What makes a Planet?

[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.

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.

Operative word being "potentially", as far as i know the current number of planets would then be around 15. While it is theoretically possible that more such objects will be found that fit the definition, i highly doubt it would be hundreds.

[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.

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.

By that logic you can disregard any planet in the system as being a planet, since there's asteroid all over the place. You would seriously consider earth a non-planet if it happened to be in the asteroid belt? That's patently absurd.

[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.

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.

Ofcourse, a stable orbit and general age would be factors. However, i am unaware of any comets massive enough to have become roughly spherical.

[CaptainChewbacca]

Earth couldn't exist in an asteroid belt, as over time it would've absorbed the majority of the bodies in there and accreted them onto its mass. For my own purposes in planetary science, I tend to use the following criteria:

1. Of sufficient size and mass to collapse into a sphere.
2. Does not orbit a parent object other than the sun.
3. Has an atmosphere or the ability to have an atmosphere.
4. Is internally differentiated.

Things like moons and magnetic fields aren't good qualifiers because there's exceptions in the inner system. I use #3 because Mercury COULD have an atmosphere if it weren't so close to the sun that solar winds blasted it away.

Under my criteria, I don't know if Pluto is a planet because I don't know if it is internally differentiated.

I know this is just my own (somewhat) educated opinion, but it helps with most of these sorts of things.

[Gil Hamilton]

Pluto wouldn't be a planet under that criteria because it has an "atmosphere" in the same way a comet has an atmosphere... it just happened to get close enough to the sun that some of the frozen gases on the surface has boiled off but hasn't quite been able to escape.

[wolveraptor]

Venus and Mercury don't fulfill criteria 4 and 3 respectively.

[King Kong]

Venus and Mercury don't fulfill criteria 4 and 3 respectively.

What makes you think that Venus is not internally differentiated? The average densities of Venus and Earth are very close, suggesting that they have similar internal structures.

The absence of a magnetic field does not necessarily mean that Venus has no large iron core. The core could be entirely solid, rather than having a liquid layer like Earth's core. Or the slow rotation of Venus could be insufficient to drive convection currents in its liquid core layer that produce a magnetic field on Earth.

As for the Mercury point, the planet is large enough to retain an atmosphere, just not for nearly as long as the Earth. Especially being that close to the sun where atoms in the atmosphere would be more likely to be heated to temperatures that would allow them to reach Mercury's escape velocity.

[GrandMasterTerwynn]

Venus and Mercury don't fulfill criteria 4 and 3 respectively.

Uhh . . . no. Venus is internally differentiated, possessing a core, mantle, and crust. And if you swapped Mercury and Mars, Mercury would likely be able to hold on to an atmosphere of roughly the same thickness as the Martian atmosphere. This is because while Mercury has half of Mars' mass; owing to its high density, it has slightly stronger surface gravity, and its escape velocity is around 88% that of Mars. (If anything, Mercury would be able to hang on to a much thicker atmosphere if it were swapped with Mars, since Mercury has a global magnetic field which is strong enough to generate an actual magnetosphere, which would serve to deflect the solar wind which would otherwise carry the atmosphere off into space, like what has happened on Mars.)

[CaptainChewbacca]

Venus and Mercury don't fulfill criteria 4 and 3 respectively.

Venus is internally differentiated with crust-mantle-core, it just doesn't have a differentiated core. In the very post where I outlined my criteria, I explained why mercury DOES qualify for #3.

[CaptainChewbacca]

P.S. Mars might not have an atmosphere because it has no mechanism to replenish its atmosphere without volcanism or other active gas cycles. The lack of a magnetic field is only part of it.