Using the Earth's fingerprint to detect habitable planets



Many astronomers have their eyes fixed on TRAPPIST-1, a system of exoplanets discovered in February 2017 which could contain potentially habitable planets. So do McGill undergraduate physics student Evelyn Macdonald and Nicolas Cowan, a professor in McGill's departments of Physics and Earth & Planetary Sciences, who have developed a spectral "fingerprint" of our atmosphere that could help determine whether these exoplanets could harbour any forms of life.

It was the methane and ozone found in the Earth's atmosphere that gave the researchers the idea for this project. 

They constructed a transit spectrum for the Earth using observations of the Earth's atmosphere collected by a detection system on the SCISAT satellite. When a planet passes – or transits – in front of its star, it blocks some of the light.  In addition, part of the light is absorbed by the planet's atmosphere. The spectrum of light passing through the atmosphere makes it possible to identify some of the molecules present, which absorb light of specific wavelengths. After compiling the SCISAT data, the researchers now know what kind of signal to look for to determine whether a transiting planet could harbour life.

It was the methane and ozone found in the Earth's atmosphere that gave the researchers the idea for this project. The simultaneous presence of these two molecules is a strong indicator of the presence of life, although they may be found in different proportions on habitable planets outside our solar system. The researchers will soon have the opportunity to put their discovery into practice on the James Webb Space Telescope, which is set to launch in 2021. As of its first year of operation, the telescope will collect data on the TRAPPIST-1 exoplanets located in the "habitable zone" and, more specifically, on transiting planets. If the presence of an atmosphere is detected, they will then compare the planet's spectral fingerprint with that of the Earth.