What is the difference between '___ Planet' and 'Planet ___'?

I am confused in the difference in meaning of these two combinations: '__ Planet' and 'Planet __'

For example, Pizza Planet and Planet Pizza. Is there any difference?

Now, both combinations are in use. There is a site PlannetPizza.com. Pizza Planet was name of a restaurant in Pixar movie Toy Story.

But in case of other words, like Water Planet, Animal Planet, the first combination is in use. I didn't find Planet Water, Planet Animal.

Similary, I see Planet Hollywood but not Hollywood Planet.

So what is the difference? Which combinations is used in what conditions?

For a site which is for students of a university, for example TPU university, should the domain name be Planet TPU or TPU Planet? Would they mean the same thing?

Thanks for your time.