Heavily defended chokepoints are the key. Defense platforms with repair stations and hangars can keep a planet safe from pirate attacks. Also if you have Entrenchment a combat specced starbase will make a planet's defenses inpenetrable as long as the enemy doesn't have torpedo cruisers. Certain Capital Ships may also be a threath (Vulkoras Desolator and Marza Dreadnought). Mines are also useful in thinning enemy numbers. Vasari don't have anti-structure cruiser, so they have to assault starbases with their own mobile starbase, so if you see a Vasari coloniser frigate near your planet, destroy it immediatly.
Faction's strengths play a role here too, TEC has powerful Gauss Cannon platforms with high HP and armor, and planetary shields to mitigate bombardment damage. Advent are strikecraft masters, their hangars can shield other structures and their defense platforms can support each other if placed close to eachother. Vasari defense platforms' missiles partially bypass shields and disable ships' abilities for a short duration.
Geez, i can write quite lengthy stories about Sins tactics. If only i could play it as well as i can talk about it :lmao:
Edit:
does anyone remember what setting had to tweaked in graphics card control panel to make the little white dots on the edges of ships and planets disappear?
Anti-Aliasing; Disable Gamma Correction, Override application setting, 8xQ, incase someone else has this problem.