Don't worry about it too much, if it's just to give you security points in your home system. Remember, at low tech levels, specialization is the key - and that you can also use PDC's to calm the population. Yes, colonists on Europa will feel safe as long as Earth has plenty of missile bases even if they are completely useless.
For RP purposes, I like to defend Sol at each "stage" or generation of tech, even if I don't have any active enemies. This is how I do it:
(1) Planetary Defence - uses PDC's. I have two PDC's that house big active sensors rated for 5,000 tons and see beyond Mars orbit. Thus the Inner System is always under surveillance. Then there's a bunch of missile silos equipped with anti-shipping missiles that have a range to that same Mars orbit. Don't worry if it seems too short. At Gen1 and Gen2, you are pretty much crippled anyway. These guys will stop any ships from getting to beam weapon range.
(2) Orbital Defence - uses OWP's. I have two types of Orbital Weapon Platforms (satellites). Keep them small in size so your N-SY can crank them out fairly quickly (and also upgrades them!). One has four railguns and a Beam Fire Control, other has two lasers and a Beam Fire Control. Third type is surveillance satellite, that houses active sensors rated for 50 tons. Don't make it too big or you'll waste a lot of research points on it. A range of million klicks is fine at this level. No need for engines or fuel or engineering spaces since they will never leave Earth orbit. These guys will intercept any missiles lobbed at Earth.
(3) JP Defence - uses Monitors. These are 10k ships that move very slowly (under 1000 m/s) but have a decent amount of armour and carry a spinal laser and particle beams. If you want to save on research points, replace the particle beams with lasers. I keep a group of these at every JP plus one extra at Earth going through rehaul. These guys need to have active sensors, as in case an enemy does come through a JP, you don't want to lose your sensor ship first, leaving the others helpless.
Depending on your population sizes, you can ignore (3) totally and still have harmonious populations.