How about having the choice to protect your top 10 or 20 planets from anyone? Or top 20 percent.
Start with a minimum of 10 for all players and switch to percent of planets when they reach a certain level or number of planets.
This would protect your core planets and would keep a lot of people in the game.

would screw up invasions -1
also, you didn't spend money on those planets (unless they're your mission Exotic or dyson)
so everyone has to right to take your core planets.

As much as I would LOVE this idea, Invasion is a part of the game Saddly its a PVP game where your planets are vunerable, just be thankful dan made mission planets uninvadable.

my top 2 planets are already protected from everyone.. that's enough for me...