Pluto's demotion as planet 'unfinished discussion'

An artist's rendition of the planet Pluto.

Science writer Dava Sobel had a hand in the events which led to last week's "demotion" of Pluto as an official planet in our solar system, but says it may take a while before people adjust to the change.

Sobel, who initially proposed promoting other "icy dwarfs" like Pluto to planet status, said that eventually people would under stand they had not so much lost a planet as gained a big family of dwarf planets.

"I'm not sure the discussion is finished," she said. "There are T-shirts selling on the internet proclaiming, 'Pluto IS a planet'."

If people gave any thought to the planets, they gave the most thought to Pluto, discovered in 1930, she said.

"People love Pluto - there's such an emotional attachment, it's very strange".

"Children identify with its smallness. Adults relate to its inadequacy, its marginal existence as a misfit".

The International Astronomical Union's (IAU) "planet definition Committee" of six astronomers, plus Sobel, met at the Paris Observatory in late June and agreed that a planet is a body in orbit around a star (as opposed to orbiting another planet) and big enough for gravity to make it round.

But the working group's proposal to retain Pluto as a planet and promote three other objects, including Pluto's largest "moon", Charon, to similar status was not accepted.

Instead, the union, meeting in Prague last week, busted Pluto down to "dwarf planet" a status to be shared with Charon, Ceres and UB313 (bigger than Pluto and also known as Xena, named after the TV character played by Lucy Lawless).

A third class - smaller solar system bodies - was created for the thousands of comets and asteroids. It was the first time the IAU had tried to scientifically define a planet.

"I'm glad that we have a definition now - though it was bound to upset some people," said Sobel. "I thought my committee's resolution was one that everyone could love - but I can see why it was changed, and I think other people will come to understand it too."

But she's not happy with the label "dwarf planet" and rather liked her committee's initial idea of calling those ice dwarfs "plutons".

Because the final definition came up in debate at the Prague meeting, it was possibly not as clear as it could have been, said Sobel, who arrived in New Zealand tonight for a speaking tour promoting her latest book The Planets.

The author of Longitude, a prize-winning best-seller, and Galileo's Daughter -- shortlisted for the Pulitzer Prize - she will speak at writers' gatherings in Auckland tomorrow night, and in Wellington on Thursday.

Sobel said the take-home message from the Pluto debate was that science did not stand still - new knowledge meant that long-held ideas could be held up to critical debate and revision.

"And it has spurred an interest in astronomy: I'm happy about that".

She feels the idea of promoting Ceres - found two centuries ago between Mars and Jupiter - from asteroid status will eventually be re-visited.

"To have Ceres and Pluto lumped together is unfortunate - I think these things may get smoothed out".

Some astronomers have suggested Ceres was promoted partly because in 2005, the Hubble space telescope showed it be spherical, which suggested it had the planetary mass required to pull itself into a ball.

But sightings since 1992 have shown several hundred other objects at the distance of Pluto and beyond, and some triggered calls from some astronomers to disqualify Pluto and other objects smaller than 2414km diameter from planet status.

- NZPA