Carbon dioxide now seen as key to finding potential life on other planets

Researchers have proposed the easiest way to spot other habitable planets may be to measure what they lack, rather than what they have: specifically, carbon dioxide.

Amaury Triaud of University of Birmingham. Picture: Facebook

The secret to determining whether distant planets host oceans, and potentially life, could be contained in their atmospheres, scientists have argued.

In a new paper, researchers have proposed that the easiest way to spot other habitable planets may be to measure what they lack, rather than what they have: specifically, carbon dioxide.

In our own solar system, Venus and Mars have atmospheres that are around 95 per cent CO2. On Earth, it is a lot less than 1 per cent. The total proportion of carbon on each is the same, but on Earth it has been removed from the atmosphere, in a large part thanks to processes involving the oceans.

Amaury Triaud, from the University of Birmingham, said it would be of great interest if one planet in a solar system has significantly less CO2 in its atmosphere than the others.

In the past two decades, scientists have discovered thousands of exoplanets – planets orbiting stars other than our own. Many are in the so-called “habitable zone”, where it would be possible to have liquid water, which it is assumed makes life far more likely.

Finding out whether they do have water, though, is extremely difficult. In the paper, published in Nature Astronomy, Professor Triaud and colleagues argue CO2 could help to narrow the search.

“People were aware that the carbon cycle removes carbon dioxide from the atmosphere of a planet. But no one expressed the idea it could be used as a signature that can tell us if an exoplanet has liquid water,” he said.

When a planet passes in front of its star, it is possible to observe patterns in the spectrum of light that reveal the chemical composition of its atmosphere. Because CO2 is a greenhouse gas, and a strong absorber of heat, it is easier to check for its presence.

The lack of CO2 on its own is not proof of life. But with so many planets to look at, it may help astronomers know where to focus their efforts. “What we want to find out is, is there life in the universe? If so … is it one planet in 10 or one in a million? In order to search for signs of biology or biosignatures, it’s easier if you know that there is already liquid water on the planet,” said Professor Triaud.

Julien de Wit, assistant professor of planetary sciences at the Massachusetts Institute of Technology, was a co-author on the paper and said current technology may be able to pick up on some of these more subtle clues.

The Times