US astronomers have found an icy, red planet-like object, orbiting the sun eight billion miles away, twice as far as Pluto, and stretching the limits of the solar system beyond anything discovered so far.
Space officials announced the discovery at NASA's Jet Propulsion Laboratory in Pasadena, California on Monday. Researchers discovered the object in November 2003.
Scientists said the object may be the first visible evidence of the Oort Cloud, a massive spherical shell of comets thought to be loosely orbiting the sun and extending outward almost halfway to the nearest star.
"There's absolutely nothing else like it known in the solar system," said Michael Brown of the California Institute of Technology in Pasadena.
The object is catalogued as 2003 VB12 and has unofficially been dubbed Sedna, goddess of the sea for Arctic dwellers.
Brown thought that appropriate given the frigid conditions under which the solar system body has probably always existed. The International Astronomical Union would have to approve the name.
It may even have its own little moon.
Sedna is estimated to be 1,770km in diameter and is about 90 per cent the size of Pluto's, making it the largest Solar System object discovered since Pluto itself in 1930.
Researchers at the California Institute of Technology discovered the planet using the new Spitzer space telescope, which made its debut last August.
Sedna lies in the Kuiper Belt, a band of ice and rock left over from the birth of the Solar System that stretches outwards from Neptune.
About 800 Kuiper-Belt Objects (KBOs) have been discovered since 1992, some with predicted orbits that would take them as far as 150 billion kilometres from the Sun.
Scientists believe the discovery of Sedna to be the most important find in our solar system since Pluto was first spotted in 1930. The temperature on Sedna never gets above minus 400 degrees Fahrenheit.
"The sun appears so small from that distance that you could completely block it out with the head of a pin," said Mike Brown, an astronomer at California Institute of Technology, who led the research team.
First detected on November 14, 2003 with the Samuel Oschin Telescope near San Diego, California, Sedna was observed within days on telescopes from Chile to Spain, Arizona and Hawaii.
Brown does not consider Sedna to be a planet. He and many other astronomers maintain that Pluto should not have ever received planet status either, since astronomers are now finding myriad round objects beyond the orbit of Neptune, and several of them are quite large.
Pluto is about 2,274 kilometers wide. Sedna is estimated at no more than 1,770 kilometers in diameter. It may be the largest object in the solar system after Pluto, but more observations are needed to pin that down.