Thursday, August 24, 2006

Not 12 planets; just 8

Speaking of numbers, the members of the International Astronomical Union have rejected a proposal by the union's leadership to expand the definition of planet, and instead have decided to kick Pluto out of the "planet" class, reducing the official number of planets in our solar system to eight.

Much-maligned Pluto doesn’t make the grade under the new rules for a planet: “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit.”

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune’s.

Instead, it will be reclassified in a new category of “dwarf planets,” similar to what long have been termed “minor planets.” The definition also lays out a third class of lesser objects that orbit the sun — “small solar system bodies,” a term that will apply to numerous asteroids, comets and other natural satellites.

At least we have a definition. And while it would be neat to have more planets, I mentioned in my previous post that the leadership's proposed definition was pretty loose -- covering objects as small as 250 miles in diameter -- and would probably cover all sorts of as-yet-undiscovered space debris. So this more exacting standard does a nice job of keeping things manageable.

My only regret is that the old system would have designated Pluto-Charon as a double-planet -- two planets orbiting each other. That would have been cool.

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Blogger Derrick Pallas said...

How does a new definition change what the Pluto-Charon system is? The definition only changes how we talk about the system. There are still two minor celestial objects that orbit each other and cross the path of Neptune. If it had made a difference to Pluto, we would have noticed about 11 hours after the IAU made their announcement.

8/24/2006 11:19 AM  
Blogger Sean Aqui said...

Well, it's a double-something system, but not a double-planet system.

Yeah, it's just semantics. All we're doing is changing the description of what is, not changing what is. But it necessarily changes how the reality is perceived by us. Yesterday we lived in a nine-planet solar system; today we live in an eight-planet-plus-some-weirdness solar system.

Is it important? No. Is it meaningful? Yes. It reflects an evolving understanding of the structure of the solar system.

Most importantly, it's fun and interesting. It reminds us how little we know of our own solar system, and how much remains to be discovered and explained.

8/24/2006 12:10 PM  

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