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Planet-sized debate on the horizon

Earth and Moon

Our Moon may qualify for the title of 'planet' if it continues to move away from the Earth (Image: NASA)

News Analysis The proposed new definition of a planet has got scientists talking, some saying it means even our Moon could one day be labelled a planet.

Astronomer Dr Charles Lineweaver of the Australian National University says he is generally in favour of the draft International Astronomical Union (IAU) definition, but it has some potential problems.

"According to the new definition of planets, our Moon ... will become a planet in the future," says Lineweaver.

Under the draft definition, which astronomers will vote on next week in Prague, a planet must orbit a star, while not being itself a star, and it must be massive enough for its own gravity to pull it into a nearly spherical shape.

Under this definition, Charon would no longer be a satellite of Pluto. Rather both would qualify as planets, of the pluton type.

Central to this double-planet system is a common centre of gravity, or barycentre, that exists in free space above the surface of Pluto.

By contrast, our Moon would remain a satellite under the new definition because the barycentre of the system is located below the Earth's surface.

But Lineweaver says as the Moon is moving further away from the Earth, this means the barycentre will one day be above Earth's surface.

"The Moon will have to be about 40% further away than it is now for it to become a planet," he says.

"Based on an extrapolation of the current recession of the Moon, this will happen in about 5 billion plus or minus 1 billion years."

Abandoned free floaters

Astronomer Dr Rosemary Mardling of Monash University in Melbourne confirms Lineweaver's claim that our Moon could one day end up technically a planet. But she is more troubled by other aspects of the proposed definition.

Mardling studies young Jupiter-like objects that float freely in space.

Because they don't orbit a star, such "free floaters" would not, under the proposed definition, be categorised as a planet.

"These things probably were given birth to by a star but were chucked out of the nest," says Mardling.

"The fact that these things are being excluded from this very special name is heartbreaking."

Mardling says while she is generally in favour of the proposal she would not vote for the proposal as it stands.

Reining in chaos

Dr Stuart Ryder of the Anglo-Australian Observatory welcomes the IAU's attempt to provide a clear-cut definition of planets based on physical properties.

In a context where new objects, especially in the Kuiper belt, are being regularly discovered and arbitrarily named, he sees the IAU as providing rules where otherwise chaos would reign.

"It's good to see we're finally taking some action on this," he says, although he acknowledges there will be people who aren't happy with the proposal.

"I suspect we haven't seen the last of this discussion, whatever the decision."

Ryder says it has been difficult to come up with a definition that has a minimum number of criteria that can be relatively easily tested.

He says originally the IAU had asked professional astronomers to come up with a definition but they could not agree.

Often, he says, astronomers would reject a definition, even if it made good sense, just because it excluded their own personal favourite class of objects from being given the favoured title of planet.

Ryder says the IAU ended up drawing on people with expertise in science writing and science history, those who might be more in touch with how society at large might feel about our heavenly bodies.

Lineweaver says the result is a compromise that allows Pluto to keep its planet status.

But planetary scientist Emeritus Professor Ross Taylor of the Australian National University thinks the "planet question" has been captured by sentiment, history and emotion, mainly over Pluto.

"It started with an 11-year-old English girl who named Pluto," he says. "Then there was [Walt Disney's] Pluto the dog."

Taylor rejects the proposed IAU definition altogether and believes that there should be just two categories of planets - major and minor - with Pluto being a minor planet, as, he says, it has always been in the eyes of the IAU.

What's in a name?

Fan or not of the new definition, most scientists say the debate has little relevance to the business of science, which relies on detailed measurements, rather than names.

The new nomenclature is more important for primary school children who want to know how many planets there are, says Lineweaver.

But names do matter beyond sentiment. For one, planets have a particular social status, and this may translate into funding opportunities.

And, like in other areas of science, the controversy will raise public awareness about the complexity of the solar system.

"It gets people thinking about the details of the solar system and that's what we're after, that's the important part," says Lineweaver. "Where those lines are drawn are not so important to me."

Tags: science-and-technology, astronomy-space, the-moon