[Dwedit]

Their ships still shoot down your troops. Then you get bombed into nonexistence. Trying to take a planet without a fleet there won't end very well.

[billyhoffman]

Get Transports Teleporters, and your troops have a 50% change to bypass the ships in orbit. If its late game, the planet you take should have high population and production, so it can build a bunch of missile bases on the turn after you take the planet.