commas: an aside

(1) Our team believes there is another hidden planet in our solar system.
(2) Our team believes there is another, hidden planet in our solar system.

What a difference a comma makes, eh?

In sentence (1), we're saying we already know of one hidden planet, and now we know there's another one! By contrast, in sentence (2), we're saying we've found another planet, probably via deduction, but it's a hidden planet, i.e., we haven't seen it directly.

This is the difference between cumulative and coordinate adjectives: case (1) is cumulative; case (2) is coordinate. And this is also why, when you see pairs of adjectives, the decision to insert a comma or not may depend on what nuance you're trying to convey. In other words: it can be a judgment call.

I deal with cumulative and coordinate adjectives here.