The challenge in the search for habitable worlds is clear. We need to be able to identify habitable worlds and distinguish between biotic and abiotic processes. Ideally, scientists would do this on entire populations of exoplanets rather than on a case-by-case basis. Exoplanets’ natural thermostats might provide a way of doing this.

“Within just a few decades, the search for potentially habitable and inhabited exoplanets has evolved from science fiction to a central scientific pursuit for the exoplanet community,” the authors of new research write. With more than 5,000 confirmed exoplanets, the scientific focus is shifting from detecting exoplanets to characterizing them. The new work shows how atmospheric carbon dioxide could play a central role in understanding exoplanets.

The new research is titled “Detecting Atmospheric CO₂ Trends as Population-Level Signatures for Long-Term Stable Water Oceans and Biotic Activity on Temperate Terrestrial Exoplanets.” It will be published in The Astrophysical Journal, and the lead author is Janina Hansen from the ETH Zurich Institute for Particle Physics & Astrophysics. The research is available on the arXiv preprint server.

Terrestrial planets like Earth have a natural thermostat called carbonate-silicate (Cb-Si) weathering feedback. The Cb-Si feedback is a geochemical cycle that regulates a planet’s atmospheric CO₂ content over long geological timescales.

When CO₂ builds up in the atmosphere, the atmosphere warms. This creates more evaporation and rainfall. Carbonic acid is a weak acid formed in the atmosphere when water combines with carbon dioxide. When a warming atmosphere creates more rain, it also creates more carbonic acid.

Carbonic acid falls on the planet’s surface, weathering silicate rocks and removing carbon. The carbon is eventually washed into the sea, where it’s taken up in the shells of marine organisms. It falls to the sediment on the ocean floor and is ultimately sequestered back into the crust with help from plate tectonics. The creatures that absorb the carbon into their shells as calcium carbonate play a key role. The carbon in their shells becomes limestone.

This process is enhanced in a warming atmosphere, meaning it eventually removes more carbon from the atmosphere until it cools and the cycle slows again. Volcanic activity can release carbon back into the atmosphere, completing the cycle. Scientists think Earth’s Cb-Si feedback has allowed our planet to maintain surface water and habitability for billions of years.

The question is, can the Cb-Si cycle be understood in terms of a population of exoplanets? If it can be, then exoplanet scientists will have a powerful new way of understanding exoplanets without spending an inordinate amount of time examining them individually. With the help of upcoming missions, the Cb-Si cycle could be the tool scientists need.

“Identifying key observables is essential for enhancing our knowledge of exoplanet habitability and biospheres, as well as improving future mission capabilities,” the researchers write. “While currently challenging, future observatories such as the Large Interferometer for Exoplanets (LIFE) will enable atmospheric observations of a diverse sample of temperate terrestrial worlds.”

The researchers explain that the Cb-Si weathering feedback is a well-known habitability marker and a potential biological tracer. The cycle creates specific CO₂ trends in terrestrial atmospheres. In their work, they explore the idea that they can identify CO₂ trends specific to biotic or abiotic planet populations. They did it by creating simulated exoplanet populations based on geochemistry-climate predictions. The exoplanets are all exo-Earth Candidates (EEC) because they’re the most conservative habitable zone planet candidates. The simulations involved EEC populations of 10, 30, 50, and 100 planets.

Their simulations include stellar flux, different F, G, and K-type stars within 20 parsecs of the sun, and various atmospheric CO₂ partial pressures. “With this, we aim to produce planet populations which remain close to an Earth-sun-like environment,” the researchers explain. The researchers then retrieved their results based on the observational power of the proposed LIFE mission, which is intended to detect atmospheric biosignatures.

“We observe a robust detection of CO₂ trends for population sizes NP ≥ 30 and all considered spectrum quality scenarios S/N = [10, 20] and R = [50, 100] in both biotic and abiotic cases,” the authors write. NP is the number of planets or population size, and S/N and R describe the quality of the atmospheric spectrum acquired by LIFE. S/N is the signal-to-noise ratio, while R is spectral resolution.

That means that Cb-Si weathering feedback trends are robustly detectable in populations of 30 or greater exo-Earth candidates, where the signal-to-noise ratio is either 10 or 20 and the spectral resolution is at least 50 or 100. S/N ratios of 10 or 20, and resolutions of 50 are modest observational capabilities.

“We demonstrate the ability of future missions like LIFE, or similar mid-infrared interferometer concepts, to enable population-level characterization of temperate terrestrial atmospheres and find that Cb-Si cycle-driven CO₂ trends, as a population-wide habitability signature, can readily be detected in a modest population of thermal emission spectra,” the authors write.

Their work had some limitations, though, which the researchers readily point out. For example, there are systematic biases in CO₂ partial pressure measurements, and those measurements are critical to identifying the trends. Their atmospheric model is also simplified and contains only H₂O, CO₂, and N₂, which are essential features of Earth’s atmosphere, but not a complete picture.

“The inclusion of additional species, such as CH₄ or O₃, would influence the self-consistent modeling of planetary atmospheres, impacting thermal structures and surface conditions,” the researchers explain.

The end result is that this method shows promise for identifying population-level CO₂ trends in populations of only 30 EECs. If scientists can do that, they can narrow down the targets worthy of in-depth study and characterization.

This is just the beginning of population-wide characterization of exoplanets and their biotic and abiotic signatures. Instead of looking for the “smoking-gun” signature of life on single worlds, we may be able to detect and identify life through large statistical patterns across numerous worlds. In that case, this work also shows how telescopes with modest observational capabilities can “filter through” the exoplanet population, sparing valuable and expensive observing time on more powerful observatories.

However, there’s still more work to do before we get to that stage. The method needs to be tested against more diverse atmospheres.

“Further studies, which test atmospheric characterization performance against broad atmospheric diversity, are essential to prepare next-generation observational facilities to provide robust and accurate constraints of atmospheric as well as planetary parameters,” the researchers explain in their conclusion.

“Efforts like these will pave the way toward assessing the commonness of habitable worlds or even global-scale biospheres outside of our solar system,” they conclude.