AI reinforcing planets?

Well I wonder who/why this work like it does? I think understanding how the AI works - and making that clear is important to keep everyone under the same umbrella.

When will an AI reinforce his planet? I think this has to do with them react “on move” but how smart art they?

So the AI has 2 very close together planets containing 60 + 30 ships…

Now I wish to attack them and capture them. If I send (say) 80 to the first planet, enough to take down all 60 but not enough to take down everything, will he reinforce? I saw that happening?

But what will happen if I send 80 to the first planet, and 1 ship to the smaller planet? Will he still reinforce?
Or even more pressing: I saw the AI reinforcing always with half the remaining army, so what happens if I send 1 ship to the bigger planet? Will he reinforce that one?