Depends do you want to spend the money on defense or re-colinizing and re-upgrading the asteroids, anyway what i do is at the choke points i defend heavily and also at planets with artifacts. At the asteroids or elsewhere the you can lighlty defend but make sure you have a fleet of ships just for defense just in case you need it, enough ships to defend at any place you want in your area.

it also depends on stratgey some people like to heavly defend all there planets from the beginning and others don't defend as much untill much later in the game when there economy is going strong.

Hope i helped some, which race to you play as?

I usually put at least one defensive structure at every "frontline" planet that could possibly be attacked. But if you're worried about the pirates, don't worry to much, they are actually very predictable, and their is usually only one or two planets they will take. All other planets I leave undefended.

The best defensive structure is the repair bay, build these at planets the pirates are attacking and put a capital ship there and the cap will gain a lot of experience for killing the pirates.

Also, repair bays allow a smaller fleet to take on a much larger one. Use them at planets your opponent likes to attack to help put your smaller fleet.

Against the AI you can usually get away with heavily fortifying every planet they like to attack to buy time to build up your economy and fleet, just remember that defenses are a method to delay your opponent and not defeat them, you will need a fleet for any defense to be effective.