ELI5: why are gas giants referred to as such instead of terrestrial plannets with giant atmospheres?

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Gas giants premiere feature is the atmosphere. For example, if you wanted to land on Jupiter, you wouldn’t find a surface in the same as on earth. You would descend farther and farther into the atmosphere as the pressure increases more and more until your craft crumpled under the weight of the air around it, likely long before you’d find any sort of surface. If you designed a craft strong enough to withstand it, you might even find a place where it can no longer descend due to *buoyancy on the air around it* because the density of the air is so great. You might be inclined to think that the gaseous atmosphere will at some point become solid due to pressure, but it would be due to buoyancy rather than a phase change because, while the pressure rising does encourage matter to become a solid, the temperature is also rising which encourages it to go the other direction. Ultimately, at the center of Jupiter, there very well might be a solid core of solid material. But it’s far from the most important aspect of the planet. As far as I know, we literally aren’t sure what’s at the center because we can’t tell. All we know about the planet is that it’s primary, distinguishing features are that it’s very big and has a lot of gas. Thus, Gas giant.


The pressure at the lower levels of gas giant atmosphere is so large that there is little difference between atmosphere and metallic hydrogen at the core. By the time any kind of craft we can imagine descends all the way down to lower levels of atmosphere, it will cease to exist. In comparison, difference between density and fluidity of, say, Venus atmosphere and rock at the surface is very clear.


Well.. I don’t think we’ve ever really CONFIRMED that they ARE terrestrial planets under all that atmosphere. We don’t know if there’s a rocky ground in there anywhere! It might just be that the gases get thicker and thicker until they become solid, there might be some rocks, or it could be something completely different and much weirder! Space can be very weird, and there’s a lot we don’t actually know. For example, we also don’t know whether our gas giants are all the same at the middle or if they vary, or whether our gas giants are different from gas giants in other systems. We DO know that gas giant are giant, and seem to be made of gas! So that name seems to work 😉


Because they’re not terrestrial planets. There isn’t even a definite boundary where the “atmosphere” stops as you do deeper towards the center. The gas just gets slowly denser and denser until it’s a liquid and then in the core probably a solid. But the liquid and even the solid core is still hydrogen and helium, *the same gas elements as the higher atmosphere*, just being held in solid form by the huge pressure. But there’s no rocks and stuff, and no defined boundary where the “atmosphere” ends solid begins – unlike terrestrial planets that have “the ground/surface” at some point.


Why can’t people here on Earth learn to spell “planets”?