I have seen raining death from space so many times but still haven't taken a planet. It's supposed to be 20%
Who's that against? Also, you should remember that it's 20% per fleet. Try sending lots of little fleets to the planet and you'll increase your chances dramatically.
EDIT: PeterKaan's home planet is finally defeated! Now to mop up his colonies, build a few upgrades and spend the 100k steel he left me.
