thermodynamics - Entropy and gravitational attraction

Any process which is spontaneous and irreversible must involve a (positive) change in entropy of the universe

This is one condition to the spontaneity of a process that the overall universe (System + Surroundings) must experience a positive entropy change.

Going back some 4.5 billion years:

• The sun is just a hot ball of gas with huge amount of debris floating around randomly in any direction


• Over a period of time a fundamental force called gravity pulls all this debris into a fiery ball of molten rock


• This ball slowly cools down and its layers start to get ordered by their own weight

Now considering all the matter on this tiny planet was once a chaotic jumble spread over many many million kilometers why would all of this rocky mess ever come together spontaneously via gravity. The earlier mess surely represents a highly chaotic state and a planet is mostly well arranged layers of solid and molten rock by their weight which by comparison seems highly ordered. So apparently a spontaneous decrease in entropy has occurred which is quite counter-intuitive to my mind. Could someone help me understand the relation between gravitational attraction, formation of planets and entropy

Answer

The collapse of an interstellar gas cloud to eventually form a star, the formation of a planet, and the gravitational differentiation of a planet are all examples of exothermic (heat-producing) processes. As is the case with other exothermic processes, these processes increase entropy.

The cooling of a newly-formed planet has the appearance of an endothermic process. The decrease in entropy as a newly-formed planet cools is more than balanced by an increase in the entropy of the rest of the universe. That cooling is accomplished by radiating energy into space. That radiated energy represents a key concept in a newer understanding of entropy: In many cases, it's better to look at entropy as energy dispersal rather than some fuzzy concept of order versus disorder. The cooling of a newly formed planet is a perfect example of that.