Astronomers using the Hubble Space Telescope discovered a fourth moon orbiting the icy dwarf planet Pluto in 2011 while searching for rings around the dwarf planet. The moon was temporarily designated S/2011 (134340) 1 (and sometimes called P4) and was officially named Kerberos by the International Astronomical Union (IAU) in 2013.
Kerberos has an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Charon, Pluto's largest moon, is 648 miles (1,043 km) across, and two other moons of Pluto, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 km).
The discovery of Kerberos was the result of ongoing work to support NASA's New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our solar system. Hubble's mapping of Pluto's surface and discovery of its satellites have been invaluable to planning for New Horizons' close encounter.
Kerberos is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.
The dwarf planet's entire moon system is believed to have formed by a collision between Pluto and similar-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto. There may be even more moons to discover.
Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago. Scientists believe material blasted off Pluto's moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.
Kerberos was discovered on 28 June 2011 by a large team led by Mark Showalter using the Hubble Space Telescope. It was confirmed in subsequent Hubble pictures taken on 3 July and 18 July. The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.
How Kerberos Got Its Name:
Originally designated S/2011 (134340) 1 (and sometime referred to as P4), Kerberos is named after the three-headed dog of Greek mythology. All of Pluto's moons are named for mythological figures associated with the underworld.