Planets that are too close to their star (such as Venus) are too hot, and planets that are too far away (such as Mars) are too cold, but planets that are within the habitable zone have just the right temperature. Although great efforts have been made to identify planets in the theoretical habitable region of stars, until now there has been no way to know whether a planet really has liquid water. Now, astronomers from the University of Birmingham and the Massachusetts Institute of Technology have found that if an exoplanet’s atmosphere has less carbon dioxide than its neighbors, it may have liquid water on its surface. It was shown that it was suggested.

Astronomers have detected more than 5,200 extrasolar worlds so far. Modern telescopes allow us to directly measure the distance from a planet to a star and the time it takes to complete one revolution.

These measurements help scientists infer whether a planet is within its habitable zone.

However, there was no way to directly confirm whether a planet was truly habitable, i.e. whether there was liquid water on its surface.

Throughout our solar system, astronomers can detect the presence of liquid oceans by observing glints, flashes of sunlight reflecting off liquid surfaces.

These glows, or specular reflections, have been observed, for example, on Saturn’s largest moon, Titan, and helped identify the moon’s large lakes.

However, detecting similar glows on distant planets is not possible with current technology.

But astronomer Julien de Witt of the Massachusetts Institute of Technology, astronomer Amaury Tryaud of the University of Birmingham and colleagues believe there is another habitable landform close to home that could be detected far away. I noticed something.

“Looking at what was happening to terrestrial planets in our own star system gave us an idea,” Tryaud said.

Venus, Earth, and Mars share similarities in that all three are rocky and live in relatively temperate regions relative to the Sun.

Earth is the only planet of the three that currently has liquid water. Researchers then noted another clear difference. That means there is significantly less carbon dioxide in Earth’s atmosphere.

“We think these planets formed in a similar way, and if we find a planet with less carbon than it does now, it must have gone somewhere else,” Tryaud said.

“The only process that can remove this much carbon from the atmosphere is a strong water cycle involving oceans of liquid water.”

In fact, Earth’s oceans have played a major and persistent role in absorbing carbon dioxide.

For hundreds of millions of years, the ocean has absorbed enormous amounts of carbon dioxide. This is about the same amount that remains in Venus’ atmosphere today.

This planetary effect has resulted in Earth’s atmosphere being significantly depleted in carbon dioxide compared to neighboring planets.

Dr. Frieder Klein, a researcher at the Woods Hole Oceanographic Institution, said: “On Earth, much of the carbon dioxide in the atmosphere is sequestered in seawater and solid rock over geological timescales; It has helped regulate climate and habitability for billions of years.” .

Astronomers reasoned that if a similar decrease in carbon dioxide was detected on a distant planet compared to a nearby planet, this would be a reliable signal of a liquid ocean and life on its surface. did.

“After an extensive review of the literature in many fields, from biology to chemistry to carbon sequestration related to climate change, we found that if carbon depletion is indeed detected, it is likely that liquid water and its effects are “We think this is likely a strong indication that this is a sign of life.” Dr. de Witt said.

In the study, the researchers developed a strategy to detect habitable planets by looking for traces of depleted carbon dioxide.

Such searches are ideal for “pea” systems, like our solar system, in which multiple terrestrial planets, all about the same size, orbit relatively close to each other.

The first step, the scientists suggest, is to confirm that a planet has an atmosphere by simply looking for the presence of carbon dioxide, which is expected to dominate the atmospheres of most planets.

“Carbon dioxide is a very strong absorber of infrared light and can be easily detected in the atmospheres of exoplanets,” Dr. de Witt said.

“The carbon dioxide signal could reveal the presence of an exoplanet’s atmosphere.”

Once astronomers determine that multiple planets in a system have atmospheres, they move on to measuring their carbon dioxide content to see if one planet has significantly less than the others.

If so, this planet is likely habitable, which means there is a large amount of liquid water on its surface.

However, habitable conditions do not necessarily mean that the planet is inhabited. To confirm whether life actually exists, the authors suggest that astronomers look for another feature in the planet’s atmosphere: ozone.

On Earth, plants and some microorganisms contribute to absorbing carbon dioxide, although to a lesser extent than the oceans. Nevertheless, as part of this process, living organisms release oxygen, which reacts with solar photons and turns into ozone. Ozone is a much easier molecule to detect than oxygen itself.

If a planet’s atmosphere shows signs of both ozone and carbon dioxide depletion, the planet may be habitable and inhabited by humans.

“If you see ozone, there’s a pretty good chance it’s related to carbon dioxide being consumed by life forms,” ​​says Tryaud.

“And if it’s life, it’s glorious life. It won’t be just a few bacteria. It’ll be a planetary-scale biomass that can process and interact with large amounts of carbon.”

The researchers believe NASA/ESA/CSA’s James Webb Space Telescope can measure carbon dioxide, and possibly ozone, in nearby multiplanetary systems like TRAPPIST-1, a seven-planet system orbiting a bright star. I’m guessing it is. Light years from Earth.

“TRAPPIST-1 is one of the few systems that can use the Web to study Earth’s atmosphere,” said Dr. de Witt.

“We now have a roadmap to finding habitable planets. If we all work together, we may make paradigm-changing discoveries within the next few years.”

of study It was published in the magazine natural astronomy.

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AHMJ triode other. Atmospheric carbon depletion as a tracer of water oceans and biomass in temperate terrestrial exoplanets. Nat Astron, published online on December 28, 2023. doi: 10.1038/s41550-023-02157-9