Using NASA's Hubble Space Telescope, astronomers discovered that the so-called "tenth planet" of our solar system a little larger than Pluto.

The object "2003 UB313," dubbed "Xena," was first discovered by Mike Brown, an astronomer at the California Institute of Technology in Pasadena, California. Brown's group spotted a moon around "Xena" later.

Previous ground-based observations suggested that "Xena" could be up to 30 percent greater in diameter than Pluto, but Hubble observations taken on Dec. 9 and 10, 2005, yielded a diameter of 2,400 km for "Xena," only 100 km larger than Pluto.

"Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Xena," said Brown on Tuesday.

"It only required a couple of Hubble images to nail Xena's diameter," he added.

Located 16 billion km away but with a diameter that is a little more than half the width of the United States, the object is 1.5 picture elements across in Hubble's view. That's enough to precisely make a size measurement.

Because "Xena" is smaller than earlier thought but comparatively bright, it must be one of the most reflective objects in the solar system, Hubble astronomers said.

The only object more reflective is Enceladus, a geologically active moon of Saturn whose surface is continuously recoated with highly reflective ice by active geysers.

The bright reflectivity of "Xena," according to the astronomers, is possibly due to fresh methane frost overlying the surface.

It is possible that the planet had an atmosphere when it was closer to the Sun, but "froze out" at its current large distance, and material settled on its surface as frost.

Another possibility is that "Xena" is also continuously leaking methane gas from its warmer interior. When this methane makes it to the cold surface it immediately freezes solid, covering craters and other features to make this object uniformly bright to Hubble.

The orbital period of "Xena," which is located in the Kuiper Belt of solar system, is about 560 years, and the planet is now very close to aphelion, the point farthest from the Sun. The Kuiper Belt is a vast ring of primordial icy comets and larger bodies encircling Neptune's orbit.

The late fingdings may further complicate the debate about whether to categorize the large icy worlds as planets.

Brown has insisted that "Xena" should be classified as the 10th planet of the solar system, since Pluto was considered to be the minimum size for a planet and "Xena" might be much larger than Pluto.

But with the fact that "Xena," the largest known object in the Kuiper Belt, is a virtual twin to Pluto, it would fulfill this criterion, too.

Source: Xinhua