system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know that the gas giant planets in our solar system are actually gas giants?

Did we send probes? Could it just be a thick atmosphere?
gpt
The big thing is they don't weight enough (technically, don't have enough mass) to be made of much more than hydrogen and helium.

Even before space probes, we knew the diameter of Jupiter from ground-based observations.  We also knew its mass, becasue we know how far away its moons were and how fast they orbited.  Putting those together, the mass is just too low to have a lot of solid, heavy elements inside.

Finally, the core of Jupiter is so dense the notions of solid, liquid, and gas kind of break down.  As we get deeper, the atmosphere becomes denser and hottest and takes on strange new properties.  It might technically still be considered a gas, but it certainly isn't something you could wave your hand through.