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The first time we find evidence of life on a planet orbiting another star (an exoplanet), it is probably going to be by analysing the gases in its atmosphere. With the number of known Earth-like planets growing, we could soon discover gases in an exoplanet’s atmosphere that are associated with life on Earth.

But what if alien life uses somewhat different chemistry to ours? A new study, published in Nature Astronomy, argues that our best chances of using atmospheres to find evidence of life is to broaden our search from focusing on planets like our own to include those with a hydrogen atmosphere.

We can probe the atmosphere of an exoplanet when it passes in front of its star. When such a transit happens, the star’s light has to pass through the planet’s atmosphere to reach us and some of it is absorbed as it goes. Looking at the star’s spectrum – its light broken down according to its wavelength – and working out what light is missing because of the transit reveals which gases the atmosphere consists of. Documenting exoplanet atmospheres is one of the goals of the much-delayed James Webb Space Telescope.

Read more: Exoplanets: how we used chemistry to identify the worlds most likely to host life

If we were to find an atmosphere that has a different chemical mix to what we would expect, one of the simplest explanations would be that it is maintained that way by living processes. That is the case on Earth. Our planet’s atmosphere contains methane (CH₄), which naturally reacts with oxygen to make carbon dioxide. But the methane is kept topped up by biological processes.

Another way to look at this is that the oxygen wouldn’t be there at all had it not been liberated from carbon dioxide by photosynthetic microbes during the so-called great oxygenation event that began about 2.4 billion years ago.

The authors of the new study argue that we should start investigating worlds larger than the Earth whose atmospheres are dominated by hydrogen. These may not have any free oxygen, because hydrogen and oxygen make a highly flammable mixture.

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Category: Science