system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What do people mean when they say “gas planet”? Like, when people refer to Jupiter as a gas planet, are they saying Jupiter has no solid surface like Earth does?
gpt
It's complicated. A gas giant, by modern definitions, is a planet composed almost entirely of hydrogen and helium - you could consider it a failed star, if you want. However, that doesn't mean it is entirely gaseous. As pressure increases - which it does as you move towards the centre of the planet - molecules are forced to change state. First, there's sufficient pressure that the hydrogen starts behaving like a liquid. Then, further down, there's sufficient pressure that it starts behaving like a solid. There *is* a solid surface to the gas giants, it's just buried deep, deep within them, and if you were to try and stand on it (and could somehow counter pressure on yourself) you would eventually become buoyant long before you touch any solid material, as you would reach a point where the pressure of the hydrogen around you is enough that it becomes too dense for you to continue falling. At this point, the hydrogen would actually still by a gas, but you'd be buoyant in it anyway.