The Hubble Space Telescope has uncovered a fifth moon – dubbed P5 – orbiting the icy dwarf planet Pluto. The new-found moon, visible as a speck of light in Hubble images, is irregular in shape and estimated to be between 10-25 kilometres across. It is in a 95,000 kilometre-diameter circular orbit around Pluto and is believed to lie in the same orbital plane as Pluto’s other known moons.
Pluto’s largest moon, Charon, was discovered in 1978. Hubble observations in 2006 uncovered two additional small moons, Nix and Hydra. In 2011 another moon, known as P4, was found in Hubble data.
“The moons form a series of neatly nested orbits, a bit like Russian dolls,” mused SETI Institute’s Mark Showalter, who led the team that found the moon. The discovery adds to the intrigue that such a tiny planet can have such a complex collection of satellites. The favored theory explaining Pluto’s numerous companions is that the moons are relics of a collision between Pluto and another large Kuiper belt object billions of years ago.
“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 Johns Hopkins University and New Horizons team member.
The team are using Hubble’s powerful vision to scour the Pluto system to uncover potential hazards to the New Horizons spacecraft. “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,” said Alan Stern of the Southwest Research Institute, the mission’s principal investigator.