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banquo's_bumble_puppy
2004-Mar-16, 12:37 PM
Here's mine-

1500 miles diameter as lower limit
must be mostly solid, (ie. 75% rock, 25% ice)
if a gas giant, must have a rocky core, (ie. metallic hydrogen does not count)

Grand Vizier
2004-Mar-16, 12:43 PM
Good thread idea. Sort of a Delphi poll. But can we assume we just refer to our Solar System? I know we cannot detect rocky exoplanets yet - but I can imagine exactly the same issue coming up in that context.

With that proviso - I'll settle on the same answer as given on another thread - because I consider this to be a cultural/linguistic issue, not a scientific one:

> Pluto (in mass or diameter) and not orbiting another, larger, non-stellar body - planet

&lt; Pluto (in mass or diameter) or orbiting another, larger, non-stellar body - not a planet

Glom
2004-Mar-16, 12:46 PM
Currently, there is a mathematical rule to the semi-major axis of the eight planets as well as the asteroid belt. For the purposes of our solar system, planet should be defined as a dominant body of an orbit corresponding to this pattern. Pluto neither corresponds to this pattern, nor lies in the ecliptic. Hence, shouldn't really be a planet.

Crazieman
2004-Mar-16, 01:41 PM
My proposal:
Any object capable of sustaining an atmosphere of measurable pressure. This would not apply to a smaller body already orbiting a planet. (moons like Titan)

Under this, all 9 would be correctly classified as planets.

gethen
2004-Mar-16, 01:47 PM
Here's mine-

2000-3000 miles diameter as lower limit
must be mostly solid, (ie. 75% rock, 25% ice)
must have a moon

Does that mean Jupiter is not a planet? I don't know if it's core accounts for 75% of its mass.
(Just nitpicking you know. :wink: )

Kaptain K
2004-Mar-16, 01:50 PM
2000-3000 miles diameter as lower limit
Leaves out Pluto

must be mostly solid, (ie. 75% rock, 25% ice)
Leaves out the gas giants.

must have a moon
Leaves out Mercury and Venus.

By your criteria, the Solar system consists of the Sun, Earth, Mars and a bunch of junk (that just happens to include 99+% of the non-Sol mass).

gethen
2004-Mar-16, 01:59 PM
2000-3000 miles diameter as lower limit
Leaves out Pluto

Round 3 on the great Pluto debate? CNN interviewed an astronomer about Sedna this a.m. (can't recall his name) who said Sedna couldn't be the 10th planet because there are only 8 planets. He didn't consider Pluto a planet. Of course, the anchor was totally stunned at the revelation that there was any debate on that subject.

banquo's_bumble_puppy
2004-Mar-16, 01:59 PM
Here's mine-

2000-3000 miles diameter as lower limit
must be mostly solid, (ie. 75% rock, 25% ice)
must have a moon

Does that mean Jupiter is not a planet? I don't know if it's core accounts for 75% of its mass.
(Just nitpicking you know. :wink: )

interesting question: my proposal...gas giants of a certain size woul be reclassified as brown dwarfs

Kaptain K
2004-Mar-16, 02:25 PM
gas giants of a certain size woul be reclassified as brown dwarfs
Brown dwarf has already been defined. Brown dwarfs are capable (or have in the past) of sustaining deuterium fusion in their cores. They range in mass from 13 to 80 time the mass of Jupiter.

tofu
2004-Mar-16, 02:44 PM
By your criteria, the Solar system consists of the Sun, Earth, Mars

Actually, I'm not sure I'd even consider Mars' moons to be moons at all. They are just captured asteroids.

So, according to banquo's criteria, the only planet in the entire solar system is Earth! wow!

banquo's_bumble_puppy
2004-Mar-16, 03:37 PM
By your criteria, the Solar system consists of the Sun, Earth, Mars

Actually, I'm not sure I'd even consider Mars' moons to be moons at all. They are just captured asteroids.

So, according to banquo's criteria, the only planet in the entire solar system is Earth! wow!

cool...I've just set science back 6 or 7 hundred years!!! 8) 8) 8)

banquo's_bumble_puppy
2004-Mar-18, 11:42 AM
I should have given this more thought when I originally posted. Didn't want to kill the thread because I think it is a valid question. I think that in the future planets will be classed differently. Maybe gas giants won't be called "planets" as such...I think that size does make a difference, (snigger). 8) 8)

Gremalkyn
2004-Mar-18, 11:54 AM
Perhaps objects will be classified by their composition and not by "planet," "comet," ect. as their primary classification. A nickle/iron body of 100km could have a "cometary" or a "planetary" orbit w/o being called a "comet" or an "asteroid." So, large gas bodies would be (whatever primary composition) of (whatever dia) in a planetary orbit. More of a "tell me about its properties" approach.

(Orbit types would probably go, too, in favor of shape and duration.)

banquo's_bumble_puppy
2004-Mar-25, 05:09 PM
http://www.spacedaily.com/news/outerplanets-04b.html

Kullat Nunu
2004-Mar-25, 05:39 PM
http://www.spacedaily.com/news/outerplanets-04b.html

How round a object needs to be accepted as planet? What is the minimum size for a body to make it round by gravity? Doesn't it depend on the composition of the body?

For example, if Mimas and Proteus were not satellites but orbited Sun and we adopted the Gravity Rules criterion. Mimas is spherical (almost pure ice, radius 199 km) and Proteus somewhat irregular (rock and ice mix, radii 218 x 208 x 201 km), then Mimas would be a planet and Proteus not, even if it is larger?

And what about large asteroids? Vesta (radius 265 km) is not very spherical either. Pallas (261 km) is more round.

And finally, that article doesn't mention satellites. Are round satellites planets too? What about if we found a KBO composed of two large, round objects rotating each other (*. Are both them planets? If they are equally sized, which one is the satellite?

*) Oops... we already have, they are called Pluto and Charon :).

informant
2004-Mar-26, 11:55 AM
Currently, there is a mathematical rule to the semi-major axis of the eight planets as well as the asteroid belt. For the purposes of our solar system, planet should be defined as a dominant body of an orbit corresponding to this pattern. Pluto neither corresponds to this pattern, nor lies in the ecliptic. Hence, shouldn't really be a planet.
Are you referrig to the Titius-Bode law? I thought Neptune did not verify it, but Pluto did. And what would be done about the asteroid belt?

informant
2004-Mar-26, 12:31 PM
Here's another idea:

Based on an object's mass, define three large categories:

comets, asteroids, etc. ||| planetoids ||| stars and brown dwarfs

Planets are a kind of planetoid. The border between planetoids and brown dwarfs is already known. The border between asteroids and planetoids could be defined by a slightly arbirtary level of mass, related to the theoretical amount necessary for ensuring a round shape in the long run.
What is a planet? I see two alternatives:

1) Using mass as sole criterion. Planets will be the largest planetoids, regardless of whether they orbit another body. Some arbitrary lower limit would have to be defined for the mass of a "planet" -- Pluto's mass? The theoretical amount needed for being able to hold a significant atmosphere?...
According to this definition, a moon can be a planet. Pluto could be a planet or not, depending on where you put the lower limit for the mass of a planet.

2) Using orbit as criterion. A planet is a planetoid that neither orbits another planetoid nor orbits close to one. It would have to be defined how close is "close", and this word should be taken in a somewhat loose sense -- moons are not considered "close", nor are objects in the Lagrange points. On the other hand, asteroid belt objects would not be considered planets, because their orbits are too "close" to those of other asteroid belt objects.
According to this definition, a moon would not be a planet. Pluto could be a planet or not, depending on whether its orbit can be considered close to that of other Kuiper Belt objects.