Science textbooks may have to be rewritten now that astronomers have found a previously unseen world wandering on the edge of our solar system.
The discovery of Pluto, in 1930, meant the sun's family of planets was officially boosted to nine.
The new world called Xena, possibly almost 3000 kilometers in diameter and thus larger than Pluto, has been dubbed the "10th planet" by NASA, which partly funded the discovery by California's Palomar Observatory.
At the moment the object is about 97 times further from the sun than Earth and three times more distant than Pluto.
Asked if textbooks would have to be rewritten, Nick Lomb, curator at the Powerhouse Museum's Sydney Observatory, said: "Yes, very much so. We will now have either 8 planets or 10. We will no longer have nine."
He said the International Astronomical Union would have to reopen the debate about whether tiny Pluto - only 2300 kilometers in diameter and thus even smaller than our moon - was really a planet or just a large member of a stream of big asteroids called Kuiper Belt Objects, reports Fairfax Digital.
According to Guardian, the trouble for astronomers is that they do not have an exact definition of a planet. Many say that, if Pluto had been discovered today, it would not have been called a proper planet. In 1999 one group from the US Minor Planet Centre proposed that Pluto be given a new joint classification so that it would keep its position among the major planets, but also be given a designation as a minor planet. The centre dropped the proposal after outcry from those who saw it as a demotion.
Gareth Williams of the centre said he still supported dual status for Pluto, but did not think Xena should be added to the registry of major planets. It should be left as a minor planet 'permanently', he said.
But Mark Sykes, director of the Planetary Sciences Institute in Tucson, Arizona, disagreed. It should be classed as a full planet, he said. 'The kinds of questions we would ask about this object Xena would be planet-like questions,' he said. For example, does it have an atmosphere and what sort of geological processes generated its apparently bright surface?
According to Professor Mark Bailey, the director of the Armagh Observatory, Northern Ireland, the discovery is likely to cast new light on the formation of the sun's family of planets about 4,500 million years ago. "It may be that the sun was once part of a cluster of stars, and these scattered the bodies making up the early solar system into the strange orbits we see today," he said, informs News Telegraph.