We’ve been wrong about the colour of aliens all along

Forget little green men.

Aliens may not be the colour years of science fiction have led us to believe. 

They could be purple, not green. 

New research from Cornell scientists has suggested that Earth-like planets could look very different to the lush green scenes we’re used to.

This is because they could be covered by bacteria that receive little to no visible light, meaning they could use invisible infrared radiation to power photosynthesis.

On Earth, similar bacteria contain purple pigments, meaning purple worlds powered by these organisms would produce a distinctive ‘light fingerprint’ which could be picked up by next-generation ground- and space-based telescopes. 

First author Dr Lígia Fonseca Coelho, from the Carl Sagan Institute (CSI), said: ‘Purple bacteria can thrive under a wide range of conditions, making it one of the primary contenders for life that could dominate a variety of worlds.’

Using life on Earth as a guide, the research team are cataloguing the colours and chemical signatures that a diverse range of organisms and minerals would present in an exoplanet’s reflected light.

Bacteria classed as ‘purple’ actually have a range of colours including yellow, orange and red due to pigments related to those that make tomatoes red and carrots orange. 

These bacteria survive on low-energy red or infrared light by using simpler photosynthesis systems than those in plants, using forms of chlorophyll – the pigment that helps plants turn sunlight into food – that absorb the infrared light.

Unlike typical photosynthesis, which uses sunlight to convert carbon dioxide and water into oxygen and sugar, this reaction does not produce oxygen. This type of bacteria is thought to have been prevalent in Earth’s early years.

Co-author Dr Lisa Kaltenegger said: ‘We need to create a database for signs of life to make sure our telescopes don’t miss life if it happens not to look exactly like what we encounter around us every day.’

The researchers suggest that these bacteria could work well on planets that circle cooler red dwarf stars – the most common type in our galaxy.

‘They already thrive here in certain niches,’ Dr Coelho said. ‘Just imagine if they were not competing with green plants, algae and bacteria – a red sun could give them the most favourable conditions for photosynthesis.

‘If purple bacteria are thriving on the surface of a frozen Earth, an ocean world, a snowball Earth or a modern Earth orbiting a cooler star, we now have the tools to search for them.’

The researchers created models of Earth-like planets with different conditions such as cloud cover, and found that both wet and dry purple bacteria produced intense colour biosignature. 

And if a purple ‘dot’ is detected in the solar system, it could trigger further observations of the planet to rule out other colour sources, such as minerals. 

‘We are just opening our eyes to these fascinating worlds around us,’ said Dr Kaltenegger.

‘Purple bacteria can survive and thrive under such a variety of conditions that it is easy to imagine that on many different worlds, purple may just be the new green.’

The study is published in Monthly Notices of the Royal Astronomical Society.