Looking for another Earth? Look for low carbon dioxide

What should we find if we want to discover another Earth? If an exoplanet is too far away for even the most powerful telescopes to look directly for water or certain biosignatures, is there anything else it can tell us about the possibility of habitability? The answer may be carbon dioxide.

Led by Amaury Triaud and Julien de Wit, an international team of researchers now proposes the absence of CO2 in the planet’s atmosphere potentially increases the chances of liquid water on its surface. Earth’s atmosphere is depleted of CO2. Unlike dry Mars and Venus, which have high CO concentrations2 in their atmospheres, our planet’s oceans have taken huge amounts of carbon dioxide out of the atmosphere because the gas dissolves in water. CO2 deficits in exoplanet atmospheres may mean the same.

Another molecule may be a sign of a habitable planet: ozone. Many organisms on Earth (especially plants) breathe in carbon dioxide and release oxygen. This oxygen reacts with sunlight and becomes O3, or ozone, which is more easily detected than atmospheric oxygen. The presence of ozone and the absence of carbon dioxide could mean a habitable and even inhabited planet.

Anyone or anything there?

There is a difference between a planet orbiting within what is considered the habitable zone and actual habitability. Habitability is defined by the researchers as “the capacity of a planet to retain large reservoirs of surface liquid water,” as they stated in a study recently published in Nature Astronomy.

Hypothetically, water can be proven to actually exist in many ways. The problem is that most existing telescopes, no matter how advanced, are unable to pick them all out. Finding liquid water light-years away is not as easy as seeing the shimmer of a lake, although it is possible at short distances, such as those in our own solar system. (When sunlight reflects off a surface liquid, one can see what scientists call a “glow,” which is how the lakes and oceans of Saturn’s moon Titan were discovered.)

Beyond water, there are other factors that could determine habitability. In addition to atmospheric properties, these include (but are not limited to) a planet’s orbit, plate tectonics, magnetic fields, and how it is affected by its star.

When less is more

Triaud, de Wit and their team argue that it is worth trying to identify potentially habitable planets that belong to a system similar to our own. If there is a system with several terrestrial planets that are close in size and have atmospheres, this makes it possible to compare the carbon dioxide content of their atmospheres and see if there is a significant deficit on one or more planets compared to the others.

While the CO2 the deficit is not a guarantee that there is liquid water on the surface, it should give scientists a reason to observe the planet or planets in question more closely. We don’t have to look far from Earth to see why this makes sense. Not only is most of the carbon dioxide in our planet’s atmosphere depleted by its oceans, but plate tectonics also buries it in the crust. The amount of atmospheric carbon dioxide on the early Earth that ended up trapped in the rocks was almost equal to the amount of CO2 throughout the atmosphere of Venus.

There is another advantage to looking for this deficiency. Because it is a particularly strong absorber of infrared light, CO2 is quite easy to detect. Telescopes that exist today, including NASA’s James Webb Telescope and ESO’s Very Large Telescope, as well as ESO’s upcoming Extremely Large Telescope, have infrared vision that can easily search for CO2 signed.

And what if we find a planet that shows a CO deficiency?2 and the presence of ozone? Researchers believe that the combination of the two could mean not just a few microbial life forms, but, at least hypothetically, a planet alive with organisms.

“Life on Earth shapes the planet,” the team said in the same study. “Planet-shaping life is really what astronomers are after.”

Nature Astronomy, 2023 DOI: 10.1038/s41550-023-02157-9

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