A study reveals that a reduced-carbon atmosphere may indicate the presence of water and potential life on terrestrial planets beyond Earth.

Researchers from MIT, the University of Birmingham, and other institutions assert that the most promising strategy for astronomers to identify liquid water and potential extraterrestrial life on other planets involves focusing on the absence, rather than the presence, of a specific chemical trait in their atmospheres.

The scientists suggest that if a terrestrial planet exhibits a significantly lower concentration of carbon dioxide in its atmosphere compared to other planets in the same system, it could serve as an indicator of liquid water and the potential for life on the planet’s surface. Moreover, this novel signature is within the observational capabilities of NASA’s James Webb Space Telescope (JWST). While other indicators of habitability have been proposed, they often pose challenges and are difficult to measure with existing technologies.

The research team contends that this new signature, characterized by a relatively diminished presence of carbon dioxide, is the sole detectable marker of habitability at present. Julien de Wit, assistant professor of planetary sciences at MIT, expresses, “The Holy Grail in exoplanet science is to look for habitable worlds and the presence of life, but all the features that have been talked about so far have been beyond the reach of the newest observatories. Now we have a way to find out if there’s liquid water on another planet. And it’s something we can get to in the next few years.”

The results of the study, co-led by Julien de Wit from MIT and Amaury Triaud from the University of Birmingham in the UK, are set to be published in Nature Astronomy. The MIT team includes co-authors Benjamin Rackham, Prajwal Niraula, Ana Glidden Oliver Jagoutz, Matej Peč, Janusz Petkowski, and Sara Seager. The collaboration also involves Frieder Klein from the Woods Hole Oceanographic Institution (WHOI), Martin Turbet from Ècole Polytechnique in France, and Franck Selsis from the Laboratoire d’astrophysique de Bordeaux.

Beyond a glimmer

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

Across our own solar system, scientists can detect the presence of liquid oceans by observing “glints”—flashes of sunlight that reflect off liquid surfaces. These glints, or specular reflections, have been observed, for instance, on Saturn’s largest moon, Titan, which helped to confirm the moon’s large lakes. Detecting a similar glimmer in far-off planets, however, is out of reach with current technologies. But de Wit and his colleagues realized there’s another habitable feature close to home that could be detectable in distant worlds. “

An idea came to us, by looking at what’s going on with the terrestrial planets in our own system,” Triaud says. Venus, Earth, and Mars share similarities, in that all three are rocky and inhabit a relatively temperate region with respect to the sun. Earth is the only planet among the trio that currently hosts liquid water. And the team noted another obvious distinction: Earth has significantly less carbon dioxide in its atmosphere. “

We assume that these planets were created in a similar fashion, and if we see one planet with much less carbon now, it must have gone somewhere,” Triaud says. “The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water.” Indeed, the Earth’s oceans have played a major and sustained role in absorbing carbon dioxide. Over hundreds of millions of years, the oceans have taken up a huge amount of carbon dioxide, nearly 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 the atmospheric carbon dioxide has been sequestered in seawater and solid rock over geological timescales, which has helped to regulate climate and habitability for billions of years,” says study co-author Frieder Klein. The team reasoned that if a similar depletion of carbon dioxide were detected in a far-off planet, relative to its neighbors, this would be a reliable signal of liquid oceans and life on its surface. “After reviewing extensively the literature of many fields from biology, to chemistry, and even carbon sequestration in the context of climate change, we believe that indeed if we detect carbon depletion, it has a good chance of being a strong sign of liquid water and/or life,” de Wit says.

A roadmap to life

In their research, the team outlines a method for identifying habitable planets by searching for a distinctive signature of reduced carbon dioxide. This approach is most effective for planetary systems resembling our own solar system, where multiple terrestrial planets of similar size orbit in close proximity, akin to “peas-in-a-pod.”

The initial step proposed by the team involves confirming the presence of atmospheres on these planets by detecting carbon dioxide, which is expected to be prevalent in most planetary atmospheres. “Certainly, carbon dioxide is a potent infrared absorber and can be readily identified in exoplanetary atmospheres,” explains de Wit. “The detection of carbon dioxide serves as an indicator of the presence of exoplanet atmospheres.” Once astronomers establish that multiple planets in a system possess atmospheres, the next phase is to measure their carbon dioxide levels. If one planet exhibits a notably lower concentration compared to the others, it suggests habitability, indicating the likely presence of significant bodies of liquid water on its surface.

However, a habitable environment does not guarantee inhabited status. To ascertain the potential existence of life, the team suggests examining another feature in a planet’s atmosphere: ozone. On Earth, the researchers observe that plants and certain microbes play a role in absorbing carbon dioxide, albeit not to the extent of oceans.

As part of this process, these lifeforms release oxygen, which, upon interacting with the sun’s photons, transforms into ozone—a molecule that is more easily detectable than oxygen itself. The researchers propose that if a planet’s atmosphere displays both ozone and depleted carbon dioxide, it is likely habitable and inhabited. “If we observe ozone, there’s a high likelihood it’s linked to carbon dioxide consumption by life,” says Triaud. “

And if there’s life, it’s substantial life. It wouldn’t be just a few bacteria; it would be a planetary-scale biomass capable of processing a considerable amount of carbon and engaging with it.” The team anticipates that NASA’s James Webb Space Telescope could measure carbon dioxide and potentially ozone in nearby multiplanet systems, such as TRAPPIST-1—a seven-planet system orbiting a bright star just 40 light years from Earth. “

TRAPPIST-1 is among the limited systems where we could conduct terrestrial atmospheric studies with JWST,” notes de Wit. “Now we have a roadmap for identifying habitable planets. Collaboratively, groundbreaking discoveries could be made in the next few years.”

This article is republished from PhysORG under a Creative Commons license. Read the original article.