Why are planets closer to the Sun more dense?

Just why are those gas giants so much smarter than us? Wait, that’s not what we mean…


Asked by Will Pittenger

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.

Answered by Sophie Allan from the National Space Centre

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