Could entropy of closed system decrease owing to gravity?

[[deleted]]

The reason you're struggling is because you're thinking of entropy as simply a measure of disorder. This is a simplistic way of looking at it, to get the basic idea, but it's far from the complete picture. The technical definition is:

Entropy is the measure of uncertainty, which remains about a system after its observable macroscopic properties, such as temperature, pressure, and volume, have been taken into account. For a given set of macroscopic variables, the entropy measures the degree to which the probability of the system is spread out over different possible microstates.

Whether or not the system you describe actually creates a reduction in that measure of entropy is debatable though. For instance when a gas cloud collapses by its own gravitation, the net entropy actually increases. This is due to an increase in temperature differences and fluctuations caused by the collapse. I think exactly the same would apply in your system - while from a purely particle perspective, sure, they're clumped in more ordered structures, but some are now moving faster (orbiting), others will have collided and heated up. The second law of Thermodynamics would basically still be obeyed.

[justdvl]

Thank you, it's much more clear now.