Alien Earths Might Be Purple! Scientists say

The hunt for alien life is on, and scientists are casting a wider net. We know Earth's green, plant-powered biosphere might not be the norm.

Our own planet offers hints. Some Earth organisms thrive in harsh environments. But what about alien "plants"?

Earth's life offers another possibility. Plants use chlorophyll (green) for photosynthesis, but purple bacteria, thriving in low light, use infrared light with special pigments.

"These purple bacteria are tough," says astrobiologist LĂ­gia Fonseca Coelho, "They could dominate many worlds."

Most Milky Way stars are red dwarfs, emitting less light than our Sun. Could life exist on these red dwarf planets, and how would we find it?

Cornell's Carl Sagan Institute is searching for life's signatures from afar.

On Earth, chlorophyll dominates plant life. This pigment actually comes from ancient cyanobacteria that became part of plant cells long ago, allowing plants to photosynthesize.

On planets with different light, a dominant life form might have a completely different color. Coelho's team studied bacteria using carotenoids (pigments) to capture light energy.

These bacteria thrive on red and infrared light, using simpler systems than plants. They don't produce oxygen either.

The researchers analyzed the pigments and modeled alien worlds with different conditions. In all cases, the bacteria gave the planets distinct colors.

Depending on the bacteria, the colors could be orange, red, or brown. Just like the variety of green on Earth, these cooler hues could still indicate alien photosynthesis.

This means if another world has life similar to our purple bacteria, we might be able to detect it!

"We're opening our eyes to these fascinating possibilities," says Lisa Kaltenegger, Institute Director. "Purple bacteria are so adaptable, they might paint many worlds a new kind of green... well, purple!"


Published May 2024 in the Monthly Notices of the Royal Astronomical Society; Purple is the new green: biopigments and spectra of Earth-like purple worlds


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