Why are the planets beyond the asteroid belt made of gas, without a surface like the inner planets?

– Xochielt Sanchez

Answer by Robert Frost:

It is thought that the outer planets do indeed have rocky metallic cores like the inner planets.  They just have a massive amount of liquid and gas surrounding those cores, so we can't see them.

About 5 AU from the Sun is a boundary called the "frost line".  Beyond this point, the Sun's energy is weak enough that ices can form from substances like ammonia and methane.  It is thought that these ices provided additional mass to the outer planet cores so that they had a deeper gravity well, preventing their captured hydrogen and helium from escaping.  It is also though that solar winds from the young, energetic Sun blasted massive amounts of hydrogen and helium into the outer solar system, providing more gas for the outer planets to sweep up.

There is a common misconception that gas giants don't have a surface.  They do, but the transition from surface to not-surface is not as distinct as it is on The rocky planets.  On Earth, for example, the ground is much, much, much denser than the air above it.

Gas lower in an atmosphere is more dense than gas higher in an atmosphere because it has the weight of the gas above it squeezing it down.  So, very deep atmospheres can be very dense near the bottom.

At Earth, if we were to start in space and lower ourselves down through the atmosphere, we would measure a gradual increase in density as we descended.  The air would get thicker, and thicker and then – suddenly – there would be ground.

But on a planet like Jupiter, the experience would be different because the atmosphere is so much larger.  We would start descending and the air would get thicker, and thicker, and thicker, and keep getting thicker.  It would become so dense it would feel like liquid, and then it would get even more dense.  By the time we reached the rocky metallic core, the difference in density between the rock and the atmosphere wouldn't be noticeable.

Now, one caveat – in a few places above, I said "it is thought".  Those are pretty well established theories, but recent observations of exoplanets (planets orbiting far away stars) have revealed planets like Jupiter much closer to their star.  Astronomers are currently working on models to explain that formation and what it means to the models we use to describe our solar system.

Why are the planets beyond the asteroid belt made of gas, without a surface like the inner planets?

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