Habitable worlds: New strategy to identify planets that may host life

A new study has introduced a revolutionary method for detecting habitable worlds and the presence of life beyond our planet.

The findings suggest that low carbon abundance in a planet’s atmosphere, observable by the James Webb Space Telescope (JWST), could indicate the presence of liquid water and potentially life.

The study was conducted by an international team of researchers led by the Massachusetts Institute of Technology (MIT) and the University of Birmingham. The research was focused on the composition of planetary atmospheres. 

Studying habitable worlds 

The experts propose that a terrestrial planet with significantly less carbon dioxide compared to others in the same system might harbor liquid water and possibly life.

“The Holy Grail in exoplanet science is to look for habitable worlds, and the presence of life, but all the features that have been talked about so far have been beyond the reach of the newest observatories,” said co-lead author Professor Julien de Wit.

“Now we have a way to find out if there’s liquid water on another planet. And it’s something we can get to in the next few years.”

So far, more than 5,200 worlds have been detected beyond our solar system. While we have the technology to determine whether a planet is within a habitable zone, there is no way to directly confirm whether a planet has liquid water on its surface and is indeed habitable.

The researchers noted a distinct feature of Earth compared to Venus and Mars – it has significantly lower atmospheric levels of carbon dioxide. Earth’s oceans have absorbed massive amounts of carbon dioxide over time, a process that is absent on Venus and Mars. 

“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,” said Triaud. “The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water.” 

Atmospheric carbon dioxide 

Carbon sequestration in Earth’s oceans and rocks has been key in maintaining its habitable climate. Over hundreds of millions of years, the oceans have taken up a huge amount of carbon dioxide that is nearly equal to the amount that persists in Venus’ atmosphere today.

“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 study co-author Frieder Klein. 

The team proposes that habitable worlds will exhibit a similar depletion of atmospheric carbon dioxide relative to their neighbors.

“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,” said de Wit.

Breakthrough strategy

The researchers outlined a strategy for finding habitable planets by looking for depleted carbon dioxide signatures. This approach would be particularly effective in “peas-in-a-pod” systems, where multiple terrestrial planets of similar size orbit close to each other. 

Initially, astronomers could confirm the presence of atmospheres by detecting carbon dioxide. Then, they would compare the carbon dioxide content of these planets to look for significant differences.

“Carbon dioxide is a very strong absorber in the infrared, and can be easily detected in the atmospheres of exoplanets,” explained de Wit. “A signal of carbon dioxide can then reveal the presence of exoplanet atmospheres.”

However, just because a planet is habitable does not necessarily mean that it is inhabited. To confirm whether life might actually exist, the team proposes that astronomers look for another feature in a planet’s atmosphere: ozone. 

Future research on habitable worlds

The researchers say that 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,” said Triaud. “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 NASA’s James Webb Space Telescope would be able to measure carbon dioxide, and possibly ozone, in nearby, multi-planet 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 JWST,” said de Wit. “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 is published in the journal Nature Astronomy.

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