this is the most likely color of life beyond Earth
If there is one color that is directly related to life here on Earth, it is green. From indoor plants to gardens, fields, forests and jungles, vast green spaces cover a large portion of the entire earth. It is not for nothing that conditions on our planet have favored the evolution of organisms that carry out photosynthesis, for which they use the green pigment chlorophyll.
But things don’t have to be the same “out there,” and green may not be the right color for discovering life beyond Earth. For example, a planet similar to ours but orbiting a star different from the Sun, such as a red dwarf, could be infected with bacteria that use infrared radiation rather than visible light to drive photosynthesis.
In search of purple worlds
These types of bacteria are not unknown to scientists as they also exist on Earth. And it turns out that their pigments are not green, but an intense violet color, so that if they were dominant in another world, they would produce a “luminous imprint” very different from the earthly one. A distinctive purple trail that a team of Cornell University researchers said in a paper published in the Monthly Notices of the Royal Astronomical Society could be detected by the next generation of telescopes, both ground-based and space-based.
“Purple bacteria,” explains Ligia Fonseca Coelho, the first signatory of the paper entitled “Purple is the New Green: Biopigments and Spectra of Violet Worlds Like Earth,” “can thrive in a wide range of conditions, making them one of the leading contenders for life and which could dominate a variety of worlds.
According to co-author Lisa Kaltenegger: “We need to create a database of signs of life so that our telescopes don’t miss the chance to detect life if it doesn’t look like what we find around us every day. “
Color catalog
To date, astronomers have confirmed the presence of more than 5,500 exoplanets, including more than 30 potentially similar to Earth. New next-generation observatories, such as the Extremely Large Telescope or the Habitable Worlds Observatory, will probe the chemical composition of these worlds that are located in the habitable zones of their stars (where conditions favor the existence of liquid water at the surface) and analyze its composition.
To make this task easier, Coelho and his team, using life on Earth as a basis, created a catalog of colors and chemical characteristics that a wide range of organisms and minerals might present in the light of an exoplanet illuminated by a star other than its star. one.
So the researchers collected and cultured samples of more than 20 species of purple bacteria, which can thrive in a variety of environments, from shallow waters, coastlines and swamps to deep-sea hydrothermal vents.
What is collectively called “purple bacteria” actually comes in a range of colors, including yellow, orange, brown and red. This is because they use different pigments that, for example, make tomatoes red and carrots orange.
All of these bacteria thrive in red or low-energy infrared light and use simpler photosynthetic systems with forms of chlorophyll that absorb infrared radiation and do not produce oxygen. According to the study, these types of organisms were likely most common on early Earth before the advent of photosynthesis as we know it, and may have been especially suited to planets orbiting cooler red dwarfs, the coldest common type of star in our galaxy. .
“They are already here in abundance in certain niches,” says Coelho. “Imagine if they didn’t have to compete with green plants, algae and bacteria: the red sun could provide them with the most favorable conditions for photosynthesis.”
Purple Power
After cataloging the different purple bacteria pigments and their light characteristics, the researchers created models of Earth-like planets with different conditions and cloud cover. In a variety of simulated environments, purple bacteria produced brightly colored biosignatures.
“If purple bacteria actually thrive on the surface of a frozen Earth, an ocean world, a snowball Earth, or an Earth orbiting a cooler star,” the researcher says, “we now have the tools to find them.”
The discovery of a “pale purple dot” in another solar system would prompt intensive observations of that planet to try to rule out other sources of color, such as colorful minerals, which scientists are also cataloguing. According to Kaltenegger, “We simply open our eyes to these exciting worlds around us. “Purple bacteria can survive and thrive in such a variety of environments that it’s easy to imagine that purple has become the new green in many different worlds.”