The search for extraterrestrial life could be on the verge of a breakthrough. A new method aims to find liquid water on exoplanets.
Cambridge – There are now more than 5,500 planets known outside our solar system – but there is currently no evidence of life on these exoplanets and liquid water is also rare in the universe. However, this does not stop scientists from continuing to search for it. A research team has now developed a new method to identify liquid water on exoplanets.
The researchers, who come from various universities, did not focus on distant planetary systems, but rather on our immediate cosmic neighborhood. Amaury Triaud from the University of Birmingham, one of the study leaders, explains that they found what they were looking for near Earth. “An idea came to us when we looked at what was happening to the terrestrial planets in our own system,” Triaud said in one notice.
Searching for liquid water on exoplanets: CO₂ as an indicator
Venus, Earth and Mars have some similarities: They are made of rock and orbit the sun in a relatively temperate zone. However, there are also significant differences: only Earth has liquid water, and at the same time Earth has significantly less carbon dioxide in its atmosphere than the other two planets. “We assume that these planets formed in a similar way,” says Triaud, adding: “If we now see a planet with much less carbon, it must have disappeared somewhere.”
But how can carbon dioxide disappear from the Earth’s atmosphere? “The only process that could remove so much carbon from an atmosphere is a strong water cycle with oceans of liquid water,” emphasizes the expert. Earth’s oceans have absorbed a lot of carbon dioxide over hundreds of millions of years, almost as much as is present in the atmosphere of Venus today. Therefore, Earth’s atmosphere contains significantly less carbon dioxide than that of our neighboring planets.
Carbon dioxide was bound in seawater and solid rock
“On Earth, much of the atmospheric carbon dioxide has been sequestered in seawater and solid rock over geological time, which has helped regulate Earth’s climate and habitability over billions of years,” explains Frieder Klein, co-author of the study. The researchers conducting their study in the specialist journal Nature Astronomy published have concluded that they have found a reliable signal for liquid oceans and possibly even life on the surface.
“After extensive review of the literature from many areas, from biology to chemistry to carbon sequestration in the context of climate change, we believe that if we do indeed see carbon depletion, it is most likely a clear sign of liquid water and/or or life,” says Julien de Wit from the Massachusetts Institute of Technology, also leader of the study.
Planetary systems with several Earth-like planets are particularly interesting
When searching for liquid water on exoplanets, the research team recommends first targeting planetary systems in which several Earth-like planets are located close to each other. The first step should be to check whether these planets even have atmospheres. Once multiple planets with atmospheres have been identified, the carbon dioxide levels of the atmospheres can be measured. “Carbon dioxide is a very strong absorber in the infrared and can easily be detected in the atmospheres of exoplanets,” explains de Wit.
If a planet has significantly less CO₂ in the atmosphere than the other planets, it could be habitable, i.e. have large amounts of liquid water on the surface. However, this does not necessarily mean that the planet is inhabited. According to the study, to find out, we should look for another component of the atmosphere: ozone.
Researchers are looking for carbon dioxide in exoplanet atmospheres
“When we see ozone, there is a pretty good chance that it is related to carbon dioxide being consumed by something living,” Triaud points out. “And if it is life, then it is magnificent life. It wouldn’t just be a few bacteria. It would be a planetary-scale biomass capable of processing and interacting with a huge amount of carbon.”
Researchers are confident that the James Webb Space Telescope (JWST) operated by the space organizations NASA, Esa and CSA will be able to measure carbon dioxide and possibly ozone in nearby planetary systems. This is good news for research, as de Wit explains: “Despite many early hopes, most of our colleagues had concluded that large telescopes like the JWST would not be able to detect life on exoplanets.”
“New hope” for the search for life in the universe
The scientist believes that the picture is changing with the new research results: “Our work gives new hope. By harnessing the signature of carbon dioxide, we can not only infer the presence of liquid water on a distant planet, but also find a way to identify life itself.”
The planetary system “Trappist-1” is particularly interesting for this method. It consists of seven Earth-like planets and is only about 40 light-years from Earth. The “Trappist-1” planets have already been the focus of the JWST space telescope and will continue to be studied in the future. It looks like the telescope could also look for carbon dioxide in the atmosphere there in the future – and possibly discover signs of liquid water or life. (tab)
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