The planet quality bonus is the most exploitable feature of the game. Basically every time you come into posession of a planet you get a 10% or 20% bonus. However their is no flag on it saying that this bonus is allready applied, so heres an easy way to exploit it that many people do. One person said, in a previous topic, he got mars up to 30 PQ doing this.

What you do is you let an alien colonise a weak planet in your territory. Then it will switch over to you, it gains the PQ bonus. After that you ship the people off the planet to one of yours, with transports, and give it back. It will switch back over to you and it will gain the bonus again, repeat until PQ >30.

Though i laugh at the fact of Mars with PQ 30-100 i think that something like this needs to be fixed.