Previous stories showed that we can use spectroscopy to detect the chemical fingerprint of an exoplanet. This story looks at one of the pitfalls we may face when making those directions.

First, let’s talk about oxygen. We’ve mentioned it a few times today, particularly in the context of Earth and our own solar system. We know we have life here. But then we talked about a world’s atmosphere that was far more carbon-based than oxygen-based. This is fine because that planet is too close in and too big to be habitable to the type of life we have here. Sure. Fine. All well and good. Does that mean oxygen is a sure-fire biosignature? No. But new research published this week in AGU Advances seeks to break down how to distinguish between potential “oxygen false positives” and actual signs of life.

The team came up with three different scenarios to potentially explain an abundance of oxygen that isn’t a result of life. Scenario one: oxygen builds up in the upper atmosphere as ultraviolet light breaks apart water molecules in the constituent oxygen and hydrogen. The hydrogen is much lighter, and it tends to escape into space more than the oxygen does. Then add in the outgassing from volcanoes, as mentioned earlier, and now we have carbon monoxide and more hydrogen to add to the mix. Carbon and oxygen like to react, so the oxygen gets pulled into more molecules, and the weathering of rocks on the surface contributes to pulling oxygen out of the atmosphere.

Scenario two: the planet doesn’t have a lot of water, to begin with, so the molten surface of the early planet solidifies quickly before the water that is there condenses. Now you have a steamy atmosphere with a lot of water molecules to break up as before, leaving oxygen behind as the hydrogen, again, escapes.

Then in scenario three, you can have a planet with a runaway greenhouse effect, like Venus, where there is more carbon dioxide than water, and then water cannot condense due to the heat, and so a whole bunch of oxygen-based molecules is just lingering in the atmosphere. It’s all a big, complicated set of processes that add up to one core issue: oxygen comes and goes for more reasons than the respiration of a planet’s inhabitants.

Now here’s the important part, as explained by co-author Jonathan Fortney: There has been a lot of discussion about whether detection of oxygen is ‘enough’ of a sign of life. This work really argues for needing to know the context of your detection. What other molecules are found in addition to oxygen, or not found, and what does that tell you about the planet’s evolution?

Remember those six molecules detected in our last story that told us the planet had to form elsewhere? That’s the kind of chemical fingerprint we’re going to need before claiming any sort of biosignature has been found. It’s more than just oxygen. It’s everything. And the team behind this paper is essentially arguing that we need the right instruments on all these upcoming next-generation telescopes to get the spectra we need from exoplanet atmospheres in order to find that first proper biosignature.

The mere thought of it makes me giddy.

More Information

UCSC press release

“Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars,” Joshua Krissansen‐Totton, Jonathan J. Fortney, Francis Nimmo, and Nicholas Wogan, 2021 April 13, AGU Advances