I was pondering the solar system the other day, when it occured to me that, while there are far larger planets, the earth is the largest ‘rocky’ planet in the solar system. Which got me to wondering - is there a limit to the size of solid planets? Is there any reason we couldn’t have a Jupiter-sized rock out there somewhere?
Also, what’s the lower-limit on gaseous planets? How much mass must you have in order to have a planet rather than a cloud of hydrogen?
Since we really only have a grasp of just our home solar system (we do have information on others, but it’s quite sketchy at this point) I’m not sure there’s a definitive answer to your question.
However, it should be possible to have a rocky planet larger than Earth. How much larger? Hard to say. And there’s the matter of density vs. volume - a planet on average less dense than Earth may have a larger volume but the same mass. And a smaller, denser planet may have more mass (and a higher gravity) even if its volume is smaller.
So… you probably could have a rocky planet with the same mass as Jupiter, with similar gravity and a much smaller volume… but if it had the same volume… I dunno. It would be incredibly massive, the core compressed by the tremendous gravity, which would be much higher than Jupiter’s because of the difference in mass.
As for the gas planet question… that depends partly on distance from the star and temperature. Far enough out, your gas planet’s gases are less volatile, more likely to be in the form of slush or embedded in slush, or even ice, allowing for a smaller “gas” planet of something like methane ice.
But if you’re talking hydrogen/helium predominating it would have to be fairly big - big enough to have enough gravity to hold onto the gasses because H and He don’t liquify at all easily. And because that stuff is less dense than anything else, you’re talking about a large volume of stuff to make up the necessary mass. So there probably is a lower size limit to a H/He planet, any smaller and the gasses would just escape into space.
If you believe the “moon collision” theory, then the earth was quite a bit bigger several billion years ago, until a Mars-sized body collided with it and knocked off enough mass to coalesce in to the moon.
There could be a theoretical limit to the size of a terrestrial planet, because the more mass you add, the more gravity tries to draw it into the center, causing collapse and compression.
A lower limit on gaseous planets would be the smallest amount of gas you could bring together that would hold itself together gravitationally against the motion of the individual molecules
Yes, but my understanding of the theory is that the original impactor was the size of Mars: considerably larger than the Moon. So it would seem that the Earth gained more mass from coalescing with the impactor than it lost by throwing off the material that became the Moon.
Computer simulations of impacts like this suggest that the dense nickel-iron core of the impactor (if it had one) would have merged with Earth’s core, and that the material thrown off to form the Moon would have come from the mantles of the two objects. So this theory not only explains the composition (devolatilised) and the isotope ratios of the lunar samples brought back by the Apollo program missions, it also explains why Earth is the densest of the planets: the collision added to its (relatively dense) core and subtracted from its (relatively light) mantle.
Regards,
Agback
What about looking at the OP question in reverse: Why haven’t the supposed “gas giants” turned into planets of just chunks of frozen gasses? - Jinx 
The really big gas giants like Jupiter and Saturn are largely hydrogen and helium. Those gases almost never liquify, much less freeze (in fact, helium requires absolute zero to freeze). Hydrogen will liquify and even solidify - under enormous pressure. So the deeper interior of Jupiter may, in fact, have liquid or even solid hydrogen. But a whole lot of the planet will be unfrozen atmosphere.
Jupiter even generates it’s own heat, making it even more unlikely it would solidify.
Now, methane worlds can freeze. But things like compressional heating and tidal forces can interfere with that, too.
[GROSS SIMPLIFICATION]There are two main infuences on the solidity of a planet-
1/distance from the central star
2/total mass
all the inner planets in our solar system are rocky, but if they had formed further away from the sun more water and gases would have remained on their surfaces and they would have been icy planets- the amount of ice increases in the outer system because it is cold.
Also the now icy planets would retain thicker atmospheres, becoming similar to small versions of Neptune or large versions of Titan.
Move an icy world with a thick atmosphere inwards toward the sun, and the hydrogen in the atmosphere would evaporate, the ice would melt, dissociate and sublime, so eventually you would be left with a large water-rock world like the Earth, or a smaller dry world like Mars.
All the planets really can be placed on a two-dimensional grid, with mass as one axis and distance from the central star as another axis, and pretty good estimates made of how much water and other volatiles are retained,
and how thick the atmosphere is, etcetera-
it is all to do with heat and gravity, basically, although the history of the planet is very important too.
What has not been seen in detail is a large planet close to the primary- they exist in large numbers, for instance at Upsilon Andromedae (images here)
Such a world, if very large, would retain a hot atmosphere, and possibly even be heated to fusion point - a little smaller and the atmosphere might be a lot smaller and even absent.
It would be possible to imagine a very hot, rocky world near to the central star- but it could not be too big, or there would be some sort of atmosphere, possibly far hotter than Venus’s.
Large rocky worlds are dense, and so have greater gravity, and so are more able to retain atmospheres.[/GROSS SIMPLIFICATION]
What I am trying to work out ATM is how big an Earth type planet can be- if the planet is slightly less dense, and slightly further away from the central star, it might be possible to have an Earth-type planet twice as big as our own- or more…if you don’t mind slightly high gravity…