planets, plutons, and planemos
by John at 8/16/2006 07:37:00 AM
Astronomers are set to vote on the meaning of the word planet, adding perhaps three new planets to our solar system. Cool!
Also, in case your wondering, Charon gets to be a planet because the center of gravity of the Pluto/Charon system is outside Pluto's sphere. Pluto and Charon will be the first binary planets (it's in the article). For completeness, "plutons" are planets with an orbit of greater than 200 years duration, and could include Pluto, Charon, Xena, and other large-ish Kuiper Belt objects that get discovered in the future. Finally, "planemos" are planetary-like bodies found wandering in extrasolar space, that is, planets that do not orbit a star.
I like this scheme, but I worry that one day school children will have to memorize a list of dozens of planets and plutons.
There will be at least 12 planets in our solar system, and probably many more, if a new definition of the word "planet" is adopted. Next week the International Astronomical Union (IAU) will vote on a draft definition of what distinguishes a planet from lesser space rocks.
The new scheme would retain Pluto – previously threatened with ejection from the club – but it would also admit several new members, including the former asteroid Ceres, and even Charon, which until now has been classed as Pluto's moon.
...
"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.
Also, in case your wondering, Charon gets to be a planet because the center of gravity of the Pluto/Charon system is outside Pluto's sphere. Pluto and Charon will be the first binary planets (it's in the article). For completeness, "plutons" are planets with an orbit of greater than 200 years duration, and could include Pluto, Charon, Xena, and other large-ish Kuiper Belt objects that get discovered in the future. Finally, "planemos" are planetary-like bodies found wandering in extrasolar space, that is, planets that do not orbit a star.
I like this scheme, but I worry that one day school children will have to memorize a list of dozens of planets and plutons.
