Amid the debate over whether Pluto is really big enough to be called a planet, Hubble Space Telescope astronomers announced Monday that they have spotted two previously unknown moons there.

Until now, Charon was Pluto's only known moon. The new moons surfaced in three days of Hubble observations in May.

"This is a whole new chapter in the Pluto story," says discovery team co-leader Alan Stern of the Southwest Research Institute in Boulder, Colo. The moons, which don't yet have formal names, also were spotted when Hubble scientists reviewed 2002 Pluto observations made by Lowell Observatory astronomer Marc Buie.

Charon, discovered in 1978, orbits much closer to Pluto at 12,024 miles and is bigger at 752 miles across. The astronomers estimate the moons range from 30 to 100 miles wide. The closer one orbits 30,000 miles from Pluto, and the more distant one is 40,000 miles out.

But all of Pluto's moons follow circular paths around its equator, on trajectories that appear to have been stable for millions, perhaps billions, of years, Stern says. That suggests all of Pluto's moons formed at the same time, perhaps after a collision between celestial bodies early in the history of the solar system, he says.

"Just to gloat a little bit for Hubble, this wasn't the first time people have looked" for more moons orbiting Pluto, says astronomer Keith Noll of the Space Telescope Science Institute in Baltimore. About 20% of the large objects in the comet-rich Kuiper Belt near Pluto show evidence of having a moon, he notes.

Among them is UB313, a planet-like body larger than Pluto that scientists also found has a "natural satellite." Similarities between Pluto's new moons and ones orbiting other Kuiper Belt objects suggest they all formed the same way, but how this happened is unclear.

Astronomers hope that further study of the new moons -- which are unobservable now until February because of Pluto's position in the sky -- will answer questions about the planet before 2015, when a NASA probe is scheduled to arrive there. Measuring the moons' orbits should sharpen estimates of Pluto's mass and density

"The really striking thing about these two new satellites is they appear to be in 'resonance' with Charon," says astronomer Hal Levison, another Kuiper Belt expert at the Boulder institute. For example, the more distant one orbits Pluto precisely once for every six times Charon orbits the planet. Such synchronized orbits occur elsewhere in the solar system, such as between Neptune and Pluto.

How the two new moons fell into step with Charon is hard to explain, Levison says. He is skeptical of Stern's suggestion that all of Pluto's moons formed at the same time after a collision.

Such a blast would have sent Pluto's new moons on skewed elliptical orbits rather than the neat, circular ones they are in now, he says.

Pluto has been caught up in astronomical debate over its status as a planet in recent months before an International Astronomical Union committee. Its detractors say it's too small and the discovery of more moons won't affect its status, says Stern, who is on the committee. But the news that Pluto "has a whole suite of companions" may boost its public image, he says.