Scientists find 'new and easier' way to identify habitable planets

Scientists say they have found a new and easier way to identify signs that a planet can support life.

Liquid water on the surface of a planet is one of the main conditions that point towards signs of life.

Until now, according to scientists, there had been no practical method of measuring the substance's presence.

The closest they had come to identifying liquid on a planet's surface was to use its glint - how star light reflects off water - which can be too weak for current observatories to detect.

Now, however, researchers say they have devised what they call a new "habitability signature", which can determine whether a planet is capable of hosting and retaining liquid water on its surface.

The process involves measuring the amount of carbon dioxide in a planet's atmosphere and comparing those measurements with that taken from neighbouring worlds.

If a planet has a reduced amount of CO2 in its atmosphere compared with its neighbours, it suggests there is liquid water on its surface, according to the researchers.

The theory is that the CO2 in the planet's atmosphere is being dissolved into an ocean - just like Earth - or absorbed by a planetary-scale biomass, the team said.

Amaury Triaud, professor of exoplanetology at the University of Birmingham, 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."

The team said its habitability signature can also determine markers of life on another planet.

Dr Julien de Wit, assistant professor of planetary sciences at the Massachusetts Institute of Technology, said: "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."

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

Prof 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's atmosphere.

"There may have been a past climatic tipping point that led to Venus becoming uninhabitable."

The research is published in the journal Nature Astronomy.

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