Although primitive life can exist without photosynthesis, the researchers argue it would be necessary for more complex multi-cellular organisms to emerge. This is because the main source for oxygen on Earth comes from photosynthetic life, and oxygen is thought to be necessary for multi-cellular life to arise.

To find such “photosynthesis-sustaining habitable zones” around stars, the researchers explain one should concentrate on where the global average surface temperature of a world in the zone stays between the freezing and boiling points of water (0 to 100 degrees Celsius).