Discovery of two new satellites of Pluto

Four's Company In May 2005 Pluto was well placed for observation and the Hubble Space Telescope's survey camera was trained in its direction. The result was the discovery of two previously unknown satellites, called S/2005 P 1 and P 2 for now, making Pluto the first Kuiper belt object known to have multiple satellites. Two papers this week cover this milestone in detail. Weaver et al . describe the discovery process, and derive preliminary orbits and size estimates for both satellites. Stern et al . present work on the potential origin of the new satellites in the Charon-forming impact, on the likelihood of a ring system at Pluto, and the likelihood that there are many more multiple satellite systems in the Kuiper belt. On the cover, images of the Pluto-Charon system on 15 May (top) and 3 days later show the newly discovered moons on the move. Charon moves from one side of Pluto to the other between the two frames. Pluto's first known satellite, Charon, was discovered 1 in 1978. It has a diameter (∼1,200 km) about half that of Pluto 2 , 3 , 4 , 17 , which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful 5 , 6 , 7 , but they were not sensitive to objects ≲150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites 6 . Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ∼38 days (P1) and ∼25 days (P2).