The relevant properties are from happiness.txt as follows:
Our Troops on Planet := -2
Natural Decrease := -50
Natural Decrease for Other Races := 20
This governs anger/happiness. Negative values make your empire/planet happier, positive values make them angrier.
There are lots of other things that affect happiness/anger which can totally overwhelm these effects, but relative to your question with "all other things being equal":
- planets with your native population get happier over time by 50 points/turn.
- planets with alien population (any amount) get angrier by 20 points/turn.
- EACH troop regardless of size and components makes the planet it is on 2 points/turn happier.
Doing the math, 10 troops of any kind will offset the negative effects of alien population exactly to zero. Add another 25 and they will act like your native population.
I find that "one turn's worth" of small troops with only a cockpit are usually enough to offset alien population. Also, I usually build an Urban Pacification Center in every system anyway just to get all planets to jubiliant. This is normally enough to prevent anger/rioting even if there are enemy ships in the system and/or battles in the system, both of which make planets angrier.
Hope this helps.
Slick.