Pluto dismissed as lump of ice

The Solar System: Can you spot Pluto?

A leading science museum in the United States has decided that Pluto should not be referred to as a planet.

The Rose Center for Earth and Space at the American Museum of Natural History in New York says Pluto has more in common with comets than planets as it is relatively small and made of ice.

A large display of the Solar System, which includes a 2.7-metre (9-ft) diameter model of Jupiter, consigns Pluto to a footnote.

"Beyond the outer planets is the Kuiper Belt of comets, a disk of small, icy worlds including Pluto," says the display.

Officials are dismissive when asked why Pluto is missing from the Solar System display.

"It's in the Kuiper Belt," said Neil de Grasse Tyson, director of the Hayden Planetarium, at the Rose Center. "What's it made of? It's mostly ice."

Museum 'in a different Universe'

The Rose Center says there is no universal definition of a planet. Instead, it divides the Solar System into the Sun and five families of objects.

For some astronomers, the museum has gone too far.

The only image of Pluto taken by Hubble Space Telecope

"Tyson is so far off base with Pluto, it's like he's in a different Universe," says David Levy, author of Clyde Tombaugh, Discoverer of Planet Pluto.

"The majority of astronomers have said that unless there is definitive evidence to the contrary, Pluto stays a major planet."

There is a precedent to demoting planets. The asteroid Ceres was called a planet in 1801 and later demoted.

But Ceres was only considered a planet for a year, while Pluto has been a major planet for more than 70 years.

Pluto fights demotion

When Pluto was discovered in 1930, it was thought to be about the same size as Earth, but astronomers have now learned that it is only 2,274 kilometres (1,413 miles) wide, smaller than the Earth's moon.

Since 1992, when astronomers discovered the first Kuiper Belt object, they have found hundreds of chunks of rock and ice beyond Neptune, including about 70 that share orbits similar to Pluto's.

The International Astronomical Union calls Pluto one of nine planets in the Solar System.

In 1999, a proposal to list Pluto as both a planet and a member of the Kuiper Belt was abandoned after it drew strong opposition from astronomers who did not want to diminish Pluto's status.

The American space agency, Nasa, is currently considering whether to send a mission to investigate Pluto.