[Nitram Draw]

One thing about defending planets is you can't hold out forever against large odds. If the enemy has 50 ships and you cannot reinforce it soon, kiss the planet goodbye. Once fleets get large, over 20 ships, you will need a fleet to run them out.

Some other tactics I've used are:

Launch mines over the planet, usually minesweepers aren't brought in to combat. This only works on a Human 1-2 times but will work against the AI.

Fighters are ok if the enemy doesn't have much PD, otherwise they get chewed up fast but can occasionally swing the battle your way.

Sats, lots of them, with missiles or beams can help. Bases are better especially Starbases and larger. They can hold out for quite a long time if there are WP on the planet.

If you are in danger of losing a well colonized system try to use engine destroying weapons to thin out, or slow down, the fleet until you can launch a counter attack.