Astronomers have announced yet more bad news for the former planet Pluto.
Kicked out of the club of planets last year, and categorized as merely the biggest dwarf planet, it may now not even be that.
Scientists have discovered a 2,400-km-wide dwarf planet called Eris, which is bigger and heavier than Pluto.
Using the Hubble space telescope and the Keck observatory in Hawaii, scientists used measurements of the orbit of Dysnomia, one of the satellites of Eris, to calculate that Eris is 27 percent heavier than Pluto.
"This is sort of Pluto's last stand," said Emily Schaller, of the California Institute of Technology, part of the research team that published its results on Friday in Science magazine.
Pluto was demoted from planet status at a meeting of the International Astronomical Union last year. The move solved an embarrassing fudge - when astronomers at the Lowell observatory announced the discovery of Pluto in 1930, they claimed it was several times larger than Earth, ensuring that it was quickly labeled the ninth planet.
But as it turned out, Pluto was substantially smaller than the moon. At 2,400 km, its width is no more than the distance from London to Moscow.
When Eris was spotted on the edge of the solar system in 2003, it forced astronomers to rethink their definition of what made a planet.
Ian Crawford, of the Centre for Planetary Science and Astrobiology at Birkbeck College, said the latest research showed that the discovery of Pluto had been a lucky accident, rather than a proper planet.
He said Pluto had been the first object discovered from the Kuiper Belt, a ring of rocks and comets that surround the outer solar system. "It goes to show that there's nothing special about Pluto."
The objects in the Kuiper Belt, which include Pluto and Eris, and the mysterious Oort cloud, were formed 4 billion years ago at the birth of the planets. They interest scientists because they preserve a record of conditions at that time, which is useful in understanding the origins and formation of the solar system.
Andrew Coates, of the Mullard space science laboratory at University College London, admitted to a tinge of sadness when Pluto was reclassified.
"I, like everyone else, had grown up through school thinking Pluto was a planet (but) science has moved on, it's definitely a Kuiper Belt object, and getting that idea across to school kids now gives them more of a chance of understanding the solar system in the future."
He said there was no reason to think that Eris was the most impressive object in the outer solar system. "It's certainly possible there are bigger objects out there."
(China Daily 06/16/2007 page1)