That means it must be a planet, so now object 2003UB313, spotted two years ago by astronomers in California, has been officially identified as the 10th planet in the solar system, and tentatively christened Xena.
The body is believed to be about 1,700 miles in diameter, about a quarter the size of the Earth, and about one-and-a-half times the size of Pluto, the ninth and last planet to be discovered, in 1930.
But at nearly 10 billion miles out, Xena is the most distant object detected orbiting the Sun, three times as far out as Pluto and 97 times as far out as the Earth. Its full orbit takes 560 years.
Like many other new wonders, Xena has emerged from California, having been discovered by astronomers at the Mount Palomar observatory near San Diego run by the California Institute of Technology (Caltech). Michael Brown, Caltech's professor of planetary astronomy, and his colleagues Chad Trujillo of the Gemini Observatory in Mauna Kea, Hawaii, and David Rabinowitz of Yale University, first photographed it on 31 October 2003, with the 48in Samuel Oschin telescope, normally used to track asteroids that might pose a collision danger with the Earth.
But it was so far away its motion was not detected until data was reanalysed in January. At present, Xena is near its farthest point from Earth, but in about 280 years, it will be as close as Neptune, (the eighth planet, discovered in 1846).
It has taken scientists this long to discover it because its orbit is tilted at a 45-degree angle to the orbital plane of the other planets. Xena is believed to be part of the Kuiper Belt, a great band of icy objects beyond Neptune, that are believed to be remnants of the material that formed the solar system.
"This is the first object to be confirmed to be larger than Pluto in the outer solar system," Professor Brown said, adding that although its exact size was yet to be determined, it was "definitely bigger than Pluto".
The new planet was rocky and icy, similar to Pluto, he said, and so far away an observer standing on its surface could cover the view of the sun with the head of a pin, although it was sufficiently bright for amateur astronomers to track it in the early-morning sky.
The astronomers have submitted their name for it - from the warrior princess in the 1990s TV series - to the International Astronomical Union and are confident it will be designated a planet, although the procedure for approval is somewhat hazy because no new bodies have received that designation since Pluto was found 75 years ago, Professor Brown said. "We hope it's fairly non-controversial among those who believe Pluto is a planet," he said. "I would say, 'Get out your pens and start rewriting the textbooks today'."
In classical times, before the invention of the telescope by Galileo in 1610, the solar system consisted of the Sun and just six planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn.
In 1781, the German musician-turned astronomer William Herschel, working in Britain, discovered Uranus, using a telescope he made himself.