In a new study, an international group of scientists has shown that the lack of carbon in an exoplanet’s atmosphere compared to other bodies in the system can be considered a clear sign of the presence of liquid water on the surface. The important thing is that this method of searching for inhabited worlds can already be used today.

More than 5,200 exoplanets and nearly 10,000 candidates are known today. The launch of the James Webb Space Telescope and the construction of the ELT observatory triggered a qualitative transition in the field of studying these other worlds: from a simple survey to the study of the properties of exoplanets.

Now, by observing how an exoplanet passes in front of a star (called a transit), astronomers can draw conclusions about the temperature difference, pressure, and composition of its atmosphere, if any. With sufficient observational accuracy, it is possible to recognize isotopes of elements, measure wind speed, and study the rarefied exosphere.

So far these methods have been applied mainly to gas giants flying close to their stars. But with the help of theoretical calculations and simulations, scientists are looking for ways to use them to study Earth-type exoplanets.

Based on the results of such a study, James Webb is able to study carbon dioxide and water in the atmospheres of the nearest Earth-like planets with moderate surface conditions in just ten passes. For example, the planets in the TRAPPIST-1 system.

But “James Webb” needs more than a thousand passes to detect oxygen O2, a well-known biological signature indicating the presence of life. In fact, he will have to spend all his working time on this object.

Another difficult problem is the presence of large amounts of liquid water on the surface of the exoplanet. Within the solar system, scientists look for it by looking at the reflection on the surface, reflected sunlight. This specifically confirmed the presence of liquid on the surface of Titan, Saturn’s largest moon. Of course, the power of our telescopes will not be sufficient for such observations in other systems. But it was the search for life in the nearest objects that led the authors of the new study to the idea of ​​carbon dioxide.

Venus, Earth, and Mars are similar in many ways in composition and location relative to the Sun. But only Earth has liquid water, and only Earth has noticeably less carbon dioxide in its atmosphere – 0.04% compared to 95% on Venus and Mars. Moreover, the decrease in carbon dioxide content is not a modern feature of our planet. Four billion years ago (the age of the Earth is about 4.5 billion years) the level of CO2 in the atmosphere was about 10%, and 2.5 billion years ago – 2.5%.

“We assume these planets formed similarly, and if we see that one of the planets today has significantly less carbon, that means it’s gone somewhere. The only process that can remove that much carbon from the atmosphere is a powerful water cycle, in which all liquid water oceans must be involved.” explains Professor Amaury Triaud (Amaury Triaud) from the University of Birmingham (Great Britain). authors of the work.

Therefore, an international group of scientists from the Massachusetts Institute of Technology (USA), the University of Birmingham (UK) and other institutes in France and the US examined the potential for using CO2 deficiency as a sign of exoplanet habitation. The results of the research were published in the journal Nature Astronomy and they gave hope.

The main “scrubber” of carbon dioxide in the atmosphere is liquid water in which carbon dioxide is dissolved. And the carbonates formed in its foundation are then “locked” into the crust and mantle of the planet. Therefore, not enough carbon returns to the atmosphere for a Venus- or Mars-type atmosphere.

The two main biological ways for life to “suck” carbon from the Earth’s atmosphere are photosynthesis and crust production. Biology accounts for about 20% of our planet’s carbon cycle, so other signatures, such as the presence of ozone, are needed to confirm its impact.

Based on the analysis, the authors developed a strategy for searching for inhabited worlds. According to their calculations, observational data from ten transits would be needed to find the atmospheres of exoplanets in the systems of interest. Forty passes to estimate the carbon dioxide deficiency on any planet. And hundreds of passes to estimate the amount of ozone, methane and carbon monoxide. Based on these data, it will be possible to determine the cause of carbon deficiency (the existence of life, liquid water or both).

Today, the James Webb space telescope can begin looking for carbon dioxide deficiency in Earth-type planets in late red dwarfs, according to the study’s authors.

“Only TRAPPIST-1 and a few other systems are suitable for studying the atmospheres of terrestrial planets with James Webb. And now we have a plan to find habitable worlds there. If we work together, we will be able to achieve groundbreaking discoveries within a few years,” said Julien de Wit, associate professor at the Massachusetts Institute of Technology (USA), one of the authors of the paper.