2024-05-01 03:14:19
When scanning the universe for potentially habitable worlds, scientists have long looked for green. Green is the primary color for life on Earth, after all. But what if life on distant planets isn’t green at all? In fact, what if the color was purple?…
When scanning the universe for potentially habitable worlds, scientists have long looked for green. Green is the primary color for life on Earth, after all. But what if life on distant planets isn’t green at all? In fact, what if it was purple?
In a study published in April in the journal Monthly Notices of the Royal Astronomical Society, scientists focused on purple bacteria — these violet-purple microorganisms found in some of the most extreme environments on our planet. The researchers collected and grew samples of bacteria and measured the wavelengths of light they reflected. The idea is to add a database of potential signatures of life that future astronomers can search for on other worlds.
“There is a great diversity of life,” says study co-author Lisa Kaltenegger, an astronomer at Cornell University and author of a new book on alien worlds. “We shouldn’t miss it just because it wasn’t green.”
Why purple?
Long before the lush forests and bright green algae blooms that color our world today, Earth was a difficult place to live. He had little oxygen. The temperatures were extreme.
But these extreme conditions are also the conditions in which organisms like purple bacteria can thrive.
Instead of using chlorophyll, the green organelle that most plants today use for photosynthesis, purple bacteria use bacteriochlorophyll and carotenoids, allowing them to perform photosynthesis in low-light, low-oxygen environments.
“So you can even imagine that another Earth in another time, an earlier time for example, might have been purple if these organisms were abundant, because they would have had the conditions to actually survive and thrive,” explains Ligia Fonseca Coelho, co-author of the study. Microbiologist at Cornell University.
In fact, scientists hypothesize that the early Earth may have been purple. In a 2018 study, researchers concluded that purple archaea, another type of microorganism that uses a molecule called retinoblastoma for photosynthesis, could have taken over our planet before it was filled with oxygen. “What this new study does is expand the possible life forms that might provide a purple signature,” says Shiladitya Des Sarma, a molecular biologist at the University of Maryland and lead author of the 2018 paper.
Now, scientists behind the latest research have added spectral data on 20 species of purple bacteria, collected from places such as swamps and deep-sea hydrothermal vents. The researchers measured the wavelengths of light reflected by the bacteria and developed a model of how these patterns would appear when seen on a distant planet.
The result is a collection of light signatures that the team adds to an ongoing database. If this data is publicly available, Kaltenegger says, scientists