Astronomers said on Saturday that they spotted a moon orbiting what they insisted to be the 10th planet of our solar system.

Mike Brown, an astronomer at the California Institute of Technology, said the the moon is so close to the planet "Xena" that all of the earth-based telescopes have completely missed it until now. Brown said his group had submitted a paper to the Astrophysical Journal to describe this finding.

In an article posted in his personal website, Brown said his group found the faint moon on Sep. 10 thanks to a new cutting-edge improvement of the instruments in the Keck Observatory in Mauna Kea, Hawaii.

Analyzing the new images, the astronomers said a small light point next to the planet "Xena" only could be a moon, Brown said.

In July, Brown announced the discovery of "Xena", or "2003 UB313", in the Kuiper belt, where most asteroids in solar system reside. It is by now the largest celestial object in Kuiper belt that astronomers have ever observed, according to Brown.

Since "2003 UB313" is larger than Pluto, the recognized 9th planet in solar system, it should be treated as the 10th planet, Brown suggested, but not all astronomers agreed with him. A panel of the International Astronomical Union is even preparing to modify the notion of planet.

Right now, the astronomers are not certain how big the moon is, but they guessed it is perhaps 350 kilometers in diameter, 8 times smaller than planet "Xena", which is temporarily named after a warrior goddess in a TV series. And the moon is nicknamed "Gabrielle", a travel mate of Xena in the series.

The planet-moon system appears similar to the Earth-Moon system, except reduced in scale by a factor of about 5-10, according to Brown.

"Xena," with a diameter of about 2800 kilometers, is 5 times smaller than the Earth, "Gabrielle" is about 8 times smaller than the Moon. And the two are separated by a distance that is about 10 times smaller than the Earth-Moon separation.

The finding of this moon will help astronomers determine the mass of its planet, "Xena," Brown said.

"Finding a moon, and then determining the distance that the moon is from the planet and how long it takes the moon to circle the planet allows us to precisely measure the mass of the planet," he wrote in the article.

A second reason that discovering the moon is important is that understanding how moons form provides insight into the history of the solar system, he said.

The astronomers suggested that the moon of "Xena" was formed from a collision between Kuiper belt objects, much like it is thought the the Earth's moon was formed from a collision between the Earth and an object about the size of Mars.

Thus, "understanding the orbit of the moon around the planet will help to show if this hypothesis is feasible," he said.

Both "Xena" and its moon "Gabrielle" are almost 16 billion kilometers from the sun, more than 3 times more distant than the next closest planet, Pluto. They make up of the most distant planet system in our solar system, Brown said.