There is usually a relic near your home world. You can put a military starbase near the relic and as close as you can get to your home world. At least your defenses get the bonus if they are within the area of effect. There are a couple of ways to extend starbase range so over time they will become more useful. Also consider putting a military starbase near a slipstream once you have secured your starting sector. You can put a fleet there to attack anyone who comes in. They're not great but they have some value.

As DivineWrath said, manufacturing starbases are a higher priority.

They have never been in the GC franchise.  Which isn't to say they never will be in GC4 (it's always a possibility, right?), but don't count on it.  If you want to defend your planets, you need a navy.  Even the military starbases' primary purpose is to support naval combat within its zone of influence. 

Luckily, it's easier to defend a small area (ie your planets) with a fleet, than it is to send a fleet across the map and successfully attack you.  There are a few mechanics to this - but the defender does have the edge here.  Plus, if you build enough warships, no one will attack you in the first place, this is probably the easiest defense - no one gets mugged when they're holding a gun (except for those two guys in the movie 'Snatch')...

Hope this helps,

-tid242