Thursday, August 24, 2006

It's official!

The solar system has eight planets. Here's the new definition of a planet from the International Astronomical Union:

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Pluto fails part (c) because it crosses the orbit of Neptune, only surviving because of its orbital resonance with Neptune such that they are never in the same place at the same time.

Whew. Now that's settled, finally.


Michael Jasper said...

How does this make you feel?

I feel so... betrayed? Or something.

Why do they have to pick on poor ol' Pluto?

Dr. Lisa said...

I feel victorious. Look, it isn't that Pluto isn't special. It's now one of the largest of a new type of object, rather than being the puniest planet. We have not demoted it, rather we have exalted Pluto.


Kirsten said...

You mean... there's no chance Pluto will collide with Neptune? Like, not ever?

Now I'm sad.

Dr. Lisa said...

I know, it's disappointing, but true. Neptune and Pluto are in a 3:2 orbital resonance - Neptune orbits the Sun three times for every two orbits of Pluto, so they don't hit. Otherwise, quite frankly, there wouldn't be a Pluto debate because there wouldn't be a Pluto to talk about.

Virtualbri said...

Aww, shucks, sometimes billions of years of history takes the fun outta even small percentage chances of things. :)

Funniest comment about Pluto so far (from my work):

Pluto and Tom Cruise:

1. Famous
2. Erratic behavior
3. Small
4. Way out there
5. Given the boot this week.

Dr. Lisa said...

Dude, I'd prefer Pluto to be classified as a planet over it being likened to Tom Cruise. :)

Charly said...


gotta change all the textbooks now - for good!

Dr. Lisa said...

One of my students tonight is a teacher and she brought up the textbook issue. Didn't seem very pleased about it!

David Moles said...

Come on, Mike, how cool is it to be able to write about "dwarf planets"?

I'm disappointed with the IAU's obvious prejudice against free-floating interstellar planemos (not to mention, on the face of it, ordinary exoplanets, although I don't think they meant to do that.) And apparently various people (specifically people involved with Pluto probe missions, gee I wonder why?) are whinging about the alleged vagueness of "has cleared the neighborhood".

Still, it's much better than the "50-plus planets, except it's not a planet until it's been ratified by a committee" definition. Or any definition that ends in "plus Pluto, for purely historical reasons."

David Moles said...

And if we could change the textbooks for good, it wouldn't be science!

Dr. Lisa said...

Good point about the textbooks, David.

And I think the term "planet" can still be used for the exoplanets. The IAU definition was for objects in our solar system, so they specified the "Sun". I also find that I have to use the terms "extrasolar planet" or "exoplanet" because otherwise my students default to thinking we're talking about our solar system.

David Moles said...

Mostly, it's the exoplanets not orbiting stars (or brown dwarfs) that I'm concerned for. Pluto has plenty of advocates, but who's speaking up for Chameleon 130540-773958?

Dr. Lisa said...

I will speak for all exoplanets. I shall be their advocate. :)

Miss you here at WorldCon, David!

Michael Jasper said...

Okay, I feel better now. :)

Dr. Lisa said...

Glad to be of service, Mike!