Uh I was attacking a planet a few minutes ago and I found out that it gives you the great experience just for attacking the planet. If your attack is lower than the defense but your defense is higher then the attack you could abuse it a bit and get the experience pretty easily. I have 24 enemy planets half of which I can attack because they are above 20 so its kinda interesting.

Good one. The bonus was supposed to only be awarded for ship-ship combat. This has been fixed.