system in 2015. The aim is to provide new insights about the worlds at the edge of our solar system.
"This is a fantastic discovery," says New Horizons’ principal investigator Alan Stern of the Southwest Research Institute. "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby."
The new moon is located between the orbits of Nix and Hydra, Pluto's second- and third-largest moons, which were discovered by Hubble in 2005. The biggest, Charon, was discovered in 1978 at the US Naval Observatory and confirmed using Hubble in 1990.
Pluto’s entire moon system is believed to have formed through a collision between Pluto and another planet-sized body early in the history of the solar system. Material was flung out, and coalesced into the four moons.
It was while looking for rings that could have been formed at the same time that the discovery of P4 was made. It was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28, and confirmed in pictures taken on July 3 and July 18.
It apparently hadn't been seen in earlier Hubble images because the exposure times were shorter; NASA says there's a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.