A carbon atmosphere could be a sign of the presence of water and life on other terrestrial planets, according to a study

Scientists at MIT, the University of Birmingham and elsewhere say astronomers’ best chance of finding liquid water, and even life on other planets, is to look for the absence, rather as the presence of a chemical characteristic in their atmosphere.

The researchers propose that if a terrestrial planet has significantly less carbon dioxide in its atmosphere compared to other planets in the same system, this could be a sign of the presence of liquid water – and perhaps life – on the surface. of this planet.

Moreover, this new signature is in the viewfinder of NASA’s James Webb Space Telescope (JWST). Although scientists have proposed other signs of habitability, these characteristics are difficult, if not impossible, to measure with current technologies. The team says this new signature, of relatively depleted carbon dioxide, is the only sign of habitability currently detectable.

“The holy grail of exoplanet science is the search for habitable worlds and the presence of life, but all the characteristics discussed so far were beyond the reach of the most recent observatories,” explains Julien de Wit, assistant professor of exoplanet science. planetary sciences at MIT. “We now have a way to find out if there is liquid water on another planet. And that’s something we can achieve in the next few years.”

The team’s findings will appear in Natural astronomy. De Wit co-led the study with Amaury Triaud of the University of Birmingham in the United Kingdom. Their co-authors at MIT include Benjamin Rackham, Prajwal Niraula, Ana Glidden Oliver Jagoutz, Matej Peč, Janusz Petkowski and Sara Seager, as well as Frieder Klein of the Woods Hole Oceanographic Institution (WHOI), Martin Turbet of the École Polytechnique in France and Franck Selsis of the Bordeaux Astrophysics Laboratory.

Beyond a glow

Astronomers have so far detected more than 5,200 worlds beyond our solar system. With current telescopes, astronomers can directly measure the distance between a planet and its star and the time it takes to complete an orbit. These measurements can help scientists determine whether a planet is in a habitable zone. But there is no way to directly confirm whether a planet is actually habitable, meaning liquid water exists on its surface.

Throughout our solar system, scientists can detect the presence of liquid oceans by observing “flares,” or flashes of sunlight that reflect off liquid surfaces. These reflections, or specular highlights, have been observed, for example, on Saturn’s largest moon, Titan, which helped confirm the presence of large lakes on the moon.

However, detecting a similar glow on distant planets is beyond the reach of current technologies. But de Wit and his colleagues realized that there is another habitable feature close to home that might be detectable on distant worlds.

“An idea came to us while looking at what is happening with terrestrial planets in our own system,” explains Triaud.

Venus, Earth, and Mars share similarities, in that all three are rocky and inhabit a relatively temperate region relative to the sun. Earth is the only planet of the trio that currently hosts liquid water. And the team noted another obvious distinction: Earth has much less carbon dioxide in its atmosphere.

“We assume that these planets were created in the same way, and if we see a planet with much less carbon now, it must have gone somewhere,” says Triaud. “The only process capable of removing this much carbon from the atmosphere is an intense water cycle involving oceans of liquid water.”

Indeed, Earth’s oceans have played a major and lasting role in absorbing carbon dioxide. Over hundreds of millions of years, the oceans have absorbed an enormous amount of carbon dioxide, almost equal to the amount that persists in Venus’ atmosphere today. This planetary-scale effect has left Earth’s atmosphere significantly depleted of carbon dioxide compared to its planetary neighbors.

“On Earth, much of atmospheric carbon dioxide has been sequestered in seawater and solid rock over geological periods, helping to regulate climate and habitability for billions of years “, explains Frieder Klein, co-author of the study.

The team reasoned that if a similar decrease in carbon dioxide was detected on a distant planet, compared to its neighbors, it would be a reliable signal of the presence of liquid oceans and life on its surface.

“Having extensively reviewed the literature in many fields ranging from biology to chemistry to carbon sequestration in the context of climate change, we believe that indeed, if we detect a decrease in carbon, “This is likely to be a strong sign of liquid water and/or life,” de Wit says.

A roadmap for life

In their study, the team presents a strategy for detecting habitable planets by looking for a signature of depleted carbon dioxide. Such research would work best for “pea-in-a-pod” systems, in which multiple terrestrial planets, all roughly the same size, orbit relatively close together, like our own solar system. The team’s first proposed step is to confirm that planets have atmospheres, simply by looking for the presence of carbon dioxide, which is expected to dominate most planetary atmospheres.

“Carbon dioxide is a very strong absorber in the infrared and can be easily detected in the atmosphere of exoplanets,” explains de Wit. “A carbon dioxide signal can then reveal the presence of exoplanet atmospheres.”

Once astronomers determine that several planets in a system harbor atmospheres, they can measure their carbon dioxide content, to see if one planet contains significantly less than the others. If so, the planet is likely habitable, meaning it has significant bodies of liquid water on its surface.

But habitable conditions don’t necessarily mean a planet is inhabited. To see if life could really exist, the team suggests astronomers look for another feature of a planet’s atmosphere: ozone.

On Earth, researchers note that plants and some microbes help attract carbon dioxide, but not as much as the oceans. However, as part of this process, life forms emit oxygen, which reacts with photons from the sun to turn into ozone, a molecule much easier to detect than oxygen itself.

Researchers say that if a planet’s atmosphere shows signs of ozone and depleted carbon dioxide, it is likely a habitable, inhabited world.

“If we observe ozone, there is a good chance that it is linked to carbon dioxide consumed by life,” says Triaud. “And if it’s life, it’s glorious life. It wouldn’t just be a few bacteria. It would be a planet-wide biomass capable of processing a huge amount of carbon and interacting with it.”

The team estimates that NASA’s James Webb Space Telescope would be able to measure carbon dioxide, and possibly ozone, in nearby multiplanet systems such as TRAPPIST-1, a system of seven planets orbiting a bright star, just 40 light years from Earth.

“TRAPPIST-1 is one of the few systems where we could perform terrestrial atmospheric studies with JWST,” says de Wit. “We now have a roadmap to finding habitable planets. If we all work together, groundbreaking discoveries could be made in the coming years.”