system
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human
Why is the inner solar system rocky (inner planets), the outer solar system gas (outer planets), and the far outer solar system rocky (Oort cloud, transNeptunian objects)?

I get why the inner planets are rocky and the outer ones gas, but why do we "go back to rocky" when you go even further out from the outer planets?
gpt
Fundamentally this comes down to the "frost line": how far out from the Sun do we need to be before water freezes?

This also depends on the reflectivity of the object in question (the more reflective it is, the less sunlight it absorbs, the colder it is), but generally speaking, it's somewhere in the outer portion of the Main Asteroid Belt separating Mars and Jupiter.

This turns out to be crucial for which planet forms where. A forming proto-planet needs to weigh in around 5 - 10 Earth-masses before it has sufficient gravity to hold onto hydrogen gas. In other words, a proto-planet needs to build up a pretty substantial mass of just solids before it can grow even faster using gas. That is far more likely to happen if you can build your planetary core out of both rock and ice beyond the frost line rather than just rock inside the frost line.

Also note that subsequent migration can then alter planetary positions later. We see a lot of exoplanet systems where Jupiter-sized planets spiraled inwards after formation to end up with an orbit closer than Mercury is to the Sun; we *think* our own Solar System was spared from this fate only thanks to a resonance between Jupiter and Saturn.

I would challenge your assertion that...

> why do we "go back to rocky" when you go even further out from the outer planets?

When you get beyond Neptune, the bodies out there are somewhat rocky, but primarily they're *icy*. For example, the density of rock is usually somewhere around 3 g/cm^(3), while the density of ice is about 1 g/cm^(3). The density of Pluto, meanwhile, is 1.88 g/cm^(3), suggesting its got a *lot* of ice. To some extent, you can think of Pluto as a giant proto-planet core that never really gathered enough mass to get started.

This also makes sense because there's more than just one kind of frost line. While we usually means the *water* frost line just inside Jupiter's orbit, there's also the CO2 line farther out (where carbon dioxide freezes into dry ice), the methane line beyond that,  and the nitrogen ice line even farther out than that. So far as we can tell, the objects beyond Neptune have lots of nitrogen ice, because that's just about the only place it can form.