An artist's impression of the dwarf planet Eris, and her moon, Dysnomia. That's the Sun at upper left, some 14 billion kilometres away.

Credit: NASA

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SYDNEY: Pluto was stripped of it's planet status last year. Now astronomers report it's not even the largest of the 'dwarf planets', a classification they created to accommodate it.

Pluto's demotion last year was prompted by the discovery of Eris: a similar Kuiper belt body made of rock and ice. Despite this, Pluto was still considered the heftiest solar system object found beyond Neptune.

But now, new mass measurements reveal that Eris – which is 27 per cent heavier – dwarfs its more-famous neighbour in both diameter and mass.

Michael Brown – leader of the team that discovered Eris – and Emily Schaller, both astronomers from the California Institute of Technology in the U.S., have taken advantage of the presence of Eris's single moon, Dysnomia, to measure its mass, using something called the 'Keplerian orbit' method.

They report their findings today in the U.S. journal Science.

Discordant dwarf

Measurements were made using Hawaii's Keck Observatory and the Hubble Space Telescope. Using the mass measurement, researchers have calculated Eris's density, which provides a significant clue about the composition of this never-visited, chilly, grey world.

Many objects in the outer solar system have lower-than-expected densities, said the researchers, but Eris's relatively high density suggests that it is more than 70 per cent rock.

"In the past we have known little about this region of the solar system because only Pluto was bright enough to allow detailed study," said Brown. "Now we have an entire population of objects out there that we can study and compare and try to use to put together the story of the solar system."

Initially, Eris – named after the goddess of 'discord' – was known as the 'tenth planet'. Not happy with the possibility of innumerable new planets being discovered, the International Astronomical Union revised its definitions and last year controversially expelled Pluto, Eris and Ceres from the planet club. This meant that any newly-discovered, similar objects would also be denied planet status.

Definition confusion

"The distinction is that a planet has to be massive enough to clear its zone of the solar system of other objects," said Jeremy Bailey, an astronomer at Macquarie University's Australian Centre for Astrobiology. "However, many astronomers disagree with this definition."

"Many of us have argued for years that Pluto is not really the same sort of planet as the other eight, as it has quite different properties," said Rachel Webster, an astrophysicist from the University of Melbourne, Australia.

"The wording was a bit hasty and stupid, but the general concepts are pretty good," said Brown. "There are eight large, dominant bodies in the solar system. In addition to these eight, there are many smaller bodies, of which Eris and Pluto are the largest and second largest. Some number of these are round and these are the dwarf planets."