If your stuck on a mission where you have to capture a planet I have found this to be a very effective tactic:
1. Build a transport.
2. Transfer control of the transport to yourself.
3. Pilot the transport to the enemy planet and park it a little bit away from the planet.
4. Build a fleet big enough to occupy the enemys guarding the planet for a few seconds.
5. when your ready order your fleet to attack the enemy planet and while they are attacking order your transport onto the planet.
This strategy has worked for me on almost all the planet capturing missions (although it's more fun to capture them by overwhelming them with ships :)).
------------------