Scientists discover new way to identify liquid water on exoplanets

The research, led by Birmingham University (UoB) and the Massachusetts Institute of Technology (MIT), suggests that a decrease in a planet’s atmospheric CO2 levels could imply that there is liquid on that planet’s surface.

According to the researchers, a decrease in CO2 levels could imply that the CO2 is being dissolved into an ocean or sequestrated by a planetary-scale biomass.

‘Habitability’ is a theoretical astronomical concept that means that a celestial body is capable of hosting and retaining liquid water on its surface, and scientists have generally agreed on which planets fall in the ‘habitable zone’, based on distances from their stars and subsequent temperatures.

The researchers devised a new ‘habitability signature’ with which they can identify whether a planet does indeed have liquid water. Previously, scientists used a planet’s glint to consider the presence of water on its surface, to measure how star light reflects off potentially present water.

However, the research team said that this signature is far too weak for current observatories to detect, whereas their method can be applied with current facilities.

In a statement, Amaury Triaud, professor of Exoplanetology at UoB who co-led the study, said: “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 researchers said that findings can be used to reveal more insights into environmental tipping points.

Triaud said: “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 research team said that their next step is to detect the atmospheric CO2 compositions of a range of exoplanets and identify which have oceans on their surface, and help prioritise further observations towards those that may support life.

The research has been published in Nature Astronomy and can be accessed here.