Astronomers have announced yet more bad news for the much-lamented former planet Pluto. Kicked out of the club of planets last year into a new category of dwarf planet, it is not even the biggest of those, scientists have found.
The same object that began Pluto's problems, a 1,500-mile-wide dwarf planet called Eris, has been confirmed as 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% heavier than Pluto. "This is sort of Pluto's last stand," said Emily Schaller, of California Institute of Technology, part of the research team that publishes its results today in Science.
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 labelled the ninth planet. But as it turned out, Pluto was substantially smaller than the moon. At 1,480 miles, 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 just been the first object discovered from the Kuiper Belt, a ring of rocks and comets that surrounds 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, which includes Halley's comet,were formed 4bn 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 schoolkids 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."
Owen Gingerich, emeritus professor of astronomy and history of science at Harvard-Smithsonian centre for astrophysics, said that astronomers were already tracking several new candidates for dwarf planet status. Within a few years, he predicted, the solar system was likely to have five or six new members.
When the American astronomer Clyde Tombaugh of the Lowell observatory first saw Pluto in 1930, he was looking for a ninth planet. By imaging the night sky in pairs of pictures taken two weeks apart, he looked to see if anything had moved, and found Pluto (artist's impression above, with its moon, Charon). In 1955, Pluto was thought to be roughly the mass of the Earth. By 1971, this had been revised to the mass of Mars. Astronomers eventually settled on a mass that is 0.002 of the Earth's. When Eris was found in 2003, astronomers had to reconsider their definition of a planet. A vote of more than 2,500 scientists last August cast Pluto into a newly defined category of dwarf planets.