[Q]

Originally posted by tbontob:
Since the "Natural decrease - Others" is 20 for all types (Peacefull, Blood Thirsty and Neutral), then all things being equal, that planet should eventually riot.

I am not sure about this. In my tests even with larger values of the natural decrease (100) the happiness went to angry but not to riot after 10-20 turns.
May be I did not wait long enough but may be the natural decrease lowers the happiness only to angry not further. You may need other events like battle losses, foreign ships, plagues to push the happiness over the riot limit. But as said I am not sure about this.