It's likely that some of the other low mass extrasolar planets that have been found are of terrestrial composition. However, without observing the planet transiting (passing in front of it's star), we can't determine the mass exactly, only a lower limit on the mass. So for Gliese 581c, we know that the mass is greater than five times that of the earth, but it could be larger. If it's a factor of three larger (similar to OGLE-2005-BLG-169Lb), it would most likely be similar to Neptune.
I knew I left that obscurely badly written when I clicked "submit." I know lots of pees are saying that "didn't life start in water?"... and, it did... but the procreation had to have some land... until the mammals came (whales,l dolphins, etc.).
JayZ750, there are people that study moving planets, even for our own Earth (which will eventually get scorched by the sun if we don't find a way to move further out.) There are some very practical ways, but they take a long, long time. (bit of gravitational tug in each of hundreds of thousands of orbits, etc.)
How about Gliese-581-d? It's not only terrestrial (it's classified as a Super-Earth), it's also within the habitable zone and could have liquid water.
For this planet they also have only the minimum mass. So it is most likely a super-Earth, but that hasn't been confirmed with a precise mass measurement and a radius measurement.
I guess a simple answer to this question is that you can't have the liquid without the solid. Even if the planet were pure H2O, it would be solid ices throughout most of the planet with a possible liquid water layer on top.
Says who? We hypothesize that all gas Giants have solid cores but we have no real proof Strong gravity could hold Gass and liquid together without a necessary solid Rocket River
It's really much more than speculation. Because the planet is stable and doesn't either contract or blow itself apart on short timescales, we know that it is in hydrostatic equilibrium, which means that the pressure and the gravity are balancing. We can also do laboratory experiments of different materials under high pressure. Combining these pieces of information with the bulk density of the planet, we can determine roughly what the composition is all the way through. While you are right that not all gas giants necessarily have solid cores, my statement about water planets is still correct. If a planet is five times the mass of the earth, then the material that is close to the center has almost five earth masses worth of material pushing down on it. We can calculate the pressure necessary to counteract this gravity, and again, we know how the material behaves under high pressure because we do laboratory experiments.
