US astronomers have discovered an object in the outer reaches of the solar system -- a rock bigger than pluto, which they labelled as the 10th planet.
If confirmed, the discovery would be the first of a planet since pluto was identified in 1930.
"Get out your pens. Start re-writing textbooks today," Mike Brown, a planetary scientist at the California Institute of Technology, said announcing the discovery.
"The new object is covered in methane ice and lies nine billion miles from earth and it's the farthest object ever discovered to orbit around the sun," he said. "I'd say it's probably one and a half times the size of pluto."
The new body, tentatively called 2003-UB313, was originally detected in 2003, but it was not regarded as a planet until scientists re-analysed their data earlier this year.
Brown first saw the new planet on January 8 using the 48-inch Samuel Oschin Telescope at Palomar Observatory, along with his colleagues Chad Trujillo of Gemini Observatory in Mauna Kea, Hawaii, and David Rabinowitz of Yale University.
It had actually been photographed first in 2003 -- hence the name -- but nobody realised what it was until its motion became apparent.
The planet 2003 UB313 has since been observed with other telescopes. Researchers have attempted to measure its heat output with the Spitzer Space Telescope, but the orbiting observatory could not find it, putting an upper limit on its size of twice pluto's diameter.
Brown said the discovery would rekindle debate over whether pluto can be regarded as a planet at all.
Astronomers have proposed a name for the "planet" to the science's governing body, the International Astronomical Union, and are awaiting the decision before announcing it.
The planet has not been noticed previously because its orbit is at a 45-degree angle to the rest of the solar system, Brown said.
"We found it because we've looked everywhere else. Nobody looks way up that high. It's tilted way out of plane," he said.
The team had planned to keep the news secret until their research was completed.
The "new planet" will be visible over the next six months and is currently almost directly overhead in the early-morning eastern sky, in the Cetus constellation.