A carbon-lite atmosphere could be a sign of habitable planet

Detecting conventional signs of habitable or inhabited environments in exoplanets, like an ocean glint or abundant atmospheric O2, is challenging with current observatories. A new proposed signature is a low carbon abundance in the atmosphere of a temperate rocky planet relative to others in the same system, indicating the presence of liquid water, plate tectonics, and biomass.

In a recent study, researchers from MIT, the University of Birmingham, and other institutions suggest that significantly less carbon dioxide in a terrestrial planet’s atmosphere than others in the system could indicate the presence of liquid water and life on its surface. This new signature is observable by NASA’s James Webb Space Telescope (JWST).

Amaury Triaud of the University of Birmingham in the UK said, “An idea came to us by looking at what’s going on with the terrestrial planets in our system.”

“Earth is the only planet among the trio that currently hosts liquid water. And the team noted another obvious distinction: Earth has significantly less carbon dioxide in its atmosphere.”

“We assume that these planets were created similarly, and if we see one planet with much less carbon now, it must have gone somewhere. The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water.”

Study co-author Frieder Klein said, “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.”

The team hypothesized that if a distant planet showed a significant reduction in carbon dioxide compared to its neighboring planets, it would be a dependable indicator of liquid oceans and potentially life on its surface.

After extensively reviewing literature across various fields, from biology to chemistry, and even carbon sequestration in the context of climate change, researchers believe that detecting carbon depletion has a high likelihood of being a strong indication of liquid water and life.

In their study, the team outlines a strategy for identifying habitable planets by searching for a depleted carbon dioxide signature. This approach is most effective in “peas-in-a-pod” systems, resembling our solar system, where multiple terrestrial planets of similar size orbit closely. The initial step is confirming the presence of atmospheres on these planets, primarily by detecting carbon dioxide, which is expected to dominate most planetary atmospheres.

Once it’s established that multiple planets have atmospheres, the next step is to measure their carbon dioxide content. If one planet shows significantly less carbon dioxide than the others, it’s likely habitable, indicating substantial bodies of liquid water on its surface.

However, habitable conditions don’t necessarily mean a planet is inhabited. To assess the potential for life, the team suggests astronomers also look for another atmospheric feature: ozone.

The researchers propose that if a planet’s atmosphere exhibits both ozone and depleted carbon dioxide, it is likely a habitable and inhabited world.

Triaud said, “If we see ozone, chances are pretty high that it’s connected to carbon dioxide consumed by life. And if it’s life, it’s a 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.”

Journal Reference:

Triaud, A.H.M.J., de Wit, J., Klein, F. et al. Atmospheric carbon depletion as a tracer of water oceans and biomass on temperate terrestrial exoplanets. Nat Astron (2023). DOI: 10.1038/s41550-023-02157-9

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