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.