New moon orbiting Pluto found P5 (circled) is the fifth moon discovered in orbit around Pluto. MOUNTAIN VIEW, Calif., July 11 (UPI) -- Astronomers say the Hubble Space Telescope has found another moon orbiting Pluto, the solar system's former ninth planet now demoted to dwarf status. The moon dubbed P5, estimated to be irregular in shape and 6 to 15 miles across, joins Charon, Hydra, Nix and P4 in orbit around the icy dwarf planet. "The moons form a series of neatly nested orbits, a bit like Russian dolls," astronomer Mark Showalter of the SETI Institute in Mountain View, Calif., said in a NASA release Wednesday. Scientists say it is unusual for a dwarf planet to have such a complex collection of satellites. A leading theory for the moons is that they are relics of a collision between Pluto and another large cosmic object billions of years ago. The discovery of P5 will help scientists plot a path for NASA's New Horizons spacecraft when it makes an historic and long-awaited high-speed flyby through the Pluto system in 2015, the space agency said. "The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system," Harold Weaver of Johns Hopkins University in Baltimore said. P5 was detected in nine separate sets of images taken by Hubble's Wide Field Camera 3 on June 26, 27, 29, and July 7 and 9. "The inventory of the Pluto system we're taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft," Alan Stern of the Southwest Research Institute in Boulder, Colo., who is the New Horizons project's principal investigator, said.