Pluto's posse.

Images taken by the Hubble Space Telescope on Feb. 15 have confirmed that Pluto has two small, previously unknown moons. First detected by Hubble last year (SN: 11/5/05, p. 291), each moon is only about 50 kilometers across, less than one-hundredth the size of Pluto's giant moon Charon.

One of the bodies lies about 48,000 km from Pluto and the other lies 64,000 km away. In contrast, Charon orbits 19,000 km away from the planet. The February observations rule out the possibility that other similarly small satellites orbit closer to Pluto than either of the new moons do.

The two moons lie in the same plane as Charon and move in synchrony with it. Those properties suggest that the tiny moons were born in the same giant collision that astronomers say spawned Charon. In this model, Charon arose when an object about the size of Pluto struck the planet 4 billion years ago. A large chunk of debris coalesced to form Charon, while smaller bits merged to form the two tiny moons, argue Alan Stern of the Southwest Research Institute in Boulder, Colo., and his colleagues in the Feb. 23 Nature. The researchers, who discovered the small moons, also note that the impact could have created an as-yet-unseen dust ring around Pluto.

The findings suggest that the handful of other denizens of the outer solar system known to have partners about as big as Charon (SN: 1/14/06, p. 26) may also harbor tinier moons and rings, the team says.

The latest Hubble observations, taken March 2, show that the trio of Pluto's moons have similar color, another clue that they have a common origin, Stern says.--R.C.