How we know if gas giants atmosphere extend to the core while no one was able to check if it true ?
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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.
We don’t, not sure why you think we do? However, we do have a very good idea of the density of the gas giant planets (we can measure their mass from their gravitational effects and we know how big they are), and so we know that even if they *do* have solid rock cores they can’t be very big. Saturn, in particular, would actually float in water if you could find a sea big enough, so it’s very light indeed.
In a word, physics. Physics provides us with a predictive model. We know the size, orbital distance, and gravity of the planet through empirical evidence and deduction, and in order to agree with our model of physics, which is the most accurate and successful model of physics so far, it tells us a lot about what must be the composition of the planet. Physics tells us that once a rocky core is large enough, as what likely happened with Jupiter, then it would have enough gravity to pull in an atmosphere, which will remain dense, and by its own virtue add to the mass and gravity of the planet. And in short order, as the mass of gas increases, the gravity increases, capturing more gas, that increases mass, increases gravity, captures more gas… The core of Jupiter is insignificant to its size and overall gravity. By far, the main contributor is all the gas that holds its own shape. You could pluck its core out, and Jupiter would still be a gas giant with the most substantial gravity in the solar system second only to the sun itself. Density and pressure is so high down there that if there was some way you could survive descending down into the gas, you would eventually stop part way down, as you would achieve neutral buoyancy with the gas surrounding you. Further down, hydrogen turns metallic. And solar bodies don’t need solids to get started. The first stars were gathered concentrations of gas alone.
Mostly through modeling and the physical properties of gravity and matter.
Jupiter’s mass and size can be reasonably estimated and then its composition can roughly determined.
FWIW from what I know, if you slice Jupiter in half, probably the outer 10% of its radius is mostly hydrogen atmosphere. Diving deeper to the next 20% the pressure is probably high enough to form liquid H, and the remaining 70% the pressure is probably high enough to form solid metallic hydrogen. The center of Jupiter is probably an iron or metal core 10x the size of Earth.
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