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Astronomers Discover Two Moons Orbiting Pluto

U.S. astronomers have discovered two moons orbiting Pluto, an icy planet smaller than our moon. Scientists found the moons while surveying the sky as part of the New Horizons mission, which was launched earlier this year to explore the solar system's most distant planet.

The New Horizon's space probe was launched in January to explore the icy planet of Pluto, and its moon Charon, 6.5 billion kilometers from Earth.

It will take almost 10 years for the probe to reach Pluto, our solar system's most distant planet.

But scientists have found two more moons for the spacecraft to inspect during its brief nine-hour fly-by.

Using the Hubble Space Telescope, astronomers detected the two small, faint moons orbiting Pluto in the Kuiper Belt, a region of the galaxy beyond Neptune that is the cradle of asteroids, comets, and icy bodies, including Pluto.

Hal Weaver of Johns Hopkins University Applied Physics Laboratory in Maryland is a scientist on the New Horizon's mission.

"It is going to give us two new Kuiper Belt objects to look at in addition to Pluto and Charon," he said. "With the New Horizons mission, it is basically four for the price of two."

The new moons are called P1 and P2. Weaver says they are 4,000-6,000 times fainter than Pluto, and two- to three-times as far away from Pluto as Charon.

For that reason, no one could see P1 and P2 with the world's best ground-based telescopes. But that did not stop Alan Stern of the Southwest Research Institute in Colorado, who began hunting for other moons around Pluto 15 years ago.

Pluto and Charon orbit around each other in what astronomers call a binary system. Experts say the twin orbits suggest that Pluto was created by a massive impact and Charon is a remnant of that explosion.

Stern reasoned, if that were the case "... Pluto would have small satellites accompanying its large one, Charon. And when we got the right tool, we put the nail through the coffin in eight minutes flat with the Hubble Space Telescope."

Stern, the principal investigator for the New Horizons mission, says P1 and P2 track Charon's orbit perfectly, which supports the idea that Pluto was created by a massive impact.

Hal Weaver believes there could be many unique planets like Pluto at the edge of the universe.

"The exciting thing about all this is we have basically discovered a new solar system," he said. "This solar system that we are talking about now is very different from the one previous generations grew up with, our parents and even us, and what we were taught in school was that the solar system ended at Pluto."

Two papers by Weaver, Stern and their colleagues describing the discovery of Pluto's two moons appear in the February 23 issue of Nature.