Scientists using the Hubble Space Telescope have discovered what they say are two more tiny moons orbiting the planet Pluto, the first evidence yet that any object in the solar system beyond Neptune has more than one satellite.
Pluto has one large moon -- Charon, discovered in 1978 -- in orbit about 12,000 miles from the planet, but until yesterday's announcement, neither Pluto nor any other object in the Kuiper Belt, a region of icy, rocky objects orbiting in the far reaches of the solar system, was known to have multiple companions.
The two moons were spotted in May by astronomers using Hubble to size up potential targets for investigation by NASA's New Horizons mission, scheduled to be launched early in 2006 on a multiyear expedition to Pluto and the Kuiper Belt.
On May 15, the New Horizons team took a series of eight-minute exposures showing two faint objects near Pluto, 3 billion miles from Earth.
"Three days later, the two objects were still there, and they were moving counterclockwise around Pluto," said team co-leader Harold A. Weaver of Johns Hopkins University's Applied Physics Laboratory. "It made for a convincing case."
Weaver said the team must confirm the discovery with a separate set of observations scheduled for mid-February. Meanwhile, a search of Pluto images made earlier by other scientists turned up a set of 2002 photographs that showed the two moons.
Weaver said early analysis suggested that the moons, currently designated "S/2005 P1" and "S/2005 P2," are in orbit about 40,000 and 30,000 miles, respectively, from Pluto. P1 could be as large as 100 miles in diameter, Weaver said, while P2 is slightly smaller.
"Around 20 percent of Kuiper Belt objects have a moon," said team co-leader Alan Stern of the Southwest Research Institute in Boulder, Colo., joining Weaver to discuss the findings in a telephone news conference. "We think a lot of astronomers are going to be looking now for multiple satellite systems."
Despite new research possibilities, the discovery is unlikely to dispel the mysteries surrounding the Kuiper Belt and the objects that dwell there, including Pluto. Many astronomers do not regard Pluto, with a diameter of only 1,400 miles, as a full-fledged planet.
Besides being relatively small, Kuiper Belt objects also have a propensity to cluster in "binary" systems in which an object and a nearby moon are so close in size that the two bodies in many respects act as one. Pluto and Charon, about half Pluto's diameter, form such a pair.
Why this happens "is a really interesting question," said astronomer Marc Buie of Lowell Observatory in Flagstaff, Ariz. "I don't know what the answer is, but it's a clue to what things were like back at the time of the formation of the solar system."
Another imponderable is that Charon and the two new moons appear to be in "resonance" in their near-circular orbits around Pluto. For every 12 times Charon circles Pluto, P2 orbits three times and P1 twice. The resonance "may give new clues to their origin," Stern suggested. "This is very important for New Horizons, because we will be able to plan a lot more observations."