Planets too close to their star are too hot (such as Venus), those too far, are too cold (like Mars), whereas planets in the habitable zone are just right. Whilst there has been much effort in identifying planets in the theoretical habitable zones of their stars, until now there was no way of knowing whether they truly have liquid water. Now, astronomers from the University of Birmingham, MIT and elsewhere have shown that if an exoplanet has a reduced amount of carbon dioxide in its atmosphere compared to neighboring planets, it suggests there is liquid water on that planet’s surface.
Astronomers have so far detected more than 5,200 extrasolar worlds. With current telescopes, they can directly measure a planet’s distance to its star and the time it takes it to complete an orbit.
Those measurements can help scientists infer whether a planet is within a habitable zone.
But there’s been no way to directly confirm whether a planet is indeed habitable, meaning that liquid water exists on its surface.
Across our own Solar System, astronomers can detect the presence of liquid oceans by observing glints — flashes of sunlight that reflect off liquid surfaces.
These glints, or specular reflections, have been observed, for instance, on Saturn’s largest moon, Titan, which helped to confirm the moon’s large lakes.
Detecting a similar glimmer in far-off planets, however, is out of reach with current technologies.
But MIT astronomer Julien de Wit, University of Birmingham astronomer Amaury Triaud and their colleagues realized there’s another habitable feature close to home that could be detectable in distant worlds.
“An idea came to us, by looking at what’s going on with the terrestrial planets in our own system,” Dr. Triaud said.
Venus, Earth, and Mars share similarities, in that all three are rocky and inhabit a relatively temperate region with respect to the Sun.
Earth is the only planet among the trio that currently hosts liquid water. And the researchers noted another obvious distinction: Earth has significantly less carbon dioxide in its atmosphere.
“We assume that these planets were created in a similar fashion, and if we see one planet with much less carbon now, it must have gone somewhere,” Dr. Triaud said.
“The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water.”
Indeed, the Earth’s oceans have played a major and sustained role in absorbing carbon dioxide.
Over hundreds of millions of years, the oceans have taken up a huge amount of carbon dioxide, nearly equal to the amount that persists in the Venusian atmosphere today.
This planetary-scale effect has left Earth’s atmosphere significantly depleted of carbon dioxide compared to its planetary neighbors.
“On Earth, much of the atmospheric carbon dioxide has been sequestered in seawater and solid rock over geological timescales, which has helped to regulate climate and habitability for billions of years,” said Dr. Frieder Klein, a researcher at the Woods Hole Oceanographic Institution.
The astronomers reasoned that if a similar depletion of carbon dioxide were detected in a far-off planet, relative to its neighbors, this would be a reliable signal of liquid oceans and life on its surface.
“After reviewing extensively the literature of many fields from biology, to chemistry, and even carbon sequestration in the context of climate change, we believe that indeed if we detect carbon depletion, it has a good chance of being a strong sign of liquid water and/or life,” Dr. de Wit said.
In the study, the researchers lay out a strategy for detecting habitable planets by searching for a signature of depleted carbon dioxide.
Such a search would work best for ‘peas-in-a-pod’ systems, in which multiple terrestrial planets, all about the same size, orbit relatively close to each other, similar to our own Solar System.
The first step the scientists propose is to confirm that the planets have atmospheres, by simply looking for the presence of carbon dioxide, which is expected to dominate most planetary atmospheres.
“Carbon dioxide is a very strong absorber in the infrared, and can be easily detected in the atmospheres of exoplanets,” Dr. de Wit said.
“A signal of carbon dioxide can then reveal the presence of exoplanet atmospheres.”
Once astronomers determine that multiple planets in a system host atmospheres, they can move on to measure their carbon dioxide content, to see whether one planet has significantly less than the others.
If so, the planet is likely habitable, meaning that it hosts significant bodies of liquid water on its surface.
But habitable conditions don’t necessarily mean that a planet is inhabited. To see whether life might actually exist, the authors propose that astronomers look for another feature in a planet’s atmosphere: ozone.
On Earth, plants and some microbes contribute to drawing carbon dioxide, although not nearly as much as the oceans. Nevertheless, as part of this process, the lifeforms emit oxygen, which reacts with the Sun’s photons to transform into ozone, a molecule that is far easier to detect than oxygen itself.
If a planet’s atmosphere shows signs of both ozone and depleted carbon dioxide, it likely is a habitable, and inhabited world.
“If we see ozone, chances are pretty high that it’s connected to carbon dioxide being consumed by life,” Dr. Triaud said.
“And if it’s life, it’s glorious life. It would not be just a few bacteria. It would be a planetary-scale biomass that’s able to process a huge amount of carbon, and interact with it.”
The team estimates that the NASA/ESA/CSA James Webb Space Telescope would be able to measure carbon dioxide, and possibly ozone, in nearby, multiplanet systems such as TRAPPIST-1, a seven-planet system that orbits a bright star, just 40 light-years from Earth.
“TRAPPIST-1 is one of only a handful of systems where we could do terrestrial atmospheric studies with Webb,” Dr. de Wit said.
“Now we have a roadmap for finding habitable planets. If we all work together, paradigm-shifting discoveries could be done within the next few years.”
The study was published in the journal Nature Astronomy.
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A.H.M.J. Triaud et al. Atmospheric carbon depletion as a tracer of water oceans and biomass on temperate terrestrial exoplanets. Nat Astron, published online December 28, 2023; doi: 10.1038/s41550-023-02157-9