In particular, an oxygen-rich atmosphere would be taken as strong evidence of a biosphere, especially if methane were also detected in the air.<\/p>
This is because free oxygen is released by the biological process of photosynthesis but isn\u2019t produced by many non-living processes.<\/p>
It\u2019s such a reactive gas (combining with methane, for example, to give carbon dioxide) that it ought to be quickly removed from any atmosphere if it\u2019s not being replenished.<\/p>
Read more from Lewis Dartnell:<\/strong><\/em><\/p> So far, Earth is the only planet known to support life<\/a>. But what makes a planet habitable<\/a>?<\/p> Our oxygen-rich, methane-tinged, atmosphere has been pushed far from the \u2018chemical equilibrium\u2019 you would expect from purely geological processes, a state known as disequilibrium, indicative of Earth\u2019s active, global biosphere.<\/p> But even though the ability to photosynthesise was developed by life on Earth at least 3 billion years ago, our planet has only had a strongly detectable oxygen biosignature for the last half-billion years or so.<\/p> It takes time for oxygen to build up in a planet\u2019s atmosphere, even if it does support plentiful life.<\/p> So how might the atmospheric disequilibrium of Earth have changed over its history, and what other biosignatures might we look for on other worlds?<\/p> Nicholas Wogan and David Catling from the University of Washington<\/a> have built a simple computer model of Earth\u2019s atmosphere and ocean.<\/p> The simulation shows how the atmospheric balance shifts with volcanism, sunlight and a simulated microbial population.<\/p> They used it to study three different phases in our planet\u2019s history: the pre-life Earth, when only geological processes influenced the composition of the atmosphere; an Earth inhabited by microbes, able to feed off simple gases like hydrogen and carbon monoxide; and the current age, with widespread photosynthesis controlling the chemistry of the planet.<\/p> On an Earth-like planet before widespread life, the atmosphere does show a modest chemical disequilibrium from the mixture of hydrogen and carbon dioxide released from geological sources.<\/p> So with the spread of primitive life, the atmospheric disequilibrium actually decreases because \u2018edible\u2019 gases like hydrogen are being consumed by microbes.<\/p> It\u2019s not until photosynthesis evolves and adds oxygen to the atmosphere that life conspicuously reveals its presence by shifting the atmospheric disequilibrium.<\/p> Spectroscopy<\/a> of terrestrial exoplanets can not only reveal strong evidence for the presence of a global biosphere, but that certain gasses \u2013 such as carbon monoxide \u2013 could also be considered an \u2018antibiosignature\u2019 indicating the absence of widespread life, because they represent a free lunch that apparently nobody is eating.<\/p> Astronomers are able to search for atmospheric biosignatures in nearby transiting exoplanets with the James Webb Space Telescope<\/a>, while scopes planned for the next decade will be capable of picking out the reflected light from Earth-sized exoplanets.<\/p> We are entering a golden age for characterising nearby worlds.<\/p>![\"Water](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/12\/GettyImages-1392358996-1024x683.jpg?fit=800%2C534\")
![\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/01\/transiting_exoplanet-dd5a1fe-e1615386399790.png\" "\\"Astronomers"){kind=spacer}