Despite Kepler’s recent end of mission the search for exoplanets continues. While their orbits and properties vary, ranging from double our Moon’s mass to thirty times Jupiter’s, their atmospheres’ compositions are harder to detect.

As of this writing this means we have mostly data from exoplanets we detected by transition and occlusion methods, but also other kind of exoplanets. However we only found what is in their air, not how much or its ratio in the atmosphere. What we did detect of their atmospheres was often hydrogen, sodium, water vapour, carbon monoxide and dioxide, methane, and even oxygen, depending on the respective exoplanet.

Sadly, the presence of O2 isn’t the smoking gun for extraterrestrial life some media like to report it for. There are atmospheric and geological processes that produce detectable—or even massive—amounts of oxygen, keeping the mere presence of O2 from being a robust biomarker. On the flip side, too much oxygen is also possible, and may prevent or hamper the development of life.

So far we lack a method that would determine the composition percentages of exoplanet atmospheres, but that doesn’t mean someone clever won’t come up with a way. And once we do, finding an atmosphere with a sizeable fraction of O2 might very well be a good indicator for extraterrestrial life.