What do you think Constitutes a Planet?

[ummmm........]

What's a KBO?

Note: not a philosophical question, I just meant what do the letters stand for?

 

[Cheezy the Wiz]

Kuiper Belt Object. It is like another Asteroid Belt, but outside of the orbit of Pluto. It is where the oldest parts of the Solar System are, the original molecules from our creation. It is where the comets originate from.

EDIT: Sorry, the Ort Cloud is where comets come from

I'd post wikis but for fear of breaking my own rule i dont

 

[Tycoon101]

Anything that is equal in size to Mars and above, with Pluto as the exception.

I really don't care about guidlines though. So I'll take anything.

 

[Perfection]

I don't mean a "significant level" of rounding, as you put it, but an imperrfect sphere, in the Earth is. From a distance it is a sphere, but it's surface is imperfect ( mountains, valleys, etc). A planet must have sufficient gravity to force itself into one of these faux-spheres. I'm afraid Ceres must remain a pretty rock in the sky. I've never heard of the other three.

You're incorrect, Ceres certainly has enough gravity to produce the near spherical rounding like those seen on the other planets. The fact is if we went by that definition, the amount of planets in the solar system would more than double.

If I were to come up with a definition here's the criteria I'd use:
1. Substeller: Incapable of fusion
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)
3. Stellar orbit: Orbits a star(s), brown dwarf(ves), or stellar remnant(s) directly (may allow exception for ejected bodies).

This is a general framework, I can think of a few cases where nitty gritty clarification is needed, but this gives one a good general idea.

 

[Cheezy the Wiz]

You're incorrect, Ceres certainly has enough gravity to produce the near spherical rounding like those seen on the other planets. The fact is if we went by that definition, the amount of planets in the solar system would more than double.

If I were to come up with a definition here's the criteria I'd use:
1. Substeller: Incapable of fusion
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)
3. Stellar orbit: Orbits a star(s), brown dwarf(ves), or stellar remnant(s) directly (may allow exception for ejected bodies).

This is a general framework, I can think of a few cases where nitty gritty clarification is needed, but this gives one a good general idea.

Then why hasnt Ceres done it yet?

I agree with most of this criteria, except for the position ones (clears away belts)

 

[Perfection]

Then why hasnt Ceres done it yet?

Umm, Ceres is a spheroid.
http://en.wikipedia.org/wiki/1_Ceres

I agree with most of this criteria, except for the position ones (clears away belts)

Why do you disagree?

Do you honestly think that we should include the large and ever increasing list of large KBOs as planets?

 

[Cheezy the Wiz]

Umm, Ceres is a spheroid.
http://en.wikipedia.org/wiki/1_Ceres

Why do you disagree?

Do you honestly think that we should include the large and ever increasing list of large KBOs as planets?

I stand proven wrong. Ceres is a planet by our defintion.

I change my mind again. I agree. I was about three-quarters of the way through a very long post arguing against it, but then I realized I was arguing your case. Planets ought to be able to clear a path of all but the largest objects.

And yes, I think the ever-increasing list of KBOs ought to be considered as planets IF they meet our standards. Minor planets, but planets nonetheless.

 

[Perfection]

I stand proven wrong. Ceres is a planet by our defintion.

By your definition, in my definition it doesn't have the gravitational oompf to singificantly impact the asteroid belt so it would not be considered a planet.

I change my mind again. I agree. I was about three-quarters of the way through a very long post arguing against it, but then I realized I was arguing your case. Planets ought to be able to clear a path of all but the largest objects.

And since two (or more) large objects cannot share the same orbit for more then a brief period, belt objects are excluded.

And yes, I think the ever-increasing list of KBOs ought to be considered as planets IF they meet our standards. Minor planets, but planets nonetheless.

They don't meet my stated standards as they don't have a significant impact on thier orbital locus (which is why they can be in a belt).

As for the term "minor planet", what do you mean by that?

 

[Cheezy the Wiz]

By minor planet, I mean as in not one of the major planets (the eight we know now), but its still more than an asteroid. We can call them planetoids,or little balls of Perfection ( only if they're pentagonal though ) or whatever, I just think they deserve to be known as something other than "a really big round asteroid."

 

[Cuivienen]

I don't mean a "significant level" of rounding, as you put it, but an imperrfect sphere, in the Earth is. From a distance it is a sphere, but it's surface is imperfect ( mountains, valleys, etc). A planet must have sufficient gravity to force itself into one of these faux-spheres. I'm afraid Ceres must remain a pretty rock in the sky. I've never heard of the other three.

Perfection already pointed out that Ceres is very spherical (the only asteroid that is large enough to be).

http://en.wikipedia.org/wiki/50000_Quaoar
http://en.wikipedia.org/wiki/90377_Sedna
http://en.wikipedia.org/wiki/2003_UB313 (Xena, not yet formally named)

I also apparently missed Orcus:

http://en.wikipedia.org/wiki/90482_Orcus

There is also "Easterbunny" that has yet to be formally named:

http://en.wikipedia.org/wiki/2005_FY9

There are numerous other known large KBOs, but many of those are likely non-spheroids.

 

[sanabas]

Anything that is equal in size to Mars and above, with Pluto as the exception.

So what do we call mercury?

 

[Perfection]

Anything that is equal in size to Mars and above, with Pluto as the exception.

In a addition to Sanabas criticism, I should note that an arbitrary size limit rather than a behavior based system strikes me as somewhat of a copout.

By minor planet, I mean as in not one of the major planets (the eight we know now), but its still more than an asteroid. We can call them planetoids,or little balls of Perfection ( only if they're pentagonal though ) or whatever, I just think they deserve to be known as something other than "a really big round asteroid."

I'll agree there, there needs to be better terminology for smaller bodies

 

[Eran of Arcadia]

Anything that is equal in size to Mars and above, with Pluto as the exception.

Yeah, Mercury is between Pluto and Mars in size; there is no way Pluto should be a planet and not Mercury.

My definition: a planet should be
a) round
b) orbiting a star directly, no other object (so no moons)
c) not part of or sharing an orbit with a larger field of objects. By this I mean that since there is a belt of asteroids, and a belt of KBOs (A Kuiper Belt, if you will) that nothing in there should be called a (major) planet. We can call everyhting in these fields (eg Pluto, Ceres) minor planets, round or not.

 

[MamboJoel]

It's historical. Our ancestors exploring the night sky noticed that all stars evolved all together night after night, year after year except some special ones that did not follow the general move.
That's what distinguishes planets from stars at the very begining.

 

[Eran of Arcadia]

It's historical. Our ancestors exploring the night sky noticed that all stars evolved all together night after night, year after year except some special ones that did not follow the general move.
That's what distinguishes planets from stars at the very begining.

Except that we consider Uranus and Neptune, which were found with telescopes, to be planets, as well as all the exosolar planets; but we no longer call the sun and the moon planets like the Greeks did.

 

[Bozo Erectus]

Ive heard that Uranus can be seen from Earth orbit.

 

[MamboJoel]

Yeah, Mercury is between Pluto and Mars in size; there is no way Pluto should be a planet and not Mercury.

My definition: a planet should be
a) round
b) orbiting a star directly, no other object (so no moons)
c) not part of or sharing an orbit with a larger field of objects. By this I mean that since there is a belt of asteroids, and a belt of KBOs (A Kuiper Belt, if you will) that nothing in there should be called a (major) planet. We can call everyhting in these fields (eg Pluto, Ceres) minor planets, round or not.

So you think it's a lone sphere orbiting a star.
Why not. Anyway the definition was much more obvious when we only knew the planets we can see without any instruments. Further knowledge has stumbled the definition itself.
In fact, that's what I like about Astronomy : Discovery ofen makes our previous definition of the world null and void.
Is it really a matter : defining what a planet is ?

 

[nc-1701]

An object that orbits a star, is round, has a regular orbit that varies in distance no more than say 25%, is atleast 2000km in diameter, and it should probably have an atmosphere.

that leaves 7 planets.

 

[Kan' Sharuminar]

Ive heard that Uranus can be seen from Earth orbit.

According to Wikipedia, it was spotted a few times with the naked eye, but it wasn't until someone caught it with a telescope that it was officially 'discovered.'

So yes, you can see it from Earth orbit.

 

[Kal'thzar]

How about linking it to a certain amount of atmospheric density?

EDIT: that is the definition of a planet.