Planet Composition

Why are the inner planets composed of rock and the outer planets composed of gas?

Because they form closer to the sun, basically.

Nebular hypothesis

I thought the gas planets were Sun Farts.

richard09 wrote:

Because they form closer to the sun, basically.

Nebular hypothesis

Yeah, the article never touches on what was, for me, the puzzling bit. Why are the near-in planets rocky, and the far-out planets gaseous? Presumably(?) they'd all have about the same raw material to build with, right? If anything, I would think the tenuous gravity out in the Lower Hebrides would lead to accretion of hard stuff -- rocks -- rather than gasses which have so little weight that collecting them must take a lot longer and be a lot more hit-and-miss.

I found this. Seems reasonable.


The planets in our Solar System formed from the solar nebula – the disc of gas left over from the formation of our Sun. Over time, this material began to collide and stick together, forming larger clumps that could collide with other larger clumps and gradually gather more and more matter. All of the planets in our Solar System began to form this way, but close to the Sun the temperature was too high for volatiles (gases like water and methane) to condense, so only the materials with a higher melting point (and higher density) were able to form at this point. The gas giants on the other hand, formed far enough away from the Sun that the temperature was cool enough for these volatile gases to condense, and form these huge, less dense planets.

Ah, that makes sense. Thank you.

And Pluto was a captured asteroid ISTR.