CUTTING EDGE

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Hunting for life, but not as we know it

Ammonia could be as important on other worlds as oxygen is on ours

Prof Lewis Dartnell is an astrobiologist at the University of Westminster

Over the coming years, as astronomers use spectroscopy to read the atmospheres of Earthsized, habitable planets, detecting the presence of one gas will be an important discovery: oxygen. On Earth, oxygen is released by life – specifically, by organisms using sunlight for energy.

Oxygen is a very reactive gas. Early in Earth's α history any oxygen released into the atmosphere was rapidly removed. It reacted with rocks, or was destroyed by photochemical reactions driven by ultraviolet rays in sunlight. Such processes are known as 'sinks', and oxygen only started to accumulate in Earth's atmosphere once its production had overwhelmed these sinks. An oxygen-rich atmosphere is thought to be a sign of flourishing life on a world, which is why astronomers would be so excited about discovering one on an exoplanet.

But is oxygen the only 'biosignature' gas that would indicate the presence of life? Might other gases produced by biochemistry also be able to overwhelm the sinks on their exoplanets and accumulate to detectable levels? Sukrit Ranjan, at Northwestern University, and his colleagues have been investigating this. The best candidate worlds for detecting such biosignature gases, they argue, are exoplanets orbiting small, cool M-class red dwarf stars. Such stars emit less ultraviolet radiation than larger, hotter stars like the Sun, and so the sinks on those planets are much weaker. Planets orbiting M-class red dwarfs offer favourable conditions for the accumulation of reactive gases to levels that we could hopefully detect with space telescopes. This is one of the reasons why the James Webb Space Telescope (JWST) will be targeting these stars.