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CUTTING EDGE
Hunting for life, but not as we know it
Ammonia could be as important on other worlds as oxygen is on ours
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 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.
“An oxygen-rich atmosphere is… a sign of flourishing life on a world, which is why astronomers would be so excited about discovering one”
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.
A noticeable accumulation
Ranjan’s team considered an exoplanet with a hydrogen/nitrogen atmosphere in orbit around a M-class red dwarf. The biochemistry of life on such a world might produce ammonia (like on Earth, where Rhizobacteria help nitrogen-fixing plants to draw nitrogen gas from the air). This scenario has been nicknamed a ‘Cold Haber World’, after the process that uses heat, pressure and a catalyst to convert atmospheric nitrogen into ammonia for fertilisers. The research team simulated a Cold Haber World with a climate suitable for oceans of liquid water, Earth-like volcanism and an atmosphere with the same surface pressure as Earth, but made up of 90 per cent hydrogen and 10 per cent nitrogen. They simulated how quickly life on such a planet would release ammonia and the rate at which atmospheric ammonia would be destroyed by the dwarf star’s sunlight. They found that for realistic biological ammonia production rates, the gas can overwhelm the sinks on the planet and accumulate to notable levels in the atmosphere. They claim the JWST could detect such an atmospheric biosignature with two transits over two months.
Such a world would be different to Earth, but this result gives astronomers hope that we’ll be able to remotely detect atmospheric biosignatures on more diverse exoplanets.
Prof Lewis Dartnell is an astrobiologist at the University of Westminster
He was reading… Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures by Sukrit Ranjan et al.
Read it online at: arxiv.org/abs/2201.08359