On Wednesday, July 11, 2012, researchers using NASA's Hubble Space Telescope announced that they had discovered a fifth moon around Pluto.
The newly discovered moon was detected using nine separate sets of images taken by Hubble's Wide Field Camera 3 during June and July in 2012.
P5, the tentative name for the newly discovered planet, measures only 6 to 15 miles (10 to 24 kilometers) across. It orbits Pluto in about 20.2 Earth days, give or take about 0.1 days.
The five known moons of Pluto are: Charon, Nix, Hydra, P4, and P5.
The NASA spacecraft New Horizons is on its way to Pluto and the Kuiper Belt, and should be able to analyze Pluto and its five moons in great detail. Its flyby of the system is scheduled for 2015.
See the family portrait of Pluto and its moons on the July 12, 2012 Space.com article "New Pluto Moon Hints at Perilous Journey for Spacecraft".