Eli5 how we know that’s gas giants like Jupiter have a very small rock mass while the rest of it is just gas?

Question

How we know if gas giants atmosphere extend to the core while no one was able to check if it true ?

In: Other

Answer 1

We know comets occasionally hit them (Shoemaker Levy struck Jupiter in 1994). Any rocky materially in that comet would have descended to the core. Over millions of years this wouldn’t have been the first.

Additionally, our best theory of their formation also involves a rocky core to be the original gravity well to collect the atmosphere around.