Leedot posted this in another thread about Outposts:

Outposts have a small amount of defense but it's really only enough to take on one or two destroyers. It's definitely recommended that you deploy some ships / defensive structures to protect them. As for what you can defend them with other than combat ships you have a couple options. 1. Mine Emplacement (T1 Trade) allows you to create a ship that you can move to any location and then convert it to an immobile mine field.
2. If you have both Cruisers (T2 Fleet) and Orbital Defenses (T1 Combat) then you can convert a cruiser into an Orbital Beam Array at an outpost. There's a similar combo with Advanced Orbital Defenses (t2 Combat) and Armed Command (T3 Fleet). It looks like the the description of those combos weren't included in the tech description so I'll updated that in the future.