Giant telescopes like the Extremely Large Telescope now being built in Chile could hunt for alien life by detecting oxygen on exoplanets – even though they were not designed with that in mind.
On Earth, plants and some bacteria are the only sources of large amounts of atmospheric oxygen. Finding oxygen on an exoplanet would therefore be a tantalising hint of life as we know it.
Current telescopes can look at the light that passes through exoplanet atmospheres and tease out their make-up, based on the substances that absorb particular wavelength bands. "We do this now for Jupiter-sized planets," says Ignas Snellen of Leiden Observatory in the Netherlands.
But current telescopes are not sensitive enough to see atmospheres on small, rocky worlds. What's more, observations made from the ground struggle to filter out Earth's own oxygen-rich atmosphere. Space missions intended to hunt for distant oxygen have been cancelled.
Flux buckets
Snellen and colleagues have now calculated that the European Extremely Large Telescope, due to be completed in the next decade on Cerro Armazones, a mountain in Chile, will be big enough to do the job. Boasting a 39-metre main mirror, this telescope is expected to see some of the most distant stars and galaxies in the universe. With its much higher resolution, oxygen from an exoplanet would appear similar to Earth's oxygen, but its wavelength band would be noticeably shifted due to the exoplanet's motion as it orbits its star.
Finding such oxygen would still be a long shot: an exoplanet has to pass in front of its star many times to gather enough data to say for sure whether oxygen is present. Depending on the planet's orbit and the size of its star, that could take between 4 and 400 years.
The team also suggests building an array of "flux buckets", cheap telescopes that collect as much light as possible. These cannot be used to produce detailed images like large observatories, but they would allow the analysis needed to find exoplanet oxygen.
"It's good to have a cheaper alternative to the big space-based missions," says Jack O'Malley-James at the University of St Andrews in Fife, UK. But he cautions that just detecting oxygen will not confirm the presence of life. Other planets with vastly different chemistry might have an alternative source, so space-based observations would still be needed to confirm the full range of chemicals in a planet's atmosphere and show whether it is truly Earth-like.
Journal reference: Astrophysical Journal, doi.org/kh6
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