What makes a Planet?

[Master of Cards]

Really what makes a planet a planet?
For a planet should it have a stable orbit?
but what about the ones captured by the sun during a pass by?
Size? Gravity?

[General Zod]

Really what makes a planet a planet?
For a planet should it have a stable orbit?
but what about the ones captured by the sun during a pass by?
Size? Gravity?

Do you really even need to ask?

[Master of Cards]

What's the definion of a large object?

[General Zod]

What's the definion of a large object?

You realize this is grade school science here, right?

[Master of Cards]

What's the definion of a large object?

You realize this is grade school science here, right?

Was watching something on Pluto on TV was listening to people fighting over it. Is Neptune a Planet?

[Gil Hamilton]

Zod, stop being a dickhead. What he asked is a valid question because there are some blurred lines between what is a planet and what might be considered not so. We consider Pluto a planet, even though it is a glorified Kuiper Belt Object with far larger objects floating around that we don't consider a planet. There are several moons in the solar system that are significantly larger than Mercury and Pluto both, such as Titan or Ganymede. Further, there are tons of Kuiper Belt Objects that are equally large or larger than Pluto or Mercury... almost certainly dozens of them.

How large is "large" is a significant question.

[Darth Tanner]

The main problem that some people see with Pluto being a planet (never heard any critisim of Neptune being a planet) is that is is rather small (only 2,300km) and also has an irregular orbit, which is likely from it being a previous moon of Neptune. However its still much bigger than any asteroid or comet and stability of orbit path shouldnt decide if something is a planet really.

Try this stuff

[GrandMasterTerwynn]

Really what makes a planet a planet?
For a planet should it have a stable orbit?
but what about the ones captured by the sun during a pass by?
Size? Gravity?

Do you really even need to ask?

It's actually not as stupid a question as it might first appear. Astronomers haven't really given too much thought over what, exactly, is a planet. Some only want worlds large enough to pull themselves into spheres, and which orbit their parent stars, to be termed planets. However, by this loose definition, the Solar System may have many tens, or even hundreds of planets, since there may be an enormous population of Kuiper Belt objects large enough to do just that. There may even be a few Earth-sized bodies on elongated orbits out there. We can narrow it down by only including bodies which all orbit in the same orbital plane, which would lose us all the Kuiper Belt objects, certainly. And then we'd have to consider the size of the objects. If we arbitrarily make it so a planet must be no smaller than Mercury, then what do we do about solar systems where there's, say, one gas giant and a few bodies in planetary orbit that are smaller than Mercury and are clearly not associated with an asteroid belt.

[Master of Cards]

The main problem that some people see with Pluto being a planet (never heard any critisim of Neptune being a planet) is that is is rather small (only 2,300km) and also has an irregular orbit, which is likely from it being a previous moon of Neptune. However its still much bigger than any asteroid or comet and stability of orbit path shouldnt decide if something is a planet really.

Try this stuff

Tanner check Zod's link and read the first definion

[Darth Tanner]

Yes but that definition isnt exactly scientific is it. As you said what is meant by 'large', theres millions of things that orbit suns but they are not all called planets. As Gil Hamilton said there are things in the various debris fields and outside what is normally considered the solar system that are larger than Pluto, but are not called planets.

One big factor that prevents the planets being reclassified is that it would require text books to be rewritten, which would cost a fortune.

[Master of Cards]

Tanner I know that
Comets
Planetiods (large bodies in KB)
Asteriods
Moons

Maybe There should be a new definion Pluto being a near Planet

[SoX]

Ill point you to here: Linky

Er yeah ill refain from repeating anything from that thread above apart from this. I don't believe Pluto should be classed as a planet.

[Master of Cards]

Ill point you to here: Linky

Er yeah ill refain from repeating anything from that thread above apart from this. I don't believe Pluto should be classed as a planet.

Me either the near planet classifaction is for pluto fanwhores to stick with it's a planet

[Solauren]

That thought Ceres was a planet until they found more asteroids

Now it will be they thought Pluto was a planet until they found more KBO's

[Ariphaos]

I imagine once they find one the size of Mars it'll be a done issue.

There is at least a good upper limit to the size of planets. At 13 Jupiter masses a gas giant begins fusing deuterium, making it a brown dwarf. At 80 or so, it can fuse hydrogen, making it a red dwarf. Around 300 (I think, forget the exact number), it can fuse helium, making it a 'normal' star, and so on.

On the smaller end, is more difficult. Certainly nothing that can't pull itself into a sphere, but that still leaves a lot of planets.

Obviously must be the local dominant gravitational partner next to its star. Probably should exclude things with orbital resonance to a major body, too, but that still leaves a few.

[Winston Blake]

How will the planetary status be decided?

The above gives my personal view on how to resolve the planetary status. The official decision will come from the International Astronomical Union. We had hoped for a timely decision but we instead appear to be stuck in committee limbo. Here is the story, as best I can reconstruct it from the hints and rumors that I hear:

* A special committee of the International Astronomical Union (IAU) was charged with determining "what is a planet."
* Sometime around the end of 2005, this committee voted by a narrow margin for the "pluto and everything bigger" definition, or something close to it.
* The exectutive committee of the IAU then decided to ask the Division of Planetary Sciences (DPS) of the American Astronomical Society to make a reccomendation.
* The DPS asked their committee to look in to it.
* The DPS committee decided to form a special committee.
* Rumor has emerged that when the IAU general assembly meets in August in Prauge they willl make a decision on how to make a final decision!

So when do we expect a decision? Back in August 2005 I used to joke that the IAU was so slow they might take until 2006 before deciding. That was supposed to be a joke. Now I joke that I hope there is a decision by the time my daughter starts grade school and learns about planets in class. She is currently 9 months old.

