Astrobiologists continue to work on figuring out which biosignatures are best to look for when searching for life on other worlds. The most common idea was to look for evidence of plants using the green pigment chlorophyll, as we do on Earth. But new work suggests that bacteria with purple pigments could thrive in a wider range of environments than their green relatives. This means that current and next-generation telescopes should look for the emissions of violet life forms.
“Purple bacteria can thrive in a wide range of conditions, making them one of the main competitors for life that could dominate a variety of worlds,” said Lígia Fonseca Coelho, a postdoctoral researcher at the Carl Sagan Institute (CSI) and first author of “Purple Is That.” new green: biopigments and spectra of terrestrial violet worlds,” published in the Monthly Notices of the Royal Astronomical Society: Letters.
Artistic concept of Earth-like exoplanets, striking the careful balance between water and landmass. Photo credit: NASA
According to NASA's Exoplanet Archive, as of this writing, 5,612 extrasolar planets have been found so far, and another 10,000 more are considered planet candidates but have not yet been confirmed. Of all these, there are just over 30 potentially Earth-like worlds, planets that lie in the habitable zones of their stars, where conditions favor the existence of liquid water on the surface.
But Earth-like has a wide-ranging meaning ranging from size, mass, composition and various chemical compositions. While being in a star's habitable zone certainly means that the potential for life exists, it does not necessarily mean that life could have originated there, or even if it did, life could look very different on this world than on the earth.
“While oxygenic photosynthesis produces modern green landscapes, bacteriochlorophyll-based anoxygenic phototrophs can also color their habitats and could dominate a much broader range of environments on Earth-like exoplanets,” Coelho and his team wrote in their paper. “While oxygenic photosynthesis gives rise to modern green landscapes, bacteriochlorophyll-based anoxygenic phototrophs can also color their habitats and could dominate a much broader range of environments on Earth-like exoplanets.”
The researchers characterized the reflectance spectra of a collection of purple sulfur and purple non-sulfur bacteria from various anoxic and oxic environments found here on Earth in a range of environments, from shallow waters, coastlines and swamps to deep-sea hydrothermal vents. Although these are collectively referred to as “purple” bacteria, they actually include a range of colors of yellow, orange, brown and red due to pigments – such as those that turn tomatoes red and carrots orange.
These bacteria thrive on low-energy red or infrared light and use simpler photosynthesis systems that use forms of chlorophyll that absorb infrared light and do not produce oxygen. They are likely to have been prevalent on early Earth before the advent of plant photosynthesis, the researchers said, and may be particularly well-suited to planets orbiting cooler red dwarf stars – the most common type in our galaxy.
A collection of bacterial samples in the Space Sciences Building at Cornell University. Ryan Young/Cornell University.
This means that this type of bacteria may become more common on more and more exoworlds.
In a world where these bacteria could dominate, they would produce a distinctive “light fingerprint” that could be detected by future telescopes.
In their work, Coelho and his team presented models for Earth-like planets where violet bacteria could dominate the surface and showed the influence of their signatures on the reflectance spectra of terrestrial exoplanets.
“Our research provides a new resource for detecting purple bacteria and improves our chances of detecting life on exoplanets with upcoming telescopes,” the team wrote.
“We need to create a database of signs of life to ensure that our telescopes don't miss life if it doesn't look exactly like what we see around us every day,” said co-author Lisa Kaltenegger, CSI director and associate professor of astronomy at Cornell University, in a Cornell press release.
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