Carl Sagan, along with co-author Edwin Salpeter, famously published a paper in the 70s about the possibility of finding life in the cloud of Jupiter. They specifically described “sinkers, floaters, and hunters” that could live floating and moving in the atmosphere of our solar system’s largest planet. He also famously talked about how clouds on another of our solar system’s planets – Venus – obfuscated what was on the surface, leading to wild speculation about a lush, Jurassic Park-like world full of life, just obscured by clouds. Venus turned out to be the exact opposite of that, but both of those papers show the impact clouds can have on the Earth for life. A new paper by authors as the Carl Sagan Institute, led by Ligia Coelho of Cornell, argues that we should look at clouds as potential habitats for life – we just have to know how to look for it.
The search for life on exoplanets has so far focused on two methods. One is to look for specific types of atmospheric gases, such as oxygen or methane, in their atmosphere. Another is to look at the spectra of their surface signatures – specifically a “red edge” that appears when vegetation is observed spectrographically, despite its traditional “green” color. Clouds have only been a hindrance to the search for either of these methods, so astrobiologists have typically ignored them, up until now.
It turns out there are indeed microorganisms that call the Earth’s upper atmosphere home. These microbes, which include species like Modestobacter, Roseomonas, and Micrococcus, float in the atmosphere between 21-29 km. To survive at that height, they have to withstand a lot of UV radiation, and to do so they produce very bright pigments particularly in the form of carotenoids. These organic molecules show up as pigments visible in the body of the microbe – specifically in very bright colors, like pink and yellow.
Carl Sagan’s talking about potential life on Jupiter during an episode of Cosmos. Credit – Brian Emerson YouTube Channel / Carl Sagan
Bright colors are one thing that spectroscopy is good at detecting, so that is what the authors decided to do. They cultivated samples of these microbes that had been isolated from the atmosphere, and created spectra of the pigments they produce. Essentially they collected data that would be visible to a space-based telescope observing an exoplanet from afar – like JWST or the upcoming Habitable Worlds Observatory (HWO). And they did so in two different states – “wet” and “dry” – to ensure they were representing all potential pigmentation states of the microbes themselves.
With this data, the authors turned to Exo-Prime II – a model used to simulate spectra of exoplanets. They set up a model of several types of Earth-like planets, including a “snowball world”, which would be very dry but still have plenty of water, and an “ocean world” which would be saturated with water but with little land. They then introduced a cloud layer, and also introduced colonies of the various microbes to that cloud layer, and looked at the different spectral outputs for each scenario.
There was a distinct difference between “wet” and “dry” microbes. Wet ones had very distinct features, with distinct spectral lines. Dry had higher overall reflectivity. But both were noticeably different from a baseline with no microbes in the clouds at all. However, the amount of colonization of the microbes does make a huge difference, though the amount of cloud cover doesn’t. In the model, the microbes had to have colonized at least 50% of the cloud cover – even if that cloud cover only covered 50% of the planet itself.
Video describing how some of the microbes end up in the atmosphere. Credit – bioGraphicMagazine YouTube Channel
That is a pretty significant amount of colonization – and nowhere near the level we have on Earth. So, while the paper proves that, at least in theory, we would be able to directly detect the spectral signatures of these microbes in the clouds of an exoplanet, the colonization level of those clouds would have to be extremely high for us to successfully do so.
Perhaps most importantly, the paper provides a database of spectral signatures to look for when missions like HWO finally take off in the coming decades. While the colonization level might be relatively high, there is certainly a possibility that we would find at least one such exoplanet out there among the stars. If we do, it’s safe to say Carl Sagan would be proud.
Learn More:
Cornell – How to spot life in the clouds on other worlds
L. F. Coelho et al – Colors of Life in the Clouds: Biopigments of Atmospheric Microorganisms as a New Signature to Detect Life on Planets like Earth