has already identified a variety of rocky entities in the cosmos. However, unlike our lush planet, many of these celestial bodies could potentially support purple life—assuming life exists beyond our world.
It’s conceivable to envision organisms on other planets exhibiting hues akin to the greens found in trees, algae, and grasses that dominate Earth’s landscape. This green life harnesses specific light wavelengths to power photosynthesis, with the green pigment chlorophyll playing a key role in this vital process. Recent studies by microbiologists indicate that extraterrestrial lifeforms might utilize different light sources and opt for purple pigments instead of green for energy production.
The notion is not implausible, considering that certain microbes on Earth exhibit a purple hue. In our oxygen-rich environment, however, green life utilizing “oxygenic photosynthesis” has thrived across most ecosystems.
“But this may not be the norm on other planets,” remarked Lígia Fonseca Coelho, a microbiologist at Cornell University’s Carl Sagan Institute, who spearheaded the research, in an interview with Mashable.
The study was recently published in the peer-reviewed scientific journal Monthly Notices of the Royal Astronomical Society. The primary objective was to advocate for the exploration of potential indicators of purple life when studying exoplanets orbiting distant stars. Future advanced telescopes like the Extremely Large Telescope in Chile and the Habitable Worlds Observatory in orbit around will scrutinize these distant worlds to assess their composition and potential for supporting life.
“Instead of fixating on green, let’s also consider purple.”
Researchers collected over 20 purple-hued bacteria from various environments such as lakes and marshes in Massachusetts and New York, analyzing their pigments and light emissions. By simulating the “light signatures” of these bacteria—distinct color and chemical markers visible in reflected light from an alien planet—they discovered that these purple bacteria would produce unique and identifiable signatures.
“Instead of fixating on green, let’s also consider purple,” Coelho stressed. “Our biases could cause us to overlook signs of life.”
Lígia Fonseca Coelho holding a sample containing purple-pigmented bacteria. Credit: Ryan Young / Cornell University
A NASA visualization of an Earth-like exoplanet. Credit: NASA Scientific Visualization Studio
Purple lifeforms may not just be a possibility; they could be prevalent.
The most common stars in our galaxy are diminutive red stars known as red dwarfs. (The nearest star to Earth, is a red dwarf.) These stars, significantly smaller and cooler than the sun, emit predominantly lower-energy light in the form of infrared or red wavelengths. Organisms near red dwarfs would likely harness this infrared light (invisible to human eyes) for cellular energy. “If life exists on such a planet, it would utilize that [infrared] energy,” explained Coelho. Notably, purplish bacteria thrive in environments with lower-energy light, akin to conditions around red dwarfs.
On a planet where green life struggles to survive, purple-hued organisms could outcompete them and become the dominant life form.
“It’s plausible that Earth was once a purple world.”
There’s even speculation that purple bacteria may have thrived on Earth in the past. Considering that our planet has only been oxygenated for half of its 4.5-billion-year history, there was a time before green photosynthetic plants covered the globe. Different bacteria, thriving on alternative chemical processes, might have flourished in such environments.
“It’s possible that Earth was once dominated by purple life,” Coelho mused.