Does a planet's population play a particular role?

It's basically a Hull for a planet - if it goes below 10, the planet can be invaded

Thanks, that sorta makes sense. As it stands, there's no visible reason for population.

Not completely true, at higher levels if you stack enough defense and pop and a reasonable amount of attack on a planet you can force people to have to repair and waste huge amounts of energy. To the point in which it can keep a single person from every having a chance of taking your planet.
2500+ population and enough defense to make each attack only do 1DMG means that there will have to be 2490+ attacks on the planet to be able to invade it.

that's why Exotica has stacked population, it makes it incredible difficult to even make it invasion-able

shouldn't that be "invadable" ??

yes....it should be...I realized it right after I posted, I couldn't think of the right word while I was writing it...lol