# 1 Protoplanetary discs

The idea that planets form from initially microscopic solid material within protoplanetary discs made predominantly of gas dates back to the Enlightenment, possibly starting with the idea that the planets of the Solar System formed out of a nebula surrounding the Sun, which featured in Kant’s Universal Natural History and Theory of the Heavens. Today, our understanding of protoplanetary discs stems from experimental observations and theoretical models of the behaviour of gases in the gravitational fields of stars.

OpenLearn - The formation of exoplanets
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