planet formation

i have a few questions on planetry formation. i understand how rocky planets formed, with rocks and asteroids and comets colliding into eachother until they had enough mass to and gravitational force to form them into a sphere. the gas giants, i believe were rocky planets that got so big they attracted the sorrounding gasses and became huge. whats the biggest rocky planet there can be without it becomming a gas giant? also , whats the minium mass ( or density, im not sure what term to use) an object has to be before it will become a sphere under its own gravity?

i have a few questions on planetry formation. i understand how rocky planets formed, with rocks and asteroids and comets colliding into eachother until they had enough mass to and gravitational force to form them into a sphere. the gas giants, i believe were rocky planets that got so big they attracted the sorrounding gasses and became huge. whats the biggest rocky planet there can be without it becomming a gas giant? also , whats the minium mass ( or density, im not sure what term to use) an object has to be before it will become a sphere under its own gravity?

I'm not much on planetary formation bit I thought the outter planets were gas because that's where the Gas was in the proto-planetary disk and that the rocky ones are on the inside because that's where the heavy stuff ended up?

Anyhow, I can be more help to you on your second question.

The answer is that it totally depends on what it's made of. If it were made of water then exceptionally little, even on the space shuttle a tiny blobs or water form into spheres. What has to happen is that the internal friction of what ever the body is made from has to be less than the force of gravity.

Bart.

I'm not much on planetary formation bit I thought the outter planets were gas because that's where the Gas was in the proto-planetary disk and that the rocky ones are on the inside because that's where the heavy stuff ended up?

The whole topic of gas giants has had to be revisited in recent years due to the jupiter sized planets found close in to other stars. The leading idea at the moment is that the gas giants formed as they did in our system, but were gravitationally dragged inwards by dust/gas/matter on the disc. When they reach a point where there is no matter left for them to attract then they reach a stable orbit. In effect they will stop moving inwards when the solar wind has cleared out any matter out to their orbit.

There's a big debate on this, but one of the points is that it has to do with a combination of the volatility of the elements in the protoplanetary disc along with gravitational and heating effects.

i.e. one of the reasons for having more terrestrial type planets closer to the star is that only the heavier elements will condense at that distance (at least in our solar system's case).

Towards the middle of the disk (in our solar system's case) it was cool enough for water, methane, and other of the lighter, more volatile elements and these would have been attracted to closeby planetary bodies in a runaway effect, which allowed the planets to expand to incredible sizes.

Further out there was simply not enough material to form large planets so we get things like Pluto. Oops, almost forgot that Pluto is not a planet (hope I don't get flamed)

The leading idea at the moment is that the gas giants formed as they did in our system, but were gravitationally dragged inwards by dust/gas/matter on the disc.

Didn't know about that theory. Had always wondered about those giant planets they found close to stars.

Take this with a pinch of salt as I'm just learning here as well!

Trev

If you're interested, theres some papers:

Lecture notes on the formation and early evolution of planetary system is a good summary
of the current theories on planet formation.

Answering fguihen's original questions, we don't know of a maximum size
for a rocky terrestrial planet. There is a hypothesised isolation size which a rocky planet grows to as it sweeps out its orbit, clearing
it; over this its believed to grow by collisions with other planets. However,
we have no good estimates as to how frequent these might be. So there is no upper limit yet.

As to mimimum size, yes, it depends on planet(ismals) formation history
and composition: an icy object can reach self-gravity to melt the ice fairly
easily, a collision-warming event can do it: the icy satellite Mimas (395km) looks round, for example. In terms of melting rock,
I dimly remember lecture notes showing Mars is about the size of the
lower limit for a 'spherical' planet; any mountain over 10km or so on
Venus will melt the mantle under it. (Olympus Mons is hence the
largest possible mountain size).

As to the whole "planets are spherical" argument, see
Soter ; current theories imply a runaway-accretion during planet formation; its thought that any planet large enough to 'clear its orbit' will automatically
be large enough to self-gravitationally reform as an approximate sphere.

Regards
Alastair

[/quote]As to the whole "planets are spherical" argument, see
Soter; current theories imply a runaway-accretion during planet formation; its thought that any planet large enough to 'clear its orbit' will automatically
be large enough to self-gravitationally reform as an approximate sphere.

This is a daft definition of a planet. A drop of water in zero G will form into a sphere, as will a gas cloud (given enough time and lack of disruption). We don't name a drop of water as a planet, why should we therefore define a planet by what it's made of (atomic structure)?, because this is what will ultimately define whether or not it ends up spherical. Planets could easily have been defined by a mass limit. Under or over the limit your either a planet or not. Forget about shape, orbit, atomic structure, ability to clear the orbit of debris etc.
A simple definition of a planet could have been simply a mass limit. Therefore massive objects that have not yet reached the "planetary mass" but were clearly accumulating mass could be accomodated and upgraded to planetary status.

IMHO. The definition of a planet therefore should have been a volume of mass X contained within a region of spacetime Y. Simple, easy to understand and scientifically sound!
Please understand that I see the difficulties with the defintion of Y here. But there are better brains than mine around to sort out these trivias.
It should be apparent to anyone that if a certain amount of matter occupies a certain amount of spactime then we can define that as a planet. The term Y therefore would have to be fairly generous to include the largest gas giants at the lowest mass densities, but to differentiate larger "gas" structures such as nebulae, etc.
This definition would also allow for planetary formations. If a quantity of mass occupies a limited amount of spacetime then a planet will be bound to be the end result (or star etc.).
The only thing left to do is define the limits, mass and spacetime volume.

Who votes for this definition of a Planet, it's simple, mass X within minimum volume Y : Clearing orbits, wiping bottoms etc are just smokescreens.

The real debate is mass X, which puts things in perspective.
Mass and spacetime have to be the only consideration when defining what constitutes a planet. Have we learned nothing? The IAU have disgraced themselves, politics and public assuagance have no place in pure science.