Astronomers using the Hubble Space Telescope have discovered a fourth moon orbiting the icy dwarf planet Pluto. The tiny satellite — temporarily designated P4 — was uncovered in a survey searching for rings around the dwarf planet. The newly found moon is the smallest discovered around Pluto. It has an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Charon, Pluto’s largest moon, is 648 miles (1043 km) across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 km). Images of the Pluto system taken on June 28 (left) and July 3 (right) show the newly discovered fourth moon, P4, circled. (Images: NASA, ESA and M. Showalter, SETI Institute) “I find it remarkable that Hubble’s cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km),” said Mark Showalter of the SETI Institute, who led this observing program with Hubble. The finding is a 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. An illustration of the Pluto satellite system orbits highlights the dwarf planet’s newly discovered smallest moon, P4. (Image: NASA, ESA, and A. Field, STScI) “This is a fantastic discovery,” said New Horizons’ principal investigator, Alan Stern of the Southwest Research Institute in Boulder, Colo. “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, which Hubble discovered in 2005. Charon was discovered in 1978 at the US Naval Observatory and resolved by 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 another planet-size body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto. Scientists believe that the 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. For more information, visit: www.nasa.gov/hubble