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Damburger
2009-Aug-11, 02:33 PM
If Venus had formed much further out in the solar system, and not suffered such a high rate of loss of light gasses such as hydrogen, would it have become a gas giant? Conversely, if a planet such as Neptune or Uranus formed in the inner solar system, would they have lost their atmospheres and become terrestrial planets? How much atmosphere does a planet need to have before it becomes considered a gas planet?

I ask because I am curious about the distinction between terrestrial and gas planets. Uranus is 14 times larger than Earth, and there have been a number of exoplanets discovered whose mass lies in that gap. There seems to be a presumption that they will be very big rocky planets, rather than small gas planets. Is there any reason for this?

korjik
2009-Aug-11, 04:20 PM
Gas giants form where the gas is cold enough to allow accretion into the planet. Basically the gas giants form far enough out that the ices (water, methane, and ammonia) are still ice, allowing more mass to accumulate into a planet, then when it gets big enough, the planet starts pulling in hydrogen and helium.

So, for the solar system, anything closer than Jupiter would be too warm for the ices.

When you get gas planets close to a star, it should have migrated after forming.

As for the distinction between gas giant and rocky, I think that the line between them really isnt known. I think that it could be pretty hard to tell also. As you get more than several Earth masses, the amount of gas and ice accreted could leave you in a situation where you cant really tell where there is a solid surface. Plus, we really dont know the conditions that you would find on a planet between the mass of the Earth and Uranus.

Damburger
2009-Aug-11, 04:53 PM
If the dividing line were 'no clear solid' surface, you could argue Earth is a gas giant, because as the pressure of the atmosphere increases liquid water forms and covers much of the surface.

SolusLupus
2009-Aug-11, 05:50 PM
Yes, but can you qualify the "giant" part of "gas giant"? The Earth is pretty darn tiny in comparison with gas giants.

aurora
2009-Aug-11, 07:05 PM
As we're able to learn more about other solar systems, we may be able to answer some of these questions.

All we really have now is our solar system, whichi has inner rocky planets and outer gas giants, and it's pretty clear which are which.

I'll bet that we'll find systems where there are very different combinations of planet types.

SolusLupus
2009-Aug-11, 07:07 PM
From what I've read before, we have seen some solar systems that suggest that there are gas giants closer to the sun than in our system (given the wobble of the sun as it reacts to the greater mass)

aurora
2009-Aug-11, 07:13 PM
From what I've read before, we have seen some solar systems that suggest that there are gas giants closer to the sun than in our system (given the wobble of the sun as it reacts to the greater mass)

That's true. Or at least, we know the mass of those planets. We don't know their composition, yet, nor do we know about small planets.

korjik
2009-Aug-11, 07:23 PM
If the dividing line were 'no clear solid' surface, you could argue Earth is a gas giant, because as the pressure of the atmosphere increases liquid water forms and covers much of the surface.

At higher pressures, the lines between gas, liquid and solid arent as clear. On the Earth, the differences are quite clear.

WayneFrancis
2009-Aug-12, 04:35 AM
From what I've read before, we have seen some solar systems that suggest that there are gas giants closer to the sun than in our system (given the wobble of the sun as it reacts to the greater mass)

Yes but it is thought that these hot Jupiters formed far out and migrated in. I imagine over time they'll shed much of their atmosphere. Kind of like us looking at Saturn now. Rings are pretty but will they stick around for very long, in astronomical times scales.

cjl
2009-Aug-13, 09:51 PM
If the dividing line were 'no clear solid' surface, you could argue Earth is a gas giant, because as the pressure of the atmosphere increases liquid water forms and covers much of the surface.
The ocean floor is a clear solid surface though, and the oceans are not deep enough to cause the water to be a significant portion of the planet's volume or radius. I would say the earth is fairly unambiguously not a gas/fluid planet.