Pluto's moon system

These two images, taken about a week apart by the NASA/ESA Hubble Space Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June.

P4 is the smallest moon yet found around Pluto, with an estimated diameter of 13 to 34 km. By comparison, Pluto's largest moon Charon is 1,043 km across. Nix and Hydra are roughly 32 to 113 km wide.

The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. It completes an orbit around Pluto roughly every 31 days.

The moon was first seen in a photo taken with Hubble's Wide Field Camera 3 on 28 June 2011. The sighting was confirmed in follow-up Hubble observations taken 3 July and 18 July.

P4, Nix, and Hydra are so small and so faint that scientists combined short and long exposures to create this image of Pluto and its entire moon system. The speckled background is camera "noise" produced during the long exposures. The linear features are imaging artifacts.

The tiny satellite was uncovered in a Hubble survey to search for rings around the frigid dwarf planet. The observations will help NASA's New Horizons mission, scheduled to fly through the Pluto system in 2015.