Oxygen in a planet's atmosphere is considered one of the best indicators we are likely to get for the presence of life. However, astronomers have thought such detections for planets beyond our solar system exceeded our current capacity, and anything coming soon. A new technique may have changed that, bringing one of the most awe-inspiring quests imaginable almost within our grasp.
Astrobiologists have put a lot of work into considering the features that might identify a world rich in life circling distant stars. An oxygen-rich atmosphere is the most obvious example. Oxygen is a very reactive element. It needs continuous replenishment if it is to last in a planet's atmosphere, something we know photosynthesizing plants and algae do well. Life, like money, changes everything, at least if you have enough of it.
Existing telescopes can't detect oxygen in the atmosphere of a planet light-years away, but if the James Webb Space Telescope meets its current 2021 launch date, after 14 years of delays, we will have something far beyond our current power.
Dr Thomas Fauchez, of NASA's Goddard Space Flight Center, said in a statement: "Before our work, oxygen at similar levels as on Earth was thought to be undetectable with Webb." As Fauchez's first three words suggest, that may have changed. In Nature Astronomy, Fauchez and co-authors describe a signal produced by oxygen-rich atmospheres that they conclude is strong enough for the Webb.
Molecules in any gas are constantly bumping into each other. When oxygen molecules do this, they block infrared radiation. The wavelengths absorbed vary, depending on whether two oxygen molecules are running into each other or if it's a collision between oxygen and some other gas.
The authors used the planets of the TRAPP1ST-1 system as they pass across the face of their star as a model, including TRAPPIST-1's spectrum. They found the James Webb would need to combine hundreds of transits to detect oxygen confidently, but detection may be possible in a system with the same characteristics within 16 light-years of Earth. There are 50 potentially suitable stars in that range.
This isn't the first oxygen signature proposed as potentially detectable by the Webb. Oxygen dimers, where two sets of O2 molecules combine, are also radiation absorbers, but Fauchez claims the collision method has a better chance.
Astronomers think alternative explanations are possible for persistent oxygen in a planet's atmosphere, such as the slow boiling away of oceans. Consequently, oxygen alone is not enough to prove the presence of life – but it would be a good indicator that might be confirmed through other gasses.