Colors of Life in the Clouds: Biopigments of atmospheric microorganisms as a new signature to detect life on planets like Earth

TL;DR

Detecting life on Earth-like exoplanets, especially in planetary clouds, remains challenging. The authors measure the first reflectance spectra of biopigments from seven cloud-derived microbial strains under wet and dry conditions and incorporate these signatures into Exo-Prime II–based simulations at Habitable Worlds Observatory resolution. They identify UV-protective pigment features in the 400–600 nm range and show that aerial biota in clouds can modify the planet's reflected light under various cloud scenarios, offering a new biosignature channel. This work expands the search for life to atmospheric ecosystems and provides spectral references to guide future cloud-focused observations with the Habitable Worlds Observatory.

Abstract

When Carl Sagan and Ed Salpeter envisioned potential Sinkers, Floaters, and Hunters living in Jupiter's clouds in 1976 (C. Sagan & E. E. Salpeter 1976), the nature of life in Earth's atmosphere remained widely unknown. Decades later, research has revealed a remarkable variety of microorganisms in our atmosphere. However, the spectral features of airborne microbes as biomarkers for detecting atmospheric life remained a mystery. Here, we present the first reflectance spectra of biopigments of atmospheric microorganisms based on laboratory cultivars of seven microbial strains isolated from Earth's atmosphere. We show their distinct UV-resistant biosignatures and their impacts on models of diverse planetary scenarios, using Habitable Worlds Observatory (HWO) parameters. The reflectance of these biopigments from aerial bacteria creates the means to detect them on other Earth-like planets. It provides a paradigm shift that moves the search for life beyond the surface of a planet to ecosystems in atmospheres and clouds.

Figures (3)

• Figure 1: Relative reflectance spectra of seven aerial microorganisms of wet (top panel) and dry (middle panel) measurements, and absolute reflectance (bottom panel - dashed lines for dry biota; solid lines for wet biota) with zoomed-in visible range wavelength panel on the right. Control (black lines in the bottom panel) refers to a filter with culture medium only, where wet control refers to fresh medium and dry control refers to dry medium. Biopigments show features between 400 and 600 nm (see "Zoom" panels), water absorption features at 1490 nm and 1900 nm are indicated by vertical blue bands. (Data set available on https://doi.org/10.5281/zenodo.17196858).
• Figure 2: Modeled reflectance spectra of different Earth-like planets: ccean worlds (left column), snowball planets (right column), both with clouds covered by strain L9-9A-1 (see Table \ref{['tab:strain_info']} and Figure \ref{['fig:general']}), resampled for an HWO resolution of R=140. Purple lines represent planets with aerial biota in clouds. Top row: 100% clouds with 100% covered by aerial biota. Middle row: 100% clouds with 50% covered by aerial biota. Bottom row: 50% clouds with 50% covered by aerial biota. Dashed gray lines show "lifeless cloudy planet", solid blue shows "lifeless ocean planet", and solid pink shows "lifeless snowball planet" scenarios. Atmospheric features are labeled O$_2$ at 690, 760, and 1260 nm, and H$_2$O at 1490 and 1900 nm. Individual spectra of albedos are shown in Figure \ref{['fig:appendix']}.
• Figure 3: Individual spectra of surface albedos used in Figure \ref{['fig:general2']}.