QUESTION: 
How can you be sure that the so-called gas-giants are really just giant
balls of swirling gas?  How do you know that they don't have a surface under
the clouds?

ANSWER from Reta Beebe on April 4, 1996:
Let us consider Jupiter as an example.

Jupiter has a mass that is 318 times as big as the mass of our earth.

Wouldn't you expect Jupiter's gravity to crush it together?  I would.

When we measure the volume of Jupiter it is more than a thousand times
greater than the volume of the earth.

This means that if I could scoop up a cup of average Jupiter and bring it to
earth and get a cup of average earth, and weigh both , the cup of earth
would weigh about 3 times as much as the cup of Jupiter.

What could Jupiter be made of that would make it so light even if it is huge?

How about the same thing as the sun? That would be Hydrogen and helium.
Those are light.

The chemists tell us that if Jupiter is mostly hydrogen and helium it will
not form a solid crust any where near the part that we can see.  The gases
will get more dense as we do down and finally be so packed together that
they behave strangely.

We are not sure that Uranus and Neptune do not have hidden oceans under the
clouds.  They are more dense than Saturn even though they are much less
massive.