The next big space telescope could spot Earth-like oxygen levels on exoplanets

Scientists generally agree that the best strategy for finding extraterrestrial life is to look for a world that has conditions similar to Earth’s, including atmospheric oxygen that could be produced by organisms running photosynthesis. But current instruments used to study potentially habitable exoplanets are woefully ill equipped to find such biosignatures.

Now a new study suggests that in a little over a year, we could be well on our way to narrowing down the search for life on another world.

“There are few bigger questions than ‘Does life exist on planets other than Earth?’” says study coauthor Edward Schwieterman, from the University of California. “Because of oxygen's link with life on Earth, we know it is important to search for on exoplanets.”

The study, led by NASA scientists and published in Nature Astronomy today, highlights an intriguing new way the upcoming James Webb Space Telescope could be used to detect and measure oxygen on exoplanets. The telescope, due to be launched in 2021 after a number of delays, was always going to be tasked with studying exoplanet oxygen, but these new findings expand those capabilities in ways no one had before realized was possible.

Moreover, this new technique could help us better ascertain how much oxygen another world contains. If a planet has oxygen levels similar to Earth’s, it raises the possibility those levels may also be driven by biology. (Although it certainly doesn’t eliminate nonbiological origins for that oxygen.)

Before this study, scientists had identified three major wavelengths on the electromagnetic spectrum (one in the visible spectrum and two near infrared) that could be observed in order to identify the presence of oxygen. But at high concentrations, like those on Earth, oxygen molecules smash into things much more frequently. Those collisions emit signals that cannot be observed using these three wavelengths, making them unsuitable for identifying denser, more abundant oxygen levels that would be more likely associated with biological activity. 

The new study identifies a wavelength at the mid-infrared level that can be used to detect collisions of oxygen molecules both with oxygen and with other gas molecules. The study’s authors suggest that the JWST’s Mid InfraRed Instrument Low Resolution Spectrometer (MIRI LRS) could search for oxygen at this wavelength around exoplanets that are transiting their host stars.

This method would potentially allow us to detect Earth-like levels of oxygen in many star systems less than 16 light-years away. In more distant systems it would be able to detect levels several times higher than those on Earth.

Since we can detect oxygen that’s colliding with other gas molecules as well, the method should allow us to learn about the atmospheric chemistry as a whole in greater detail, and whether it’s amenable to life or may have been shaped by past or present extraterrestrial life. For example, Schwieterman points out that oxygen features measured alongside atmospheric methane would suggest biochemical processes on the surface that are similar to what’s found on Earth.

Schwieterman suggests that the best exoplanets to study with this technique are those orbiting M dwarf stars, which puts the planets in the TRAPPIST-1 system at the top of the list. Forty light-years away,TRAPPIST-1 has multiple exoplanets that could support life, including three that are right within the habitable zone. At the very least, we can use the mid-infrared band to figure out whether the oxygen we have spotted on a distant exoplanet is something to get excited about.