When searching for life in the Universe (aka. astrobiology), 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” (or “biomarkers”), referring to evidence of biological activity and processes.
This search has been enhanced by next-generation instruments like the James Webb Space Telescope (JWST), and others will soon follow.
This includes the Habitable World Observatory (HWO), the first telescope designed specifically for astrobiology surveys (which will launch by the 2040s).
In preparation, scientists are refining their strategies for identifying these signatures on exoplanets. In a recent paper, geophysicists from the University of Chicago ran simulations to determine how telescopes like the HWO could detect oxygen gas (O2) and ozone (O3) via direct imaging studies.
Their findings indicate that the presence of clouds, contrary to expectations, could enhance the detection of these biosignatures.
The research was led by Huanzhou Yang, a Graduate Student with the Department of Geophysical Sciences at the University of Chicago. He was joined by Michelle Hu, a student with the DeMille Group atomic/molecular/optical (AMO) physics group at UChicago, and Dorian S. Abbot, a Professor of Geophysical Sciences at UChicago.
The paper that describes their findings recently appeared online and has been accepted for publication in the Astrophysical Journal (scheduled to be released in a few weeks).
The field of exoplanet studies has grown by leaps and bounds in the past 20 years, with more than 5,900 confirmed planets to date. So far, the vast majority of those detected were found using indirect methods, most notable the Transit Method (Transit Photometry) and the Radial Velocity Method (Doppler Spectroscopy).
So far, only a small percentage (1.4%) have been discovered using the Direct Imaging Method, where astronomers analyze light directly reflected by an exoplanet’s atmosphere or surface.
This is changing thanks to next-generation telescopes, like the James Webb Space Telescope (JWST) and future space- and ground-based observatories (a la HWO) 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 the Community Aerosol and Radiation Model for Atmospheres (CARMA).
This general-purpose sectional microphysics code simulates the presence of various aerosols in planetary atmospheres. They further simulated clouds with the Planetary Spectrum Generator (PSG), 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 (O2 and O3) that are well-studied, and for which the results can be transferred to other biosignatures. Said Yang:
“The potential to detect biosignature (like oxygen and ozone) depends on the exoplanets’ distance to us, the temperature of the host stars, biosignature occurrence frequency, the atmosphere condition of the exoplanets (including clouds), 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.
The implications of this research could be far-reaching for upcoming missions, which include the HWO, the Nancy Grace Roman Space Telescope (RST), and ground-based observatories like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT). 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.”
Written by Matthew Williams/Universe Today.
