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Why the color purple can refer to extraterrestrial life

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In the search for extraterrestrial life, scientists have long focused on the color green: the color we often associate with life on Earth. Research published in April 2024 / Monthly Bulletins of the Royal Astronomical Society However, it turns out that we can focus better on the purple color.

In the study, astronomers from Cornell University in the US studied purple bacteria, microorganisms that are often found on Earth in places where little life can thrive. The researchers collected and grew bacteria and measured the wavelengths of light they reflected. The findings may help (future) astronomers search for alien worlds.

Why purple?

Long before lush forests and bright green algae turned Earth green, the conditions for life were far from ideal: oxygen was low and temperatures were extreme. But it is precisely in these harsh conditions that organisms such as purple bacteria know how to survive well.

Because while photosynthesis in plants takes place in chloroplasts with the help of the pigment chlorophyll, purple bacteria use other pigments (bacteriochlorophyll and carotenoids). This allows them to carry out photosynthesis in places with little light and oxygen.

In fact, in 2018, scientists suggested that the earth itself may once have been purple. The study found that purple archaea, single-celled microorganisms that use a molecule called retinol for photosynthesis, would have conquered our planet before there was enough oxygen.

Signs of alien life

Cornell University researchers took samples of twenty different types of purple bacteria from swamps and lakes, among others. They studied the wavelengths of light reflected by the bacteria and modeled what those patterns would look like when observed from a distant planet.

When astronomers look for life on other planets, they use certain signs called biosignatures. The color of the planet’s surface can be such a biosignature. But “current telescopes cannot measure the light reflected from the planet’s surface,” says astronomer Edward Schwieterman, who was not involved in the study. For example, the James Webb telescope can only detect biosignatures in an exoplanet’s atmosphere to see if it contains oxygen, methane, or other gases.

“It’s still difficult to translate what we study in the lab into astronomical measurements,” says Shiladitya DasSarma, lead author of the study. Researchers hope that new telescopes, such as Chile’s Extremely Large Telescope and NASA’s Habitable World Observatory space telescope, will take images that will enable these measurements of reflected light. Both telescopes are expected to be in use by the end of 2030.

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Esmee van Dijk works as an editor for National Geographic Magazine, Historia and Traveller. As he loves history and culture, he prefers to travel around the world both physically and mentally. At the top of his wish list is a tour of Taiwan, Japan and South Korea, where he hopes to finally see his favorite artists in real life.

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