Magazine article Science News

Exoplanet Oxygen May Not Signal Life: Water and Ultraviolet Light Could Create the Gas

Magazine article Science News

Exoplanet Oxygen May Not Signal Life: Water and Ultraviolet Light Could Create the Gas

Article excerpt

The first sign of extraterrestrial life probably won't be a spaceship landing in a cornfield or a radio transmission from deep space. Most likely, the announcement will be encoded in the chemistry of a distant planet's atmosphere. On Earth, oxygen betrays life's presence. But oxygen in an exoplanet's atmosphere wouldn't necessarily indicate alien shrubbery. Two researchers argue that an ocean-bearing planet zapped by its sun's ultraviolet light could masquerade as a living world.

Photosynthesis produces Earth's oxygen. But Robin Wordsworth and Raymond Pierrehumbert, geophysicists at the University of Chicago, wondered whether there is another way for a rocky planet to have an abundance of the gas. The pair considered a deceptively simple scenario: a planet devoid of certain other gases, such as nitrogen. One of nitrogen's roles on Earth is to help form a low-temperature layer in the atmosphere that water can't get past, called a cold trap. On a planet without nitrogen gas, Wordsworth says, water would drift to a height where the planet's atmosphere would no longer shield it from UV radiation from the planet's sun. UV light would break water molecules into hydrogen, which would escape to space, and oxygen, which would stay behind.

Wordsworth and Pierrehumbert argue, in a paper posted March 11 at arXiv.org and in press at Astrophysical Journal Letters, that eventually enough oxygen could build up to create a different kind of cold trap that would stabilize and hold on to an oxygen atmosphere for billions of years. Without the trap, the oxygen would eventually either follow the hydrogen into space or be absorbed by rocks.

"What they have done is clever," says Jonathan Lunine, an astronomer at Cornell University. Astrobiologists have until now thought water was a potential oxygen source only on planets such as Venus, where a runaway greenhouse effect drove temperatures to spiral higher and higher, eventually boiling away the oceans. …

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