Wednesday, August 16, 2006

As of today, 12 planets

A while back I wrote about the debate over Pluto's planetary status. This was followed by a proposal by some astronomers to strip Pluto of its designation.

Well today, the International Astronomical Union went the other way. It's executive panel has proposed a definition of "planet" that is expected to be approved by the membership. It adds three new planets to our solar system -- including Pluto's largest moon, Charon.

The panel suggests retaining Pluto and immediately adding three new planets to the nine that are familiar to any schoolchild: Ceres, currently considered a large asteroid; Charon, now considered a moon of Pluto; and Xena, a recently discovered object that is larger than Pluto.

The definition of "planet" is complicated but interesting:

The proposal defines a planet as an object that circles the sun and is massive enough that its own gravitational forces compress it into a roughly spherical shape. Depending on its composition, a planet would have to be at least roughly 250 to 500 miles in diameter to qualify. It designates a new subcategory of planet, the ``pluton," a Pluto-like planet that takes at least 200 years to circle the sun. Pluto, Charon, and Xena are all plutons, and scientists expect many more to be discovered. Under the proposal, Ceres is an ordinary planet.

Moons are excluded from planetary status, using a criterion that depends on the relative mass of two bodies that are gravitationally tied. If one body is much smaller than the other, then it is considered a moon. Pluto and Charon are closer in mass, and so they are dubbed a double planet. The Earth's moon is round and much larger than Pluto, but it is so much smaller than Earth that it is considered a moon, not a planet.

250 miles in diameter isn't very big from a solar perspective -- the Earth, for instance, is about 8,000 miles across. But at least the definition is clear, even if it changes the colloquial meaning of the word "planet", making it a far less exclusive club. Indeed, astronomers expect to discover even more bodies meeting the criteria for planet as new and more powerful telescopes probe the outer reaches of the solar system.

One strength of the scientific method is that old dogmas are shed relatively easily in the face of new evidence. This is a prime example of that. I grew up learning about 9 planets; my children will grow up learning about 12 or more. And human knowledge marches on.

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Blogger Dyre42 said...

Yeah, but now I'll have to throw out that "My very educated mother just sent us nine pizzas " thing I learned in grade school to help me remember the order of the planets.

8/17/2006 12:46 AM  
Anonymous Anonymous said...

It's about time they came up with a definition of "planet". Pluto lovers will rejoice, for their beloved aggregate of matter is in with the big boys.
- Caracarn

8/17/2006 1:26 PM  
Blogger Sean Aqui said...

That "nine pizzas" thing always made me hungry....

Ijust hope they haven't set the bar so low that we end up with 37 planets or something. That would make the term all but meaningless. I suppose if it gets too crowded they can downgrade plutons as a group, leaving us with 8 planets and a whole mess of trans-Neptune objects.

8/17/2006 1:41 PM  

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