Id like it expanded as well. I did a write up with some ideas to improve it. 

https://forums.sinsofasolarempire2.com/518611/planetary-assault

My thinking was this would stop players only sending invasion fleet only consisting of transport ships (In current game mechanic Colony Ship) without backup to invade the planet, planetary missile defense can deal a blow to undefended transports or colony ships which have little to none offensive capabilities. This in turn will make players to plan out how to invade the planets, for example, first send your main offensive fleet to take out the enemy fleet defending the planet, after that send your specialized ground attack fleet (In this case specialized ground attack Frigates - for now there is little to no use as your capital ship can attack ground) to take out ground based defenses while taking some hits and this would also prevent main fleet ships without actual ground attack ability coming close to the planet, after that send your transports to deal with enemy ground forces. If the player neglect to send ground attacking ships to take out missile-ground based defenses of the planet, transport fleet or invasion fleet could fail or be wiped out. This will be very interesting change to the game.

Quoting Smarti12,

reply 4

There's always a 'versus' to be made in what's fun and what's realistic. And what is really fun, is that you guys want more realism but end up on planetary-side missiles. Those are extremely unreleastic Having to leave a gravity well is just too energy consuming for anything, whether it be missiles or ships.

We can leave planetary missile defense if it is hard to implement or unrealistic. But at least ground invasion mechanic I think is doable, and can be displayed via stationary troops on the planet and vs invading force via different stats how it progresses.

Well, a colony ship can't take over an undefended colony without some bombing ships, and even if a planet had a basic defense system if you have enough bombardment ships it should still be able to overwhelm the defenses. Either way, a planetary basic turret or missile launching system could be fun to defend against a stupid random annoying pirate pillage that is bombing your planet. 

That said, we do have the garrison defense and that alone can be enough to defend against invasions, though it is very expensive to get going. You can also build hangar defense platforms which can also defend against an invasion so you do have options.