Question: why is Jupiter round but a gas planet shouldn't the gas escape into the universe

Alexander Allen answered on 11 Mar 2019:

Lovely question!

Let’s imagine that we can explain both gases and liquids by calling them fluids. Jupiter is a huge collection of gas that is attracted to the centre of the planet. The gas is all attracted so the outside layers of gas push down on the inside layers. This puts more and more pressure on the gas, the deeper we go.

As you keep compressing the gas it acts more and more like a liquid that gets denser and hotter. At some point it would get so dense that people could float at this level. (Unfortunately the pressures would be so great that you would likely be crushed as well.) Overall this makes Jupiter into a large very tangible object. It has a big radius and a big mass.

As you head out though the gas does get very thin, to the point where you can literally imagine molecules of the gas bouncing up and down, being attracted by the planet but then bouncing off other molecules… This is the same as how Earths atmosphere works.

In short, you should imagine that Jupiter has an atmosphere like the earth that is pretty much held in place by gravity. Unlike the Earth, however, Jupiter doesn’t have a definitive ground boundary. It’s just a sea of gas that gets thicker and thicker all the way to the core… This atmosphere also means that lots of huge storms are common on the surface, like the Great Red Spot.