Gray Population Squares: The gray planets (identified by both their color and rings) represent worlds that are suitable for any Production. The gray Population Squares may receive a Population Cube from any of the three Population Tracks. Population Cubes removed from gray Population Squares are returned to any Population Track.
Advanced Population Squares: Population Squares with a star symbol (Advanced Population Squares) represent harsher worlds that are difficult to colonize. You may only place Population Cubes in Advanced Population Squares if you have Researched the related Advanced Tech.
Gray Advanced Population Squares may receive a Population Cube from any Population Track for which you have Researched the related Tech (Advanced Mining, Advanced Labs, Advanced Economy, or Metasynthesis).
Orbitals: Orbitals are artificial living environments that you may Build. An Orbital may only receive a Population Cube from the Science or Money Population Tracks.