The hunt for Earth-like planets in the universe, which usually looks for where water might exist, should consider photosynthesis instead, German scientists say.

Researchers at the Potsdam Institute for Climate Impact Research argue while primitive life can exist without photosynthesis, it would be necessary for more complex multicellular organisms to emerge, Astrobiology Magazine reported.

"Photosynthesis-sustaining habitable zones" around stars, the researchers say, would be where the average surface temperature of a world in the zone stays between the freezing and boiling points of water, 32 degrees to 212 degrees Fahrenheit.

The search should also be for planets where there are sufficient levels of carbon dioxide in the atmosphere, which photosynthetic life would consume to make oxygen and create organic matter, the scientists say.

Institute physicist Werner von Bloh estimates our Milky Way galaxy might host up to 2.5 million worlds suitable for complex multicellular photosynthetic life.

"Finding signs of life on other planets might be the most challenging goal in astrobiology," von Bloh said.