Cutting edge
Next generation telescopes could help astronomers find distant biosignatures
“Many of the biochemical processes of life release gaseous by-products that can build up to detectable levels in a planet’s atmosphere”
Every now and then in ‘Cutting Edge’ we return to advances in the search for life beyond Earth. Current efforts are focused both on finding single-celled extraterrestrials within our own Solar System, as well as on exoplanets orbiting other stars in the Galaxy. Both strategies have their own strengths and weaknesses. We are able to physically explore the Solar System close-up with robotic probes, but even places like Mars or the moon Europa seem inhospitable compared to Earth. On the other hand, there are an enormous number of habitable planets across the Galaxy, but we can only observe them remotely. The signs of life, or biosignatures, on an exoplanet would have to be globally conspicuous to be detectable.
McCullen Sandora and Joseph Silk, both associated with the Institut d’Astrophysique de Paris, have been taking stock of our current knowledge of promising exoplanet biosignatures which upcoming telescopes could let us search for.
Many of the biochemical processes of life release gaseous by-products that can build up to detectable levels in the planet’s atmosphere. These could help reveal how advanced life has evolved, as over the history of life on Earth biology has progressed through a sequence of evolutionary transitions, including the development of photosynthesis (growing by the energy of sunlight and releasing oxygen), multicellular life like trees and animals, and technological civilisation – all of which give off different biosignatures.
The methane gas in Earth’s atmosphere was overwhelmingly released by life. Methane-producing biochemistry appears to have evolved early on Earth, and so detecting this gas could be a good indicator that an exoplanet is hosting microbial life. Other biosignature gases include methyl chloride or dimethyl sulfide – the characteristic smell of the sea that’s released in large amounts by plankton and coral.
Signs of a biosphere
Once a biosphere has evolved photosynthesis it will lead to a slow accumulation of oxygen in the atmosphere. This began on Earth around 2.5 billion years ago, but the levels of atmospheric oxygen wouldn’t be directly detectable until they have built up to near present-day levels of 20 per cent. It’s actually much easier to detect its by-product, ozone, which has the added benefit of shielding the planet’s surface from high levels of ultraviolet radiation.
As for intelligent alien life – if SETI (Search for Extraterrestrial Intelligence) efforts are successful, and we detect an unambiguously artificial radio signal from an extrasolar planet – that would be an open-and-shut case. However, we could detect more general signatures of technological civilisations that have an effect on their homeworld’s atmosphere. Industrial air pollution like CFCs have no known geological source and stick around for tens of thousands of years. We could detect the spectral signature of solar panels, if the planet is covered with enough, or even see the artificial lights from cities. Crucially, say Sandora and Silk, we will soon be advancing from merely speculating about tell-tale signs, to having the capability to detect such signatures with sophisticated telescopes. For example, Sandora and Silk anticipate that the Extremely Large Telescope (ELT) under construction on the top of Cerro Armazones in Chile will be able to target 10 to 20 rocky habitablezone planets for atmospheric signs of microbial life or photosynthesis. And if the proposed 15m LUVOIR space observatory is funded and built, it could survey over 100 Earth-like planets for technological signatures.
Lewis Dartnell was reading… Biosignature Surveys to Exoplanet Yields and Beyond by McCullen Sandora and Joseph Silk. Read it online at https://arxiv.org/abs/2005.04005