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Astronomers reject the term ‘planet’

Controversial plan would end debate over number of planets in Solar System.

An expert panel charged with ending the debate over what is and isn't a planet has come up with a radical solution: end use of the term altogether, unless it is accompanied by a qualifier.

Debates on nomenclature are common in science, but the planet question is one of the few to have spilled into the public arena. Researchers have argued over the status of Pluto for decades, for example, with some claiming that it is not a fully fledged planet. Similar rows have raged in recent years over how to describe new additions to the Solar System. The panel could now be close to settling such matters. If it succeeds, works ranging from encyclopedias to children's books will have to be updated.

The panel's proposal, a copy of which has been seen by Nature, contends that the collection of objects currently dubbed planets, from rocky worlds on the outer shores of our Solar System to free-roaming objects in deep space, is too diverse to justify a single moniker. Instead, the researchers want to define different types of ‘planetary object’, such as terrestrial planets, including Earth, and extrasolar planets, which orbit stars other than the Sun.

“If we're going to use the word planet we should put an adjective in front of it,” says Brian Marsden, a panel member and an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Name game: some say the word ‘planet’ is used too widely for it to be a useful definition. Credit: NASA

The 19-strong group was convened last year by the International Astronomical Union (IAU), but speeded up its work this July when a media debate broke out over the status of another addition to the Solar System. The object, known as 2003 UB313, orbits near Pluto. One of its discoverers, Mike Brown of the California Institute of Technology in Pasadena, who has recently been courting controversy regarding another discovery (see ‘Planet spotters compete’), says it should count as a tenth planet, in part because it is larger than Pluto. But other astronomers say both UB313 and Pluto are simply large members of the Kuiper belt, a jumble of rocky and icy objects that orbits some 10 billion kilometres from the Sun.

The proposal, e-mailed to group members on 12 September, would end such arguments. UB313 and Pluto would be known as Trans-Neptunian planets, a class roughly defined as large objects that orbit the Sun beyond Neptune. Other members of the Solar System would fall into the categories of terrestrial planets or gas giants, although Iwan Williams, the group's chair and an astronomer at Queen Mary, University of London, says that his team plans only to define the Trans-Neptunian class, and will leave other definitions to the IAU.

Williams hopes to send a final version of the proposal to the IAU within two weeks, after the team has reviewed it. But whereas the broad definition of planetary objects is uncontroversial, at least one member plans to dispute the names for subtypes. “I don't believe we should classify planetary types by location,” says Alan Stern of the Southwest Research Institute in Boulder, Colorado. “We should use properties of the objects as a guide.”

UB313 and Pluto would be better known as “ice dwarfs”, Stern suggests, because such a definition “tells us more about the objects”. He points out that stars are classified by their physical properties, not their location.

If the group can reach a consensus, it will be up to the IAU's executive committee to decide whether to accept the proposal. But will the public and scientists then change the names they use for Mercury and Mars? “Old habits die hard,” says Jacqueline Mitton, an author of popular astronomy books based in Cambridge, UK. She points out that some astrophysicists still describe stars as either ‘early’ or ‘late’ types, terminology that was officially abandoned around 50 years ago. “Committees can make pronouncements, but they can't always change things,” she adds. “It will take a very long time.”


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Giles, J. Astronomers reject the term ‘planet’. Nature 437, 456–457 (2005).

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