Old debate re-opened - What is a planet?

finnjim2001

stepryan wrote:
voyager wrote:
Quote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.


would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.


By that logic stephen wouldn't mercury need to be reclassified as well

finnjim2001
not necessarily i said that it could have the mass to retain to retain it would not necessarily have one. besides that i think i read somewhere that it does have a trace atmosphere but it is very thin.
stephen.

voyager wrote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.

OK, allow me to rephrase that some what better:

A planet is a large circular body in a near circular orbit around the Sun.

That rules out the asteroids and any moons and makes Pluto and EKBO.

The real planets all orbit in the same plane, in the same direction and in similar near circular orbits. This to me implies a common origin and hence these objects should bea class to them selves.

Pluto and the other EKBOs all have inclines and highly eliptical orbits, they too seem to share a common source but it does not seem to be the same as that of the planets and hence these objects should not be called planets IMHO.

voyager wrote:

OK, allow me to rephrase that some what better:

A planet is a large circular body in a near circular orbit around the Sun.

That rules out the asteroids and any moons and makes Pluto and EKBO.

The real planets all orbit in the same plane, in the same direction and in similar near circular orbits. This to me implies a common origin and hence these objects should bea class to them selves.

Pluto and the other EKBOs all have inclines and highly eliptical orbits, they too seem to share a common source but it does not seem to be the same as that of the planets and hence these objects should not be called planets IMHO.

just to keep the arguement going. by what rule says that a planet has to travel :-
(A) in a circular orbit
(b) around the sun
(c) in the same direction as all the other planets

if you look at the solar system venus axis rotates backwards and the pole of uranus is at 90 degrees from where it should be with respect to the ecliptic so it faces us. this with our own planets inclination should show us that this narrow definition is simply silly. we have yet to survey any of the other solar systems around any other star other than our own in any detail. who says that the universe has to obey the rules of our system. there could be other systems where planets rotate at different inclinations to each other. this could be the norm rather than the exception. planets could orbit planets, i.e. who said you could not have an earth size planet with a moon the size of mars. it would probably not last that long unless the mars object was very light but it could happen. all models of the origin of the solar system are just that models. no one can prove it happened. no one has a time machine that can go back there. they only way that we can get an idea is to explore other systems particularly ones that are forming and that is not likely for a good while. a nice and neat solar system with all the planets behaving in the one way as stated above would be the exception rather than the rule giving the chaos of the event and the likelyhood of it occuring. diversity seems to be the key to nature not clones. it was not that long ago that the earth was the center of the solar system, comets were imperfect objects in the atmosphere and the planets were heavenly gods. while ekbos are far out and may not be circular to dismiss them as debris is silly as seeing we did not have any proof that they existed until recently so we know nothing about them, for most astronomers the did not exist.
stephen.
p.s.:- the earth is slightly oblate and not pefectly circular .

Hi All,
You can see why the IAU are having trouble with this.

QUESTION, would it be a good idea to split the solar system into 2 sections, one area is contained inside a sphere which is inside the kuiper belt, and the second zone would be the kuiper belt itself.

A planet would be defined as a body greated then 1000km within the area before the kuiper belt, ALL bodies in the kuiper belt would be KBOs.
After all, every objects within the asteroid belt is called an asteroid, albeit, there are asteroids all over the solarsystem.

BUT, are are we just mincing words here, what if the Kuiper belt was not called the Kuiper belt and called just another asteroid field, then the easiest thing to do would be to make EVERYTHING with a diameter greater then say 1000/2000 km a planet and all else an asteroid,
which bring us back where we started. :lol:

Dave Lillis wrote:

Hi All,
You can see why the IAU are having trouble with this.

QUESTION, would it be a good idea to split the solar system into 2 sections, one area is contained inside a sphere which is inside the kuiper belt, and the second zone would be the kuiper belt itself.

A planet would be defined as a body greated then 1000km within the area before the kuiper belt, ALL bodies in the kuiper belt would be KBOs.
After all, every objects within the asteroid belt is called an asteroid, albeit, there are asteroids all over the solarsystem.

BUT, are are we just mincing words here, what if the Kuiper belt was not called the Kuiper belt and called just another asteroid field, then the easiest thing to do would be to make EVERYTHING with a diameter greater then say 1000/2000 km a planet and all else an asteroid,
which bring us back where we started.

david,
then by your definition the moon, galilean satellites and titan are all planets as they all exceed your definition. plus the are circular go round the sun (as well as around their respective planets) inside the plane of the solar system and in the same direction as the rest of the planets as suggested by bart. this would still mean that pluto is a planet as it exceeds all definitions here and it has you to be proven conclusively that it is an ekbo. . so i am happy to announce that there are 6 more planets in the solar system the moon,galilean satellites and titan .
stephen.

[img size=320] :twisted: Just to stir things up a little, There are a number of definitions orbiting the science communitity which I personnally find to be flawed. Here are a couple of examples: 1.) From the discovers of sedna: "We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the earth. Yet the earth is more massive than all of those put together. Thus, the earth is a planet. Ceres, the largest asteroid, is not greater in mass than the sum of the masses of the remaining asteroids. Hence, not a planet." If this where the case, then if an earth sized object was discovered in the same orbit as Jupiter, it would not be classed a planet. - Nope, this can't be right! 2.) From Gibor Basri of the University of California, Berkeley: "A planet is a spherical object never capable of core fusion, which is formed in orbit around an object in which core fusion occurs at some time." This definition gives no reference at all to mass, - are we then to class all the Ceres, Pallas, Vesta and all the other asteroids as planets? This debate will probably rage on forever and a day. I think that the only way this can be settled is if the definition of a planet is split into a number of catagories, for example (and these are ONLY SUGGESTIONS, I'll leave the actual definitions to people with bigger brains than me!) A possible catagory could be (a) class 1 planets - planets (above a certain mass or size to be defined) that orbit around a star. (b) class 2 planets - (of the same sized definition) that orbit around a primary planet (basically moons, this gets over the fact that these also orbit the sun and may be larger than some primary planets). Class 3 planets - free floaters, planets that do not appear to orbit anything. I think that the core fusion element must also be considered or stars will also be thrown into the mix and really bugger thing up. or maybe we're just making it all too complicated......... [/img] :!: :!: :!: