Inhabited worlds; worlds whose atmospheres might contain oxygen, a by-product of life as we know it here on Earth. It seems to make sense to search for oxygen in the atmospheres of potentially habitable planets, right? Wrong. The logic offered is that photosynthesis, which produces oxygen on Earth, is a hallmark of life. In other words, wherever you find oxygen in an alien atmosphere, you could have a habitable world. Wherever you don’t, you do not. But this would be a serious error.

Oxygen as a Biosignature

Oxygen could point to existing life (called a biosignature): high levels of oxygen in an atmosphere should mean that we could look for photosynthetic creatures and, thus, a biosphere. Here’s where it gets interesting, because that’s not all it could point to.

Habitable vs. Inhabited

A habitable planet is a planet that supports life under the prevailing conditions: liquid water, a stable climate, and a source of energy. The theory, habitable planets have life on them but not necessarily. It is certainly possible that Earth is a planet that could support life but didn’t have any for a long time. When you think of it like that, you might expect to see oxygen. But what if the planet’s atmosphere is in a different state – like a nascent Earth – and the planet is habitable but it doesn’t have enough oxygen to make it past our filter? It’s possible that we won’t be able to detect life yet.

Non-Biological Oxygen Production

Besides biological processes, it can also be generated abiotically. Vigorous UV radiation from the host star can split water molecules in the planetary atmosphere, generating oxygen. Abiotic oxygen can trigger false-positive results The presence of oxygen, therefore, is not a reliable signal of a truly habitable planet.

A Multi-Faceted Approach

As such, while oxygen remains a viable target for the search for extraterrestrial life, it should not be the sole arbiter of habitability. A more complete look at habitability would include more atmospheric gases and more planetary characteristics, such as temperature, pressure and other potential biosignatures, including methane and nitrous oxide.

The Quest Continues

The quest for habitable worlds is an iterative and longgame process, and going forward scientists should broaden their perspectives, recalibrating their view of oxygen and other gases as biosignatures to be less incautious, more balanced and more versatile. By allowing for other potential biological effects in other atmospheres to slip in through the edges of our detectors, we drastically improve our prospects of locating worlds that could host or once hosted life – whether or not they do.

©️ The Rocky Mountain Dispatch LLC. 2024