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During an observing campaign aimed at searching for possible rings around Pluto, a team of researchers led by Mark Showalter (SETI Institute, Mountain View, California) using the Wide Field Camera 3 on the Hubble Space Telescope, has found a new satellite in orbit around this freezing dwarf planet. Provisionally called P4, it has a diameter between just 13 and 34 km and its orbit lies between those of two other small moons, Nix and Hydra, that together with the larger Charon, complete, for now, the satellite system of Pluto.
The new discovery strengthens the hypothesis that the system was formed as the result of an impact between the primitive Pluto and another body of similar size. Some of the material thrown out by the impact would have been retained by the gravitational pull of the body that is now Pluto, and would have been able to accrete into moons. It looks ever less likely that the moons are the result of gravitational capture of bodies that formed elsewhere.
The first observation of P4 was on 28th June (left image) and was confirmed by further observations made on 3rd July (right image) and 18th July. In fact, it could have been discovered earlier, but the exposure times of previous images were too short to detect the satellite with certainty, only the longer exposures suitable for searching for rings (thought likely to exist and maintained by material released from the moons by micrometeorite impacts) could the new moon be clearly seen. Some images taken in 2006 actually show evidence for the presence of P4, but at a level very close to the background noise level in the images.
This new discovery is the result of the increasing focus on Pluto, and efforts to gain as much information as possible on the system, in the run up to the arrival at Pluto of the New Horizons probe in 4 years time. Alan Stern, who leads this mission, has already made it clear that the program of detailed observations that the probe is to carry out will be modified to include P4.
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