In just over three decades, our knowledge has expanded from the limited understanding of the planets within our solar system to the detection of over 5,000 distant exoplanets scattered throughout the vast cosmos. Despite this remarkable progress, none of these newfound worlds bear a resemblance to our own Earth. However, if an Earth-like exoplanet were to exist in orbit around a distant star, it might exhibit a striking deviation from the familiar pale blue appearance—it could potentially sport a bold hue of purple.
What Prompted the Return 28 Years Later?
A team of scientists affiliated with the Carl Sagan Institute has advocated for the exploration of purple-hued worlds in the quest for extraterrestrial life. In a recent publication in the Monthly Notices of the Royal Astronomical Society: Letters, these researchers posit that purple bacteria could flourish on alien planets under diverse environmental conditions, potentially emitting a unique “light fingerprint.”
Lisa Kaltenegger, the director of the Carl Sagan Institute and a co-author of the study, emphasized the importance of establishing a comprehensive database of potential life indicators to ensure that our telescopic observations are not limited to life forms resembling those found on Earth.
Purple bacteria, which are photosynthetic microorganisms capable of surviving in environments with minimal visible light and oxygen, can be found in various habitats on Earth such as shallow waters, coastlines, marshes, and deep-sea hydrothermal vents. These ancient microorganisms trace their origins back to the emergence of microbial life on Earth approximately three billion years ago, as suggested by previous research.
The research team conducted experiments involving more than 20 species of purple sulfur and purple non-sulfur bacteria, analyzing their biopigments and light signatures. By modeling Earth-like planets with different atmospheric conditions and cloud coverage, the researchers observed that both moist and arid environments supported the production of vividly colored biosignatures by purple bacteria.
The scientists propose that purple bacteria may thrive particularly well on planets orbiting cooler red dwarf stars, which are the most abundant stellar type in the Milky Way galaxy. Lígia Fonseca Coelho, a postdoctoral associate at the Carl Sagan Institute and the lead author of the study, highlighted the potential advantages of these microorganisms in environments where they are not in direct competition with green plants, algae, and bacteria. The unique conditions provided by a red-hued star could offer optimal circumstances for photosynthesis to occur.
Drawing from their research outcomes, the team is compiling a catalog of the distinctive colors and chemical signals that various organisms and minerals could exhibit in the reflected light of exoplanets. This initiative aims to equip upcoming telescopes with the necessary capabilities to detect diverse forms of life beyond our planet, including those that do not mirror terrestrial life forms.
As Kaltenegger aptly summarized, “We are just opening our eyes to these fascinating worlds around us.” The adaptability of purple bacteria across a spectrum of environmental settings leads to the intriguing possibility that, on numerous alien worlds, the color purple might indeed surpass green as the hallmark hue of life.