When it comes to detecting the presence of liquid water on planets outside the solar system, and thus the conditions necessary for life, the James Webb Space Telescope (JWST) may want to look for what's missing rather than what is is. This is what that means.
A team of researchers, including scientists from the Massachusetts Institute of Technology (MIT) and the University of Birmingham, suggest that if rocky worlds like Earth outside the Solar System have lower amounts of carbon dioxide in their atmospheres than other planets in the same system, this could be a sign that they harbor liquid water. And as we know from the formation of life on our own planet and the conditions necessary to support life here, the presence of liquid water is an important indicator of potential habitability.
While the search for key chemical components that indicate habitability on extrasolar planets, or exoplanets, is only just within the reach of current technologies, depleted carbon dioxide is a feature the JWST now wants to note.
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"The holy grail in exoplanet science is the search for habitable worlds and the presence of life, but all the features discussed so far are beyond the reach of the latest observatories," said Julien de Wit, member of the discovery team and researcher. assistant professor of planetary sciences at MIT, said in a statement. "Now we have a way to find out if there is liquid water on another planet. And that's something we can find out in the coming years."
A roadmap for discovering life on exoplanets
Currently, scientists are very good at using instruments to determine how far a planet is from its host star and therefore whether it is in that star's 'habitable zone' - defined as the region that is neither too hot nor too cold to the existence of liquid water.
However, in our own solar system, Earth, Mars and even Venus are all in the habitable zone around the sun. Yet only one of those planets currently has the capacity to support life as we know it. That means that habitability and liquid water conservation for exoplanets isn't just location, location, location. So currently, scientists don't have a robust way to confirm whether a planet is habitable or not.
Thinking about Earth, Mars and Venus and the differences between the trio, the team realized that the only one with habitability, Earth, also has an atmosphere depleted of carbon dioxide compared to its neighbors in the habitable zone.
"We assume that these planets formed in a similar way, and if we now see one planet with much less carbon, it must have gone somewhere," Triaud noted. "The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water."
Over billions of years, our planet's oceans have been responsible for depleting enormous amounts of carbon dioxide from the atmosphere, meaning there is now less of it than on Venus or Mars.
"On Earth, much of the atmospheric carbon dioxide has been fixed in seawater and bedrock on geological timescales, helping to regulate climate and habitability for billions of years," said Frieder Klein, co-author of the study and scientist at the Woods Hole. Oceanographic Institution (WHOI), the statement said.
That led the team to think that a similar depletion of carbon dioxide in an exoplanet's atmosphere could also indicate the presence of a liquid ocean.
Conducting a search with these parameters would be best suited to 'peas-in-a-pod' planetary systems that, like the Solar System, host multiple rocky or terrestrial worlds of similar size orbiting their star at similar distances .
The first step in the team's proposed research is to look for carbon dioxide and use it as an indicator that the exoplanet targets have an atmosphere. Once it is determined that multiple planets in a single system have atmospheres, the next step would be to determine exactly how much carbon dioxide is in the atmosphere.
This should reveal whether one or more of the planets has significantly less carbon dioxide than the others, indicating that it likely has oceans of liquid water and could therefore be habitable.
Of course, this method involves a little more than just comparing the amounts of carbon dioxide. 'Habitability' is not the same as 'inhabited'. To check whether life actually exists on an exoplanet characterized by a lack of carbon dioxide, the team proposes to look for another molecule: ozone.
Made up of three oxygen atoms, ozone is a molecule created when life forms such as plants and microorganisms take carbon dioxide from the Earth's atmosphere and then emit oxygen molecules that are struck by sunlight. Ozone is a good proxy for these processes on alien worlds because it is easier to detect in the atmospheres of distant exoplanets than oxygen itself.
The team says that if a planet's atmosphere shows signs of depleted carbon dioxide combined with an abundance of ozone, it could be either habitable or inhabited.
"If we see ozone, there's a pretty good chance it's related to carbon dioxide consumed by life," Triaud added. "And if it is life, it is glorious life. It wouldn't just be a few bacteria. It would be a planetary-scale biomass capable of processing and interacting with an enormous amount of carbon."
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The researchers believe that the JWST is already capable of measuring the amounts of carbon dioxide and ozone in multi-planet systems close to Earth.
This includes the TRAPPIST-1 system, which is located 40 light-years away and is known to host seven Earth-like planets, several of which are in the habitable zone of their cool star.
"TRAPPIST-1 is one of the few systems with which we can conduct terrestrial atmospheric studies with JWST," concludes De Wit. "Now we have a roadmap for finding habitable planets. If we all work together, paradigm-changing discoveries can be made in the years to come."
The team's research was published December 28 in the journal Nature Astronomy.
