A California astronomer has discovered what he believes is the 10th planet in our solar system, a group of NASA-funded researchers said.
An artist's concept shows the planet catalogued as 2003UB313 at the lonely outer fringes of our solar system.
The new planet, known as 2003UB313, has been identified as the most distant object ever detected orbiting the sun, California Institute of Technology astronomer Michael Brown said.
Mr Brown and colleagues Chad Trujillo and David Rabinowitz have submitted a name for the planet to the International Astronomical Union and are confident it will be designated a planet. Mr Brown did not reveal the proposed name.
The procedure for approving the new planet is somewhat hazy as no new bodies have received that designation since Pluto was discovered in 1930, Mr Brown said.
"We hope that it's fairly noncontroversial among those who believe Pluto is a planet," Mr Brown said. "I would say get out your pens and start rewriting the textbooks today."
The planet is located about 9.7 billion miles (14.5 billion km) from the sun and is about 1 1/2 times the size of Pluto, the researchers said.
The new planet orbits the sun once every 560 years and is now at its farthest point from Earth, he said. In about 280 years, the planet will be as close as Neptune, he said.
Like Pluto, the object's surface is believed to be predominantly methane, but its size -- about 1,700 miles (2,700 km) in diameter -- qualifies it as a planet, Mr Brown said. Earth is about 7,900 miles (12,700 km) in diameter.
The new planet is believed to be part of the Kuiper Belt, a large ring of icy objects that orbit beyond Neptune and are believed to be remnants of the material that formed the solar system.
Mr Brown said the new planet was detected in January by the Samuel Oschin Telescope at the Palomar Observatory near San Diego.
The Caltech team, funded in part by NASA, had been waiting to announce the find until they had completed their studies, but changed their minds after a hacker threatened to go public with their data, Mr Brown said.
Their finding comes a day after a Spanish team of astronomers announced the discovery of another relatively large object orbiting in the solar system's outer reaches. That object, Mr Brown said, was about three-quarters the size of Pluto.
The new planet went undiscovered for so long because its orbit is tilted at a 45-degree angle to the orbital plane of the other planets, and travels in an elliptical orbit, Mr Brown said.
The team had been scanning the skies with the 48-inch (120-cm) telescope for five years, searching for large bodies orbiting in higher planes than that of the Earth and other planets.
The new planet is so far away that an observer standing on its surface could cover the view of the sun with the head of a pin, Mr Brown said. It was sufficiently bright, however, for amateur astronomers to track it in the early morning sky, he said.