Definition of a Planet

The IAU will publish beginning of September 2006 the definition of a "Planet".

So nobody will know what a planet is until September, assuming they don't delay the decision again.

[outcast]

I don't really understand why a body can't be considered a planet simply because it's within an asteroid belt or such. If the object is large/massive enough to become roughly spherical under it's own gravity, and orbits a star, then it's planet in my view. The fact that this increases the number of planets in our own solar system is hardly relevant.

[Ariphaos]

I don't really understand why a body can't be considered a planet simply because it's within an asteroid belt or such. If the object is large/massive enough to become roughly spherical under it's own gravity, and orbits a star, then it's planet in my view. The fact that this increases the number of planets in our own solar system is hardly relevant.

Increasing the number of planets in the Solar System doesn't quite describe the problem. We're not goign from 8 to 9 or 10 or 11 or 12, here. We're going from 8 to potentially several hundred. That is an issue because it negates the concept of a planet being one of the important celestial bodies moving around a star.

[SoX]

I don't really understand why a body can't be considered a planet simply because it's within an asteroid belt or such. If the object is large/massive enough to become roughly spherical under it's own gravity, and orbits a star, then it's planet in my view. The fact that this increases the number of planets in our own solar system is hardly relevant.

Then calling one of the asteroids a planet negates calling it an asteroid belt. You'd have a planet belt instead. Pluto is either just a big asteroid that happened to get captured around the same distance as the KB or more likely its just a big asteroid.

[Master of Cards]

I don't really understand why a body can't be considered a planet simply because it's within an asteroid belt or such. If the object is large/massive enough to become roughly spherical under it's own gravity, and orbits a star, then it's planet in my view. The fact that this increases the number of planets in our own solar system is hardly relevant.

Then calling one of the asteroids a planet negates calling it an asteroid belt. You'd have a planet belt instead. Pluto is either just a big asteroid that happened to get captured around the same distance as the KB or more likely its just a big asteroid.

Or a rogue moon from Neptune

[General Zod]

I don't really understand why a body can't be considered a planet simply because it's within an asteroid belt or such. If the object is large/massive enough to become roughly spherical under it's own gravity, and orbits a star, then it's planet in my view. The fact that this increases the number of planets in our own solar system is hardly relevant.

Asteroids rarely have a stable orbit. I'd imagine something's age and stability mean a lot more than the fact it's been orbiting a solar system for centuries. Otherwise by your logic a comet is a planet.

[Gil Hamilton]

Asteroids rarely have a stable orbit. I'd imagine something's age and stability mean a lot more than the fact it's been orbiting a solar system for centuries. Otherwise by your logic a comet is a planet.

Tons of asteroids have stable orbits. Most of the big ones in the main Asteroid belt have as stable orbits as any planet you could as for (and on average more regular than Pluto, which is pretty eccentric) and there are groups of asteroids captured by certain planets that are really predictable, like the asteroids that camp Jupiter's Lagrange points.

[General Zod]

Asteroids rarely have a stable orbit. I'd imagine something's age and stability mean a lot more than the fact it's been orbiting a solar system for centuries. Otherwise by your logic a comet is a planet.

Tons of asteroids have stable orbits. Most of the big ones in the main Asteroid belt have as stable orbits as any planet you could as for (and on average more regular than Pluto, which is pretty eccentric) and there are groups of asteroids captured by certain planets that are really predictable, like the asteroids that camp Jupiter's Lagrange points.

True, but how long have they been around, and what's the likelihood of them being broken apart by incoming asteroid impacts? Most planets don't have this problems to worry about because they've gathered enough mass to make worry from an asteroid impact minimal.

[Master of Cards]

The orbit and it's divetion from the eliptic show if the planet was formed with the other planets

[GrandMasterTerwynn]

Asteroids rarely have a stable orbit. I'd imagine something's age and stability mean a lot more than the fact it's been orbiting a solar system for centuries. Otherwise by your logic a comet is a planet.

Tons of asteroids have stable orbits. Most of the big ones in the main Asteroid belt have as stable orbits as any planet you could as for (and on average more regular than Pluto, which is pretty eccentric) and there are groups of asteroids captured by certain planets that are really predictable, like the asteroids that camp Jupiter's Lagrange points.

True, but how long have they been around, and what's the likelihood of them being broken apart by incoming asteroid impacts? Most planets don't have this problems to worry about because they've gathered enough mass to make worry from an asteroid impact minimal.

The asteroids we have nowadays have been around pretty much since the beginning of the solar system. The likelihood of them disintegrating from collisions is, likewise, extremely low. An asteroid belt is nothing like what bad sci-fi has taught you to believe. If I dropped you on the surface of a mile-wide asteroid smack-dab in the middle of the belt, with enough resources to keep you alive for a year, you'd be lucky if you saw another asteroid in your time there. You might notice a large number of 'stars' which change their position from night to night, and every so often (in timescales of months,) one of them might get much brighter than the rest. And if you were on that asteroid for a decade or so, you might be treated to a much more spectacular close pass from a neighbor . . . but on the balance your life on this hypothetical mile-wide rock would be about as exciting as you'd expect life on a mile-wide rock to be . . . better hope the supplies I left with you included a lot of porn.

Most of the debris which could've spawned significant collisions has already either been swept up by other planets and asteroids, or ejected from the solar system. On the balance, the asteroids whom you worry will impact Earth tend to be those residing in the population of Earth-crossers, since these do pass close enough to Earth that one of their number will, from time to time, be nudged onto a trajectory which results in it intercepting Earth the next time around.