Are you looking for another land? Look for low carbon dioxide

What would we need to find if we wanted to discover another Earth? If an exoplanet is too far away for even the most powerful telescopes to directly look for water or some biosignature, is there anything else that might tell us about its potential habitability? The answer could be carbon dioxide.

Led by Amaury Triaud and Julien de Wit, an international team of researchers now suggests that the absence of carbon dioxide₂ In the planet's atmosphere would likely increase the chances of liquid water being present on its surface. Earth's atmosphere Depleted of carbon dioxide₂. Unlike dry Mars and Venus, which contain high concentrations of carbon dioxide₂ In their atmosphere, our planet's oceans take in huge amounts of carbon dioxide from the atmosphere because the gas dissolves in water. a company₂ A shortage of exoplanetary atmospheres could mean the same thing.

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

Is there anyone – or anything – there?

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

Proving that water actually exists can be done theoretically in several ways. The problem is that most existing telescopes, no matter how advanced, are unable to achieve all of these goals. 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 within our solar system. (When sunlight reflects off a surface liquid body, what scientists refer to as “scintillation” can be seen, which is how lakes and oceans were discovered on Saturn's moon Titan.)

In addition to water, there are other factors that can determine habitability. Besides the properties of the atmosphere, these include (but are not limited to) the planet's orbit, plate tectonics, magnetic fields, and how they are affected by its star.

When less is more

Triaud, De Wit and their team believe 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 similar-sized terrestrial planets that have atmospheres, this makes it possible to compare the carbon dioxide content of their atmospheres and see if one or more planets have a significant deficit compared to the other.

While company₂ The deficit does not guarantee the presence of liquid water on the surface, but rather should give scientists a reason to closely monitor the planet or planets in question. We don't have to look far from Earth to see why this makes sense. Not only has most of the carbon dioxide in our planet's atmosphere been depleted by the oceans, but tectonic plates have also buried it in the Earth's crust. The amount of carbon dioxide in early Earth's atmosphere that ended up trapped in rocks is roughly equal to the amount of carbon dioxide₂ In the entire atmosphere of Venus.

There is another advantage to looking for in this deficit. Because it is a strong absorber of infrared radiation, CO₂ It's easy to spot. Today's telescopes, including NASA's James Webb Telescope and the European Southern Observatory's Very Large Telescope, as well as the European Southern Observatory's upcoming Very Large Telescope, have infrared vision that can easily search for carbon dioxide.₂ Signatures.

So what if we found a planet that showed a lack of carbon dioxide?₂ And the presence of ozone? The researchers believe that combining the two means not only a few microbial life forms, but, at least in theory, a planet inhabited by living organisms.

“Life on Earth shapes the planet,” the team said He said In the same study. “Planet-forming life is actually what astronomers are after.”

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