The planet is about one and a half times the size of Pluto and was discovered orbiting about 14.5 billion km from the Sun, an astronomer at the California Institute of Technology in Pasadena said on Friday.
The discovery in the outlying regions of solar system was made with the Samuel Oschin Telescope at the Palomar Observatory near San Diego, planetary scientist Mike Brown said in a statement.
The new planet, known as 2003UB313, is a typical member of the Kuiper belt, but its sheer size in relation to the nine known planets means that it can only be classified as a planet, Brown said.
The new planet was detected
by the Samuel Oschin telescope
It went undiscovered for so long because its orbit is tilted at a 45-degree angle to the orbital plane of the other planets, Brown said.
The new planet, a body of rock and ice, orbits the Sun once every 560 years, he said.Currently about 97 times further from the Sun than the Earth, the planet is the farthest known object in the solar system, and the third brightest of the Kuiper belt objects.
But Brown conceded that the discovery would likely rekindle debate over the definition of "planet" and whether Pluto should still be regarded as one.
Brown discovered what could be a new addition to the universe known to man along with colleagues Chad Trujillo, of the Gemini Observatory in Mauna Kea, Hawaii, and David Rabinowitz, of Yale University, on 8 January.
Scientists said they 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.
Reflectance still unknown
The reflectance on the planet is not yet known so it is 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, Brown said.
"Even if it reflected 100% of the light reaching it, it would still be as big as Pluto," said Brown.
"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. We are 100% confident that this is the first object bigger than Pluto ever found in the outer solar system"
scientist from the California Institute of Technology
"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. We are 100% confident that this is the first object bigger than Pluto ever found in the outer solar system."
The astronomers have proposed a name for the "planet" to the science's governing body, the International Astronomical Union, and are awaiting the decision of this body before announcing it.
"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.
News of the discovery was announced earlier than expected after hackers broke into Brown's website and stole news of it, he charged.
The team had planned to keep the news secret until their research was completed, but a Spanish team said on Thursday that it had identified a large, bright object in the Kuiper belt surrounding the solar system.
Brown said "somebody with more cleverness than scruples" had uncovered what had been under wraps, forcing a public announcement.