No, no no no no no...
Don't JETTISON their population. They now belong to your empire, and you don't want to kill them.
Just move your population onto the planet from your transport, then move the alien population from the planet to your ship. They are now YOURS. You can transport them to your other worlds that breathe Oxygen (that up to this point have been domed), put them on the planet, and then relocate your current population to another planet.
They are still YOUR planets, they just have an alien race living there. And magically, a Huge-sized domed planet that had space for 5 facilities now has space for 25 facilities!!!
The reason I suggested trading TRANPORTS instead of trading PLANETS is simply logistics. I would rather have all my planets in my existing systems populated with races that can actually breathe the air (and therefore not have domed colonies) than have several planets spread throughout the galaxy in my ally's space (who could become my enemy, of course).
Slightly off topic:
Population is a valuable asset. The higher your population, the better your resource production and shipyard build rate. (See table below)
I currently have one of my planets with over 6000 (million) population, and the bonus is +70% !!! My shipyard II can crank out over 4400 per turn, and over 6600/turn in emergency mode!! (Let's see what happens when I upgrade it to a Shipyard III... Oooo. The upgrade will only take 1 turn
!! )
Here are the correct bonus levels:
Min Pop--Bonus % to resource production and build rate:
100--5%
500--10%
1000--20%
2000--30%
3000--40%
4000--50%
5000--60%
6000--70%
7000--80%
8000--90% bonus (this is the max pop on a huge-sized, non-domed planet)
Hope this helps clarify.
[Edited bonus %'s after I went and checked]
[ 05 December 2001: Message edited by: MegaTrain ]