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 point to alien shrubbery. Two researchers argue that an ocean-bearing planet zapped by its sun’s ultraviolet light could masquerade as a living world.
Plant photosynthesis produces Earth’s oxygen. But Robin Wordsworth and Raymond Pierrehumbert, geophysicists at the University of Chicago, wondered whether there was another way for a rocky planet to have an abundance of the gas in its atmosphere. 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 atmospheric nitrogen, Wordsworth says, water would drift higher, eventually reaching 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, 2014 at arXiv.org and in press at Astrophysical Journal Letters, that eventually enough oxygen could build up to create a new 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. On Venus, hot temperatures broke down the planet’s cold trap long ago, causing the planet to lose its water and hydrogen to space. Hypothetical alien astronomers, if they had had telescopes pointed our way several billion years ago, could have briefly detected oxygen in Venus’ atmosphere, but it wouldn’t have been a sign of life. “It actually would have been the signature of a dying planet,” Lunine adds.
Wordsworth’s work extends previous ideas about water destruction from Venus-like worlds to any rocky planet within a star’s habitable zone, the region around a star where a planet could comfortably sustain liquid water.
Lisa Kaltenegger, an astronomer at the Harvard-Smithsonian Center for Astrophysics, agrees that it’s an interesting idea. But most scientists don’t think oxygen alone would be enough to claim the presence of life, she says. Astrobiologists, she says, look at the bigger picture: the planet’s orbit, its composition and the presence of oxygen in conjunction with other gases produced by life such as methane.
One issue with Wordsworth and Pierrehumbert’s scenario, Lunine adds, is how a planet without nitrogen would come to exist. While the origin of Earth’s nitrogen is a mystery, he says, it was probably delivered by asteroids and comets. Scientists expect other solar systems to be similar. “Still,” he says, “the universe surprises us all the time.”
Unfortunately, current telescopes aren’t up to the task of testing these ideas. Getting a chemical reading from an exoplanet atmosphere is no easy feat. Currently, scientists can only crudely measure the atmospheres of Jupiter-like giants orbiting nearby stars. But upcoming telescopes, including the Transiting Exoplanet Survey Satellite due to launch in 2017 and the James Webb Space Telescope in 2018, could let astronomers knock on the door of a sister Earth and see if anyone’s home.