Green or Purple Aliens
When prompted to envision an alien, what often comes to mind is a humanoid figure, perhaps with green skin. However, today’s astrobito may challenge that stereotype: seems like purple could be the new green in space!
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Paper details:
Title: Purple is the new green: biopigments and spectra of Earth-like purple world
Authors: Ligia Fonseca Coelho, Lisa Kaltenegger, Stephen Zinder, William Philpot, Taylor L. Price, Trinity L. Hamilton
First-author Institution: Department of Astronomy, Cornell University, Ithaca, USA
Status: Published in Monthly Notices of the Royal Astronomical Society
Colour green is a synonym of life, particularly evident in the flourishing ecosystems of lush tropical forests, sprawling jungles, and extensive valleys. Remarkably, the green hue of these landscapes is visible even from space, serving as a potential biosignature when searching for life on other planets.
Yet, life on Earth predates its verdant appearance. For millennia, several bacteria and microorganisms have flourish on Earth, dying the planet with a spectrum of colours shaped by the microorganisms’ metabolic processes and the conditions of early Earth. So, could the presence of colours different than green offer insights into potential forms of life on other planets?
Photosynthesis and purple bacteria
The greenery in our planet’s landscapes is a consequence of one core biological process: photosynthesis. Simply put, photosynthesis is the process of converting light into chemical energy. Plants and some microorganisms possess chlorophylls (Figure 1), which are pigments that absorb sunlight to power a series of chemical reactions that split water to produce oxygen (later released into the atmosphere) and glucose (the primary energy source for these organisms).
Like a picky eater, chlorophylls absorb light at specific wavelengths, primarily in the red and blue parts of the spectrum, reflecting green light and giving plants their characteristic colour. It is not surprising then that the astronomy and astrobiology communities have searched for “green exoplanets” as a sign for potential extraterrestrial life.
But there's another side to photosynthesis: anoxygenic photosynthesis. Certain bacteria and microorganisms also use this process, but instead of producing oxygen, they convert hydrogen sulphide into sulphur. These organisms employ another family of pigments called bacteriochlorophylls, which are structurally distinct from chlorophylls and absorb at different wavelengths, such as green, yellow, or even infrared light (Figure 1). With these spectral features, anoxygenic bacteria and microorganisms can thrive in light-limited environments like underwater or in soil.
Figure 1. Chlorophylls and bacteriochlorophylls absorb and emit light at different wavelengths due to their structural differences.
As most of the light reflected from bacteria and microorganisms using anoxygenic photosynthesis is purple (hence the name purple bacteria), there is the possibility that life beyond Earth might not be exclusively green. However, most models for terrestrial exoplanets tend to favour green pigments due to a lack of reference reflectance spectra from purple bacteria.
Purple biopigments and terrestrial exoplanets
To address the prevailing bias towards green-like pigments in the search for extraterrestrial life, the authors of this study undertook a fascinating experiment. Growing a set of 23 purple bacteria strains in the laboratory, they measured the reflectivity of their pigments to simulate the spectra of various terrestrial exoplanet environments dominated by purple bacteria. Particularly, using pigments from Gloeobacter violaceus, Blastochloris viridis, and E53 PSB, the researchers explored how these pigments influenced the overall spectra of Earth-analogue exoplanets and ocean worlds, as well as other exoplanet environments (Figure 2).
Figure 2. Modelled exoplanet spectra with biopigments from different purple bacteria. Adapted from Figure 3 in the main publication.
Their findings shed light on the distinctive spectral features of these biopigments within the modelled exoplanet spectra. Notably, bacteriochlorophyll b exhibited a pronounced peak around ~1000nm, distinct from other major bands, making it a very attractive candidate to look for when analysing future terrestrial exoplanet spectra. On the other hand, biopigments like chlorophyll-a and bacteriochlorophyll-a exhibited overlaps with certain spectral features attributed to O2 and O3, particularly below 700nm.
These results carry significant implications for the advancement of terrestrial exoplanet atmospheric modelling. They showcase the necessity of incorporating a diverse array of surface biopigments into such models, not only for the detection of potential life indicators but also for accurately identifying atmospheric constituents like O2, O3, and even H2O, whose signatures may overlap with those from surface biopigments.
New observing strategies
One key difference between chlorophylls and bacteriochlorophylls, as seen in Figure 2, lies in their light absorption capabilities. While chlorophylls predominantly absorb within the visible spectrum, bacteriochlorophylls can use red to infrared light (above 800nm) to fuel photosynthesis. This allows purple bacteria to flourish in challenging and harsh environments here on Earth.
But in the astrobiology context, this spectral characteristic could also allow purple bacteria to thrive on exoplanets orbiting M-dwarf stars. These stars, although cooler and smaller than our Sun, emit radiation that aligns with the optimal absorption and reflectance range of bacteriochlorophylls. This finding implies that while chlorophyll-based photosynthesis thrives under the Sun’s radiation, a range of purple bacteria could likewise thrive under the light of an M-dwarf star.
Whether the first sing of life beyond Earth is green, purple, or perhaps both, the astrobiology community is gathering more and more evidence of what could and could not indicate life. This reservoir of knowledge will be instrumental in shaping new observing strategies for upcoming telescopes like the Extremely Large Telescope and the Habitable Worlds Observatory, enhancing our quest for extraterrestrial life. Until then, we just need to keep doing cool science. Who knows, maybe Hollywood might find inspiration here when realising new alien movies.
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