US astronomers said Friday they had discovered the tenth planet in the solar system.

The planet, larger than Pluto, was discovered in the outlying regions of the solar system with the Samuel Oschin Telescope at Palomar Observatory, according to Mike Brown, a professor of planetary science at the California Institute of Technology.

The planet is a typical member of the Kuiper belt, a suburb region in solar system with asteroids and comets. But the size of this planet in relation to the nine planets already known means that it can only be classified as a planet, Brown said.

Currently about 97 astronomical units from the sun (an astronomical unit is the distance between the sun and Earth), the planet becomes the farthest-known object in the solar system, and the third brightest of the Kuiper belt objects.

"It will be visible over the next six months and is currently almost directly overhead in the early-morning eastern sky, in the constellation Cetus," said Brown.

Brown and colleagues first photographed the new planet with the 10-meter Samuel Oschin Telescope on Oct. 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known, in other words, it's not yet possible to tell how much light from the sun is reflected away. But the amount of light the planet reflects puts a lower limit on its size.

"Usually when we find these we don't know their size for certain, only lower limits. The lower limit to this object is the size of Pluto. This object is at least the size of Pluto and likely a bit larger," Brown said.

"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," he said. "I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size.

"But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system."

Determination of the upper limit of the size of the planet is constrained by results from NASA's Spitzer Space Telescope, which has already proved its mettle in studying the heat of dim, faint, faraway objects such as the Kuiper-belt bodies.

Because the Spitzer is unable to detect the new planet, the overall diameter must be less than 3,000 kilometers, Brown said. A name for the new planet has been proposed by the discoverers to the International Astronomical Union, and they are awaiting the decision of this body before announcing the name.

Source: Xinhua