Which exoplanets could actually have oceans? To identify Earth-like worlds in space, astronomers say one could compare the carbon dioxide levels in their atmospheres with those of their neighboring planets. Their study shows that a planet with remarkably low CO₂ in the gas envelope suggests that there could be liquid water or even life on its surface. The low content may be due to the gas being bound in oceans or in extensive biomass. Atmospheric CO₂ findings in distant planetary systems could thus bring the most promising candidates into focus for more detailed investigations, say the researchers.
A view from space makes it clear what characterizes our home planet: shimmering blue oceans - the earth's surface is characterized by liquid water. In view of the more than 5,000 exoplanets that have now been discovered in space, the question arises as to whether there are more blue jewels among them. Life forms as we know them could also have developed on such water-rich exoplanets. When searching for Earth-like planets, astronomers generally focus on celestial bodies in the so-called habitable zone around distant stars. This is an area in which an orbiting planet receives a level of radiation at which the formation of liquid water on its surface is theoretically possible.
Possible – but is there actually water?
However, it remains unclear whether the life-friendly water is actually there. With the current possibilities of astronomy, water surfaces cannot be recognized on exoplanets. Therefore, indirect sources of information are required. The astronomers led by Amaury Triaud from the University of Birmingham came to their idea by taking a comparative look at the rocky planets in our solar system. Venus, Earth and Mars have some similarities and orbit the Sun in a region of moderate radiation. However, Earth is the only planet in the trio that still harbors bodies of water today. Another noticeable difference is the level of carbon dioxide in the atmosphere. In our case it is only small quantities - on Mars and Venus, however, the gas forms over 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,” says Triaud. It is clear that the intensive water cycle on Earth has removed CO₂ from the atmosphere. Over the course of Earth's history, the oceans have swallowed about the same amount that is present in the atmosphere of Venus today. “On Earth, much of the atmospheric carbon dioxide has been stored in seawater and rocks over geological timescales, helping to regulate climate and life quality over billions of years,” says co-author Frieder Klein of the Woods Hole Oceanographic Institution.
A clear indication
As part of their study, the researchers compiled the relevant chemical, geological and biological influencing factors and analyzed the processes that, in the case of Earth, are associated with CO₂ reduction. Using model simulations, they also analyzed the extent to which other causes could be responsible for atmospheric carbon dioxide poverty on planets. “We examined the likelihood of false positive signals and found that all of the ones we could imagine seemed unlikely,” the scientists write. The team therefore concludes that CO₂ poverty in an exoplanet compared to its neighbors is a clear signal of liquid oceans or even life on its surface.
According to the researchers, this can now be translated into a concrete strategy for the search for Earth-like planets. Systems would be best suited for this if several candidates are already known: rocky planets of approximately the same size and neighboring ones - analogous to Venus, Earth and Mars. The first step then consists in the basic detection of atmospheres by examining the light shining through them as they transit in front of the star. Once astronomers determine that multiple planets in a system host atmospheres, they can move on to measuring their carbon dioxide levels. “Carbon dioxide is a very strong absorber in the infrared range and can be detected in the atmosphere of exoplanets,” explains co-author Julien de Wit from 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 CO₂ poverty becomes apparent, this would then be a clear indication that there are significant amounts of liquid water on the surface of an exoplanet. But that 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 JWST could also detect ozone, which consists of three oxygen atoms. This is indirect evidence of oxygen, which has so far been difficult to detect spectroscopically in its diatomic form. The concrete idea is that certain alien organisms could release oxygen during photosynthesis processes, which is converted into ozone with the photons from star radiation. “If we see ozone, the probability is quite high that it is related to the carbon dioxide consumption of life,” says Triaud. “It would then also have to be a planet-scale biomass that can process a large amount of carbon.”
The scientists have already set their sights on a suitable system for applying their method: the seven known planets that orbit the bright star TRAPPIST-1, which is only 40 light-years away from Earth. “It is one of the systems where we were able to carry out terrestrial atmospheric studies with the JWST,” says de Wit. “We now have a roadmap for finding habitable planets. If we all work together, groundbreaking discoveries could be made in the next few years,” concluded the scientist.
Source: Massachusetts Institute of Technology, University of Birmingham, specialist article: Nature Astronomy, doi: 10.1038/s41550-023-02157-9