New research shows that a lower amount of carbon dioxide gas in a planet’s atmosphere compared to that of its neighbours could hint at the presence of liquid water on that planet. Researchers said that the drop in the carbon dioxide levels relative to the neighbouring planets implied a possible absorption of the gas by an ocean or isolation by biomass on a planetary scale. While multiple studies have made attempts to identify planets lying in the habitable zones of the stars they orbit, the researchers said that until now there was no way of knowing whether they truly have liquid water. The international team of researchers, led by the University of Birmingham, UK, and Massachusetts Institute of Technology (MIT), US, said that they had devised a new ‘habitability signature’ and that it was a practical method for detecting habitability. They have published their findings in the journal Nature Astronomy. Planets that are neither too close to their star and thus too hot, nor too far from their star and thus too cold are considered to be in the just right ‘habitable zone’. The planets could therefore be ‘habitable’ and capable of hosting and retaining liquid water on its surface. The carbon dioxide, or CO2, in a planet’s atmosphere is a strong absorber in the infrared region of the light spectrum, the same property through which it is currently causing Earth’s temperatures to rise, explained co-lead researcher Amaury Triaud, Professor of Exoplanetology at the University of Birmingham. ‘It is fairly easy to measure the amount of carbon dioxide in a planet’s atmosphere. 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,’ said Triaud. Earth’s atmosphere too used to be mostly CO2, but then the carbon dissolved in the ocean, making the planet capable of supporting life for the last roughly four billion years, Triaud said. The researchers said examining CO2 levels of other planets and measuring their habitability could reveal insights about Earth’s environmental tipping points and the carbon levels at these points that could make our 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,’ said Triaud. The ‘habitability signature’ devised by the team could serve as a biosignature as well, they said, because living organisms too capture carbon dioxide. ‘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,’ explained Julien de Wit, Assistant Professor of Planetary Sciences at MIT and co-leader of the study.