Alien life may be purple

The image of the “little green man” may have been wrong for decades. A study from Cornell University, in the United States, suggests that life on other planets may have a very different hue than what popular imagination tends to picture: purple. The study was published in the journal Monthly Notices of the Royal Astronomical Society.

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Why purple and not green?

On Earth, plants are green because they use chlorophyll — a molecule that absorbs red and blue light but reflects green light. This process, photosynthesis, is the foundation of plant life on our planet. But on worlds with different conditions, especially those orbiting red dwarf stars (which are cooler and emit light quite unlike our Sun), this mechanism may never have emerged. Instead, another type of organism could dominate: purple bacteria.

What are purple bacteria?

These microorganisms already exist on Earth, but occupy restricted ecological niches. They carry out a process similar to photosynthesis using low-energy red and infrared light — exactly the type of radiation most abundant around cooler stars. There are two main groups studied: purple sulfur bacteria and purple non-sulfur bacteria. According to Cornell doctoral candidate Lígia Fonseca Coelho, who led the study, these bacteria “already thrive here in certain niches — imagine if they weren’t competing with green plants, algae, and bacteria: a red sun could give them the most favorable conditions for photosynthesis.”

How the study was conducted

To understand the chemical and visual signature that a planet dominated by purple bacteria would emit, Coelho and her team collected 20 samples of these bacteria from different locations around the world — including hydrothermal vents and even ponds near Cornell’s own campus. From these samples, the researchers developed computational models of Earth-like planets in different environments, both wet and arid, to simulate how these worlds would appear to distant telescopes.

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The light “fingerprint” that could reveal life

The most significant finding of the research is the identification of a specific spectral signature — a kind of luminous “fingerprint” — that purple bacteria would emit on a planetary scale. This signature, according to the study, would potentially be detectable by next-generation observatories, such as the Extremely Large Telescope, operated by the European Southern Observatory and currently under construction in Chile. In many of the simulated models, this fingerprint came back with a purple hue.

Was Earth once purple?

Interestingly, the idea of a purple planet is not just science fiction for the future — it may be part of Earth’s own past. A separate study from the University of Maryland, published in 2022, argued that before the emergence of chlorophyll, our planet was dominated by a molecule called retinal, which absorbed green light and reflected red and violet. To human eyes, this ancient planet would have appeared purple. When chlorophyll emerged — favored by rising oxygen levels in the atmosphere — it absorbed the wavelengths that retinal was no longer taking advantage of, and Earth gradually turned green.

Where to look for these purple worlds?

The focus of the search should be low-oxygen exoplanets orbiting red dwarf stars — the most common type in the Milky Way. Coelho simulated a variety of scenarios: from a frozen Earth to an ocean world, a “snowball” planet, and a terrestrial analog orbiting a cooler star. In all of these scenarios, purple bacteria showed favorable conditions to thrive and potentially dominate the planet’s surface.

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With the right tools already in development, science is getting ever closer to being able to distinguish, from billions of kilometers away, whether a planet harbors life — and what color that life might be. If Coelho and her team are correct, the first biological signature detected on another world may not be the familiar green of Earth’s forests, but rather the vibrant purple of bacteria thriving under a light our eyes can barely see.

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