Pluto may no longer be classified as a planet, but that doesn’t mean it isn’t still getting its fair share of attention. NASA’s New Horizons spacecraft is currently making its way to the icy dwarf planet to study it and its moons. To search for potential hazards to the spacecraft and help plan a safer trajectory before its scheduled arrival in 2015, a team of astronomers has trained the eye of the Hubble Space Telescope on the system and discovered a fifth, previously unknown moon orbiting the planet.
The newly discovered moon, which has been provisionally designated as P5, was detected in nine separate sets of images taken by Hubble’s Wide Field Camera 3 between June 26 and July 9. It is estimated to be irregular in shape, measuring six to 15 miles (9.6 to 24 km) across, and is in a 58,000-mile (93,342 km) diameter circular orbit. The orbit is assumed to be on the same plane as other satellites in the system, resulting in the five moons creating what team leader Mark Showalter of the SETI Institute describes as “a series of neatly nested orbits, like Russian dolls.”
That such a complex collection of satellites is orbiting such a small planet – it’s roughly 1,433 mile (2,306 km) diameter is around 66 percent that of the Moon – has intrigued the team, who say this latest discovery provides additional clues as to how the Pluto system formed and evolved. The smart money is on the theory that Pluto’s moons are the result of a collision between the dwarf planet and another large Kuiper Belt object billions of years ago.
Following the discovery of Charon, Pluto’s largest moon, in 1978 in observations made at the United States Naval Observatory in Washington, D.C., Hubble uncovered two additional small moons, Nix and Hydra, in 2006, and another, P4, in 2011.
“The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system,” said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
This could pose problems for the New Horizon spacecraft, which will be moving past Pluto at a speed of 30,000 mph (48,280 km/h) and could be destroyed in a collision with orbital debris the size of a ball bearing.
“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,” added Alan Stern of the Southwest Research Institute in Boulder, Colo., the mission’s principal investigator.
In the years following New Horizon’s Pluto flyby in 2015, astronomers plan follow-up observations using NASA’s James Webb Space Telescope. The Webb telescope, which is the planned successor to Hubble, will be able to measure the surface chemistry of Pluto, its moons, and many other bodies that lie in the Kuiper Belt along with Pluto.