Aug. 16th, 2006

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For a bar located across the street from the Art Gallery of Ontario, the Village Idiot Pub (126 McCaul Street) manages to combine atmosphere and value with affordability quite nicely. Good beer selection, too.
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At least a half-dozen different people on my friends list have commented about the impending redefinition of what a planet is. The class' expansion seems to be ienvitable.

"We came to the conclusion that we should base our definition as strongly on the science as possible. The word 'planet' was invented millennia ago, and modern science tells us so much more," says committee member Richard Binzel, an astronomer at MIT, US.

The result is radical. "If gravity can make it round, it's a planet," Binzel told New Scientist. That is not quite the whole story--planets also have to orbit a star, and not be either stars themselves or satellites of other planets--but the new part of the definition is roundness.

Whereas very small heavenly bodies tend to be irregular rocks, larger ones are crushed by their own gravity into a spherical shape. The committee decided that the threshold of roundness should distinguish planet from non-planet. "We let nature decide," says Binzel.

The threshold size depends a bit on the material, but for ordinary ice or rock it turns out to be well below 1000 kilometres. So the new definition includes Ceres, previously classed as the largest asteroid, which is 950 kilometres across.



The 12 Planets, from MSNBC.
Originally uploaded by rfmcdpei.


The new categories of dwarf planets like Ceres, to this point the solar system's largest asteroid, and "plutons," Pluto-type icy objects in the outer solar system, will complicate things. I have to admit, here, that I'm quite excited by Ceres' elevation to the status of planet. That's how it started out, after all.

Ceres would become the fifth planet from the Sun and the outermost of the five rocky planets, orbiting between Mars and Jupiter. "It was considered a planet once before, after it was discovered in 1801," says Binzel. "It is again."

Ceres might not be the only asteroid to be promoted. If the definition passes, the IAU will also have to consider the claims of Vesta, Pallas and Hygiea, as all of them are round-ish. The smallest, Hygiea, has a diameter of only about 400 kilometres.


I'd written last September about Ceres' potential to be reclassified as a planet. I'm pro: Ceres is small, true, but as the astronomers have pointed out, it is a roughly spherical object that orbits Sol independently of any other body. Certainly, if Ceres had been recognized as a planet before now, it certainly would have received more attention from astronomers and scientists. Even Mercury got Mariner 10 three decades ago and has two more probes en route. Poor Ceres, interesting a body as it is, can only expect the oft-delayed Dawn mission to Ceres and Vesta sometime next decade, if Dawn isn't cancelled again.
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