Scientists discover new way to identify liquid water on exoplanets

Scientists have devised a new way to identify habitable planets and potentially inhabited planets, by comparing the amount of carbon dioxide in their atmosphere, to neighbouring planets.

An international team of researchers from the University of Birmingham (UK), the Massachusetts Institute of Technology (MIT) (US) and elsewhere, have shown that if a planet has a reduced amount of CO₂ in its atmosphere compared to neighbouring planets, it suggests there is liquid water on that planet’s surface. The drop in CO₂ levels implies that the carbon dioxide in the atmosphere of the planet is being dissolved into an ocean or sequestrated by a planetary-scale biomass.

The research is published today (28 December 2023) in Nature Astronomy.

Habitability is a theoretical astronomical concept that means that a celestial body is capable of hosting and retaining liquid water on its surface. Planets too close to their star are too hot (such as Venus), those too far, are too cold (like Mars), whereas planets in the ‘habitable zone’ are just right. The habitable zone is sometimes referred to as the Goldilocks zone.

The researchers devised a new ‘habitability signature’ with which they can identify whether a planet does indeed have liquid water. Before this, the closest scientists had come to identifying liquid on a planetary surface was to use its glint, how star light reflects off water. However, this signature is far too weak for current observatories to detect whereas the new method can be applied with current facilities.

Amaury Triaud, Professor of Exoplanetology at the University of Birmingham, who co-led the study said: “It is fairly easy to measure the amount of carbon dioxide in a planet’s atmosphere. This is because CO₂ is a strong absorber in the infrared, the same property causing the current rise in global temperatures here on Earth. By comparing the amount of CO₂ in different planets’ atmospheres, we can use this new habitability signature to identify those planets with oceans, which make them more likely to be able to support life.

“For example, we know that initially, the Earth’s atmosphere used to be mostly CO₂, but then the carbon dissolved into the ocean and made the planet able to support life for the last four billion years or so.”

As well as developing a new way to identify habitable planets, the research can be used to reveal more insights into environmental tipping points.

Amaury Triaud continues, “By examining the levels of CO₂ in other planets’ atmospheres we can empirically measure habitability and compare it to our theoretical expectations. This helps gather context for the climate crisis we face on Earth to find out at which point the levels of carbon make a planet uninhabitable. For example, Venus and Earth look incredibly similar, but there is a very high level of carbon in Venus’ atmosphere. There may have been a past climatic tipping point that led to Venus becoming uninhabitable.”

The new method is not just a signature for habitability, but it can serve as a biosignature too, since biology captures carbon dioxide as well.

Dr Julien de Wit, Assistant Professor of Planetary Sciences at MIT and co-leader of the study explains: “Life on Earth accounts for 20% of the total amount of captured CO₂, with the rest mainly being absorbed by the oceans. On another planet, this number could be much larger. One of the tell-tale signs of carbon consumption by biology, is the emission of oxygen. Oxygen can transform into ozone, and it turns out ozone has a detectable signature right next to CO₂. So, observing both carbon dioxide and ozone at once can inform us about habitability, but also about the presence of life on that planet.”

An important element of the new study is that those signatures are detectable with current telescopes. Julien de Wit concludes “Despite much early hopes, most of our colleagues had eventually come to the conclusion that major telescopes like the JWST would not be able to detect life on exoplanets. Our work brings new hope. By leveraging the signature of carbon dioxide, not only can we infer the presence of liquid water on a faraway planet, but it also provides a path to identify life itself.”

The next step for the research team is to detect the atmospheric carbon dioxide compositions of a range of exoplanets, identify which have oceans on their surface, and help prioritise further observations towards those that may support life.