Astronomers using the Hubble Space Telescope have discovered a new moon orbiting the icy planet Pluto. Designated S/2011 (134340) 1 by the International Astronomical Union — but nicknamed P4 — the newcomer orbits Pluto every 32 days at a distance of about 37,000 miles (59,000 km). This puts it between the paths of Nix and Hydra, two moons found circling Pluto in 2005.
The discovery brings the tally of Pluto's moons (including Charon, its largest) to four, and the total of all known planetary satellites to 171.
A team led by Mark Showalter (SETI Institute) and Douglas Hamilton (University of Maryland) spotted P4 in images taken by Hubble's Wide Field Camera 3 on three separate occasions — June 28, July 3, and July 18, 2011. The observations took place when Pluto was in opposition to Earth, at a distance of just over 31 astronomical units. After the first two sessions, "we were 99% sure that we had found a new moon," Hamilton tells Sky & Telescope. "We then requested Director's Discretionary Time, which was approved rapidly and the additional July 18th data confirmed our discovery."
The astronomers have been using HST to map Pluto and its surroundings in preparation for New Horizons, a spacecraft steadily making its way toward the outer reaches of the solar system to study Pluto and other cold, dark inhabitants of the Kuiper Belt.
According to today's announcement by the IAU's Central Bureau for Astronomical Telegrams, P4 is only magnitude 26 — about a tenth as bright as Nix. Assuming that its reflectivity is the same as Charon's (35%), then P4 would measure only 9 miles (14 km) across; if it's much darker, it could be as large as 25 miles (40 km). Either way, it's small in comparison to Charon, which is 750 miles (1,210 km) in diameter.
The discovery will mean that New Horizons will have more to do when it reaches its destination in July 2015. "It's going to make the encounter even more exciting, and even busier than the pace we planned with just three moons to observe," says Alan Stern, the mission's principal investigator.
Astronomers think that P4 and its siblings formed when a large object collided with Pluto in the distant past. The resulting rocky debris began orbiting the dwarf planet and eventually coalesced into satellites. Earth's own Moon is believed to have formed in the same manner.
And what about a fitting name for the new find? "Mark and I like the name Cerberus, the three-headed dog that guards Pluto's realm," Hamilton admits. "But the name is unofficial for now!"
According to experts, the name P4 will stick at least for a month. First, more observations are needed — both to move the naming process along and to aid New Horizons mission planning. So the discovery team plans to submit a new HST proposal to help nail down the orbit.