In Planetoid 3, all organic based life requires Oxygen to live; when you start a game your planet has plenty of trees and oxygen, but as your planet grows full and population grows, the oxygen level will lower. It's usually easy to keep your planet(s) full of oxygen, trees and keep it working, but before you go off colonizing new worlds, you need to look into terraforming, because most worlds out there have no trees, and thus no oxygen. So, you will have to first fly a planter ship out to the planet and spread trees on it. Then, once that is done, the an atmosphere and oxygen will form. Then, you can send a transport out and begin colonizing.
All planets except for gas giants can be terraformed by planters and form oxygen, but established planets can also lose their atmosphere and trees whether from an attack, over population, or temperature changes like meteoroids or ice comets hitting the surface of the planet.