New Partners: Hubble finds more moons around Pluto

Already deemed the oddball among planets, Pluto just got a new wrinkle. Two, actually. This week, astronomers announced that the Hubble Space Telescope has spied a pair of previously unrecognized moons orbiting Pluto, giving this outer solar system body a total of three satellites. If the finding is confirmed, Pluto will be the only object beyond Neptune known to have more than one moon. About 20 percent of the objects in the Kuiper belt, a reservoir of cometlike objects even farther from the sun than Pluto is, have single partners.

PLUTO COMPANIONS. The imagined surface of one of two newfound moons of Pluto shows the planet above the horizon. The previously known moon, Charon, lies to the right of the planet, and the other new moon is at the far left. Inset is Hubble’s image of Pluto and its moons. Weaver, Stern et al., NASA, STScI

Researchers “will have to take these new moons into account when modeling the formation of the Pluto system,” says Alan Stern of the Southwest Research Institute in Boulder, Colo. He and Hal Weaver of the John Hopkins Applied Physics Laboratory in Laurel, Md., reported the findings during a NASA press briefing.

“If real, these two new satellites of Pluto point in the direction of the richness of small, icy bodies on the edge of the solar system, bodies that we are just now being able to detect,” comments Alan Boss of the Carnegie Institution of Washington (D.C.).

Hubble found the two tiny satellites last May 15 and May 18 during a search for Pluto moons. The researchers caution that more Hubble observations, scheduled for February, are required before they can be certain that the bodies are orbiting Pluto.

However, the scientists already have some corroborating evidence. Last week, team members identified the two objects in Hubble images taken 3 years ago. The objects show up in about the positions that would be expected if they were orbiting Pluto.

One of the moons, provisionally named S/2005 P1, lies about 48,000 kilometers from Pluto and has an estimated diameter of 56 km. The other newly discovered moon, dubbed S/2005 P2, lies about 64,000 km from Pluto and has a diameter of about 48 km. By comparison, Charon, Pluto’s previously known moon, is 19,600 km from the planet and has a diameter of 1,270 km, about half that of Pluto.

Initial estimates indicate that for every 12 times that Charon circles Pluto, S/2005 P1 goes around 3 times and S/2005 P2 goes around twice. That orbital relationship suggests that the two moons didn’t start out as debris that Pluto captured but instead arose from the same violent collision that formed Charon.

As theorist Robin Canup of the Southwest Research Institute describes it, a massive object walloped Pluto soon after its birth some 4.5 billion years ago. Debris from the collision coalesced into Charon, with a bit left over to make the newfound moons. As interactions with Pluto pushed Charon’s orbit outward, that moon’s gravity forced the orbits of the two outer moons to synchronize with it.

“We see [the Hubble findings] as a whole new chapter in the Pluto story and the Kuiper belt story,” says Stern.

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