Edge of the Milky Way | The Conservation Report

Image via Eric in SF on Flickr

Calculating where photosynthesis might be possible in the universe, in addition to star size and whether a planet might have a moon, may help scientists find life on other planets. Of course, these factors are based on observations from our own solar system. However, these factors are probably good factors, since Earth not only lies within a habitable zone, but it is protected by two large gaseous planets, Jupiter and Saturn, from devastating comet and asteroid impacts (though, the gravity of Jupiter has the opposite effect of actually hurtling asteroids towards the Earth). The moon is an important factor too as is Earth’s own magnetosphere. For these reasons, complex life was able to evolve on Earth over time. Another interesting factor for life on Earth (and maybe on other planets) is that our solar system lies towards the edge of the Milky Way, so we’re protected from harmful galactic phenomena like radiation. However, our heliosphere, which is the “immense magnetic bubble containing our solar system, solar wind, and the entire solar magnetic field,” protects our solar system from harmful high-energy galactic radiation. Via msnbc.com:

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).

They also say to look 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. They assume these planets experience plate tectonics to help replenish vital supplies of key minerals.

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Given these limitations, von Bloh and his colleagues estimated our galaxy might host up to 2.5 million worlds suitable for complex multi-cellular photosynthetic life. Moreover, they calculated that up to 690 million worlds could host more basic single-celled life that could also be photosynthetic, similar to cyanobacteria on Earth. They detailed their findings in the June issue of the journal Plant Science.

The researchers note their calculations as to the prevalence of complex life might get narrowed down even further if other factors are considered. For instance, large moons around planets in these zones might help the planets stabilize their tilt, leading to a stable climate. In addition, the presence of giant worlds elsewhere in these systems could help shield habitable planets from cosmic impacts.

Image: Habitable zone relative to size of stars (via Wikipedia)