Which exoplanets could actually have oceans? To identify Earth-like worlds in space, astronomers say they can compare the levels of carbon dioxide in their atmospheres with those of their neighboring planets. Their study shows that a planet with a significantly low level of carbon dioxide in its gaseous atmosphere indicates the possibility of liquid water or even life on its surface. The low content may be due to the gas being associated with oceans or extensive biomass. Detections of atmospheric carbon dioxide in distant planetary systems could bring promising candidates into focus for more detailed investigations, researchers say.
The view from space shows what's so special about our home planet: Sparkling blue oceans – Earth's surface features liquid water. Considering the more than 5,000 exoplanets discovered in space, the question arises whether there are more blue gems among them. It is also possible that life as we know it evolved on such water-rich exoplanets. When searching for Earth-like planets, astronomers generally focus on celestial bodies located in the so-called habitable zone around distant stars. This is the region where the orbiting planet receives a level of radiation that would theoretically make it possible for liquid water to form on its surface.
Possible – but is there really water?
However, it is still unclear whether habitable water actually exists. With current astronomy capabilities, watery surfaces on exoplanets are unrecognizable. Therefore, indirect sources of information are needed. Astronomers led by Amory Triwood from the University of Birmingham came up with their idea by taking a comparative look at rocky planets in our solar system. Venus has some similarities to Earth and Mars, and they orbit the Sun in a region of moderate radiation. However, Earth is the only planet in the trio that still has bodies of water today. Another noticeable difference is the level of carbon dioxide in the atmosphere. In our case, they are only small quantities – on Mars and Venus, however, the gas makes up more than 95 percent of the atmosphere.
“We assume that these three planets formed in a similar way, so if we now see one with much less carbon dioxide, it must have gone somewhere,” Triode says. It is clear that the intense water cycle on Earth has removed carbon dioxide from the atmosphere. Over the course of Earth's history, the oceans have swallowed up roughly the same amount as is in the atmosphere of Venus today. “On Earth, much of the atmospheric carbon dioxide is stored in seawater and rocks over geological timescales, helping to regulate climate and quality of life over billions of years,” says co-author Frieder Klein of Woods Hole Oceanographic Institution.
As part of their study, the researchers compiled relevant chemical, geological and biological forcing factors and analyzed the processes that, in the Earth's case, are associated with carbon dioxide reduction. Using model simulations, they also analyzed the extent to which other causes could be responsible for poor atmospheric carbon dioxide on planets. “We examined the possibility of false positive signals and found that all of the ones we could imagine seemed unlikely,” the scientists wrote. Therefore, the team concludes that the lack of carbon dioxide in an exoplanet compared to its neighbors is a clear indication of the presence of liquid oceans or even life on its surface.
According to the researchers, this can now be translated into a concrete strategy for searching for Earth-like planets. The systems would be better suited to this if several candidates were already known: rocky planets of approximately the same size and neighboring planets – similar to Venus, Earth and Mars. The first step then is basic detection of the atmosphere by examining the light shining through it as it passes in front of the star. Once astronomers determine that multiple planets in the system host atmospheres, they can move on to measuring their carbon dioxide levels. “CO2 is a very strong absorber in the infrared range and can be detected in exoplanetary atmospheres,” explains co-author Julian de Wit of the Massachusetts Institute of Technology in Cambridge. NASA's James Webb Space Telescope (JWST) has already demonstrated this capability.
On the trail of living environments
If the lack of carbon dioxide becomes apparent, this will be a clear indication that there are large amounts of liquid water on the surface of the exoplanet. But this alone does not indicate life. In order to obtain evidence of this, the team suggests that the identified candidates should then be specifically searched for the presence of another substance in the atmosphere: the James Webb Space Telescope can also detect ozone, which is made up of three oxygen atoms. This is indirect evidence of the presence of oxygen, which has until now been difficult to detect using spectroscopy in its diatomic form. The concrete idea is that some aliens can release oxygen during photosynthesis, which turns into ozone with photons from star radiation. “If we see ozone, the likelihood is very high that it is linked to the consumption of carbon dioxide in life,” Triaud says. “And then it also has to be planet-scale biomass that can process a significant amount of carbon.”
Scientists have already set their sights on a suitable system for applying their method: the seven known planets orbiting the bright star TRAPPIST-1, which is only 40 light-years away from Earth. “It's one of the systems through which we've been able to do studies of the Earth's atmosphere using the James Webb Space Telescope,” says de Wit. “We now have a roadmap for finding habitable planets. If we all work together, groundbreaking discoveries can be made in the next few years,” the scientist concluded.
Source: Massachusetts Institute of Technology, University of Birmingham, specialized article: Natural Astronomy, doi: 10.1038/s41550-023-02157-9
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