A team of geophysicists from the University of Chicago showed how clouds on exoplanets could enhance the search for biosignatures. Their findings could have significant implications for the Habitable Worlds Observatory (HWO) and other next-generation telescopes that will study exoplanets via direct imaging.
An Artist's concept shows Earth-like world with clouds and liquid surface water orbiting a red dwarf star. Credit: Lynette Cook When searching for life in the Universe , scientists use strategies that go by various names.
For example, there's the "follow the water" approach, looking for signs of oxygen gas, carbon dioxide, methane, ammonia, and other compounds associated with life here on Earth. Collectively, these are known as "biosignatures" , referring to evidence of biological activity and processes. This search has been enhanced by next-generation instruments like the, the first telescope designed specifically for astrobiology surveys . In preparation, scientists are refining their strategies for identifying these signatures on exoplanets. In a, geophysicists from the University of Chicago ran simulations to determine how telescopes like the HWO could detect oxygen gas via direct imaging studies. Their findings indicate that the presence of clouds, contrary to expectations, could enhance the detection of these biosignatures., a Professor of Geophysical Sciences at UChicago. The paper that describes their findings recently appeared online and has been accepted for publication in theYour browser does not support HTML5 video. and future space- and ground-based observatories that employ coronographs and spectrometers. Whereas coronographs block out light from parent stars, enabling astronomers to study light reflected directly from the atmospheres or surfaces of orbiting exoplanets, spectrometers allow astronomers to detect absorption features that show what chemical signatures are present, allowing them to place tighter constraints on planetary habitability. Thanks to these sophisticated instruments and thousands of exoplanets available for study, the field is transitioning from discovery to characterization. However, clouds are often considered a barrier for detecting biosignatures on exoplanets, which is surprising considering that they are part of Earth's water cycle and intimately related to our planet's habitability. As Yang told Universe Today via email: "The detection for atmospheric components on exoplanets is based on the fact that the radiative absorption by gas is highly sensitive to wavelengths, which is a unique feature for each gas species. This allows us to analyze what components are present. The optical features of clouds are less sensitive to wavelength than gas absorption because it’s mainly contributed by Mie-scattering. Also, clouds are very opaque compared to gas. They therefore universally block absorption features of the gases in every wavelength, weakening our ability to detect the gas species." As Yang added, this applies to transit detections, where astronomers analyze light transmitted through an exoplanet's atmosphere during a transit. But for direct imaging studies, clouds could increase the observational signal of biosignatures by increasing reflected light. The high reflectivity of clouds allows more photons to be detected by telescopes, which can compensate for their effect of blocking the gas absorption information. To assess this potential, Yang and his team conducted simulations using theThis general-purpose sectional microphysics code simulates the presence of various aerosols in planetary atmospheres. They further simulated clouds with the, a radiative transfer model suite that synthesizes spectra from planetary atmospheres and surfaces based on varied planetary parameters. For their purposes, they selected two simple biosignatures that are well-studied, and for which the results can be transferred to other biosignatures. Said Yang: "The potential to detect biosignature depends on the exoplanets’ distance to us, the temperature of the host stars, biosignature occurrence frequency, the atmosphere condition of the exoplanets , etc.,. The researchers can make relatively good assumption for the some of these factors, but clouds are highly uncertain. Some works therefore use cloud-free scenarios to evaluate this potential. With our work, we can confidently use these cloud-free scenarios as a lower-bound for the predicted detections of oxygen and ozone.. As Yang stated, one of the most notable takeaways from their results is the way it provides additional confidence in direct-imaging surveys and their ability to detect biosignatures on exoplanets: "These surveys, compared to transit surveys which observe mostly planets around M-stars, are better for observing larger planets orbiting hotter stars like the Earth in the solar system," he said. "These planets are more likely to have atmospheres and are better candidates for habitable planets. Second, further analysis on some transit survey targets is limited because of the existence of clouds. Now we can see this as an advantage for direct imaging surveys and put priority on these targets. This can potentially help observers select their targets." Matt Williams is a space journalist, science communicator, and author with several published titles and studies. His work is featured in
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