How would you define "planet"?

[Bill McBlueState]

Pretend you're part of the International Astronomical Union and you're debating this week about the definition of the term "planet." What definition would you propose?

I would go with "An object in orbit around a star that has never sustained nuclear fusion and is gravitationally dominant in its neighborhood."

The first part provides a clear upper mass limit, and the second part provides a somewhat more ambiguous lower limit. Under that definition, our solar system would have eight planets, and Pluto would be off the list. Pluto is simply one of the larger Kuiper Belt objects, and the particulars of its discovery really shouldn't accord it more importance than objects with similar mass, composition, and distance from the sun.

[SnoopDog]

1. I would adopt the terms planets and orbiting masses

... or something like that. Masses may not be the best word.

Planets should have a minimum mass and spherical diameter whereas orbiting masses would constitute the rest. Pluto obviously should not be labeled a planet.

[BlooInBloo]

10. Neither is mercury then, right?

[Bill McBlueState]

23. actually, Pluto is much smaller than Mercury

One of the interesting things about Pluto is that from its discovery in 1930 to the present, our idea of its size has gotten much smaller.

In an encyclopedia from the 1960's, it's listed as almost as big as Earth.
In books from the 1980's, it's listed as the second smallest planet after Mercury.

Today, the accepted diameters are about 3000 miles for Mercury and about 1400 miles for Pluto.

[MADem]

2. Pluto, off the list? Oh, the inhumanity!!!

http://www.herlitz.de.nyud.net:8090/fileadmin/Downloads/Kids/Pluto.jpg

I mean...look at that adorable face!!

Seriously, I have no opinion, but it seems like an interesting topic, so this is about the only way I can think to give it a kick in hopes of reading the opinions of people who have some expertise in this area...!

[longship]

3. This is a reasonable definition.

And I agree that Pluto is suspect as a planet. I'm not sure that I would call it such.

* It is very small.
* It has a highly eccentric orbit, dramatically different than the near circular orbits of the other true planets.
* It's orbit takes it within the orbit of one of the planets. So part of the time it's the ninth body, and other times it's the eighth.

Pluto is a planetoid from the Kneiper belt, not a planet.

[bananas]

4. It would be defined in terms of "Janet" and "dammit" nt

[Sinti]

5. Planet Shmanet Janet n/t

[TechBear_Seattle]

13. "Interplanet Janet, she's a galaxy girl!"

Sorry. I'm indulging in a bit of pointless nostalgia. :hi:

http://www.schoolhouserock.tv/Interplanet.html

[Sinti]

17. So was I - only mine is a lot less innocent :)

:hi:

[TheWraith]

6. I would say...

A roughly spherical object in orbit around a star, which has measurably less than the mass required for nuclear fusion, and is above 1,000 miles in diameter.

Anything under 1,000 miles, but above 223 miles and matching the other criteria, would be called a "minor planet." This is the term already used for small planetlike bodies such as Ceres, Vesta, Juno, et al.

I'm reluctant to let go of Pluto, it having been our ninth planet for so long, it seems kind of rude to simply redact the title. I'd rather promote the larger discoveries to full planethood, rather than adopt an unneccessarily stringent requirement for definition.

[anarch]

26. I'd say a 30,000 mile diameter minimum

I don't like having lots of so-called "planets" to keep track of. In my solar system, there are only four planets, and life is much simpler.

[Eugene]

7. Planets plan boosts tally to 12 (BBC)

Last Updated: Wednesday, 16 August 2006, 06:05 GMT 07:05 UK

Planets plan boosts tally to 12
By Paul Rincon
Science reporter, BBC News

The number of planets around the Sun could rise from nine to 12 -
with more on the way - if experts approve a radical new vision
of our Solar System.

An endorsement by astronomers meeting in Prague would require school
and university textbooks to be rewritten.

The proposal recognises eight classical planets, three planets belonging
to a new category called "plutons" and the largest asteroid Ceres.

Pluto remains a planet, but becomes the basis for the new pluton category.

-snip-

Full article: http://news.bbc.co.uk/2/hi/science/nature/4795755.stm

[muriel_volestrangler]

8. Calling the new category "pluton" is a bad idea

because that already has a scientific meaning:

Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or plutons, of magma that traveled toward the surface from a zone of partial melting at the base of the Earth's crust.

http://en.wikipedia.org/wiki/Batholith

[Solon]

9. Here's a basic definition...

A celestial body that both orbits a star, and has enough mass to maintain a spherical shape. This means that both Pluto and Charon are STILL out, individually(they orbit each other, their bary center orbits Sol), but Ceres is put in, and the Pluto/Charon system is still technically in.

[mccoyn]

12. The earth wobbles in its orbit because of the moon.

You'll have to draw the line between binary system and planent/moon system.

[Solon]

14. The Center of the orbit between the Earth and the Moon is still...

within the Earth, so its NOT a double planet system in the sense that they orbit each other, because they don't. Also, ALL objects that orbit larger objects cause them to "wobble". How do you think the first extra-solar planets were discovered? From the wobbling of Stars.

[TechBear_Seattle]

16. That is also why the first extra-solar planets were so vast

To cause a wobble noticeable at interstellar distances, the barycenter would have to be fairly close to the star's surface. That means that, with a star similar to our sun, a Jupiter sized mass in Mercury's orbit or something 7 to 10 times Jupiter's mass in Mars' orbit.

[TechBear_Seattle]

15. That line is already drawn, and it is pretty specific

The distinction is: where is the barycenter?

If the barycenter is within the sphere of one of the bodies, then that body is the primary and the other is a sattelite (example: The Earth and the Moon.)

If the barycenter is outside the sphere of both bodies, then neither is the primary and you have a double-body system (example: Pluto and Charon.)

Interesting factoid: The barycenter of the Earth-Moon system is only about 1,448 km (900 miles) beneath the earth's crust. The center of the earth is another 4,925 km (3,060 miles) away.

[rman]

20. Then Earth is also not a planet

Even though the bary center of the Earth/Moon system lies within the volume of Earth, it does not coincide with the rotational axis of the Earth.

On the other hand, it's hard to argue that Earth/Moon but also Pluto/Sharon do *not* orbit the sun. Taking that into account, both systems are (double) planets.

[Solon]

22. Actually yes it is...

Even though the bary center of the Earth/Moon system lies within the volume of Earth

What you said right there is ALL that is needed to distinguish between a Planet and its Moon, everything else you said doesn't matter. Pluto doesn't have its barycenter within it, therefore, neither Pluto nor Charon are Primaries in their particular system.

[rman]

25. Still, both do orbit the sun

It's just the the orbits are perturbed. But then again, any orbit is.

[rman]

11. This is a source of some confusion for the public,

but astronomers don't really care.

[Bill McBlueState]

24. you're telling me!

Scientifically it's not important whether we call something a planet or whatever.

The only reason I as an astronomer am interested is because I'm going to have to explain the IAU definition to students and the public. And "sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape" (from the proposed definition) is not the most intuitive thing in the world for people without a physics background.

[bemildred]

18. The present nine, only, everything else is a satellite, or something.

Make up a nomenclature for classes of satellites.

[Nihil]

19. Everything else is a planetoid (in this solar system)

And while we're at it, if the astronomers want to invent a term
for an angels-on-the-head-of-a-pin category they've adopted,
please do so instead of nicking a perfectly valid geological term!
"Pluton" indeed! :eyes:

[bemildred]

21. I like "satellite", or something without planet in it.

They all orbit something, some center of mass, eh? How about "orbiton" or "orbitoid"? :-)

I agree about the theft of the word "pluton". Given the greek mythological origins of that word, it seems most inappropriate for a celestial body.