Astronomers say they have found a new moon orbiting our solar system's recently discovered 10th planet.

Scientists at an observatory on the U.S. island of Hawaii discovered the moon three weeks ago. They estimate it is slightly more than 250 kilometers in diameter, about one-10th the size of its far-flung planet, which is known informally as Xena.

Later this year the U.S. space agency will train the Hubble Space Telescope on the new object to help scientists calculate the moon's mass and map its orbit around Xena, which it circles about twice a month.

Xena, the most distant object ever seen around the sun, moves in an highly elliptical orbit about 16 billion kilometers away, beyond the frozen plant Pluto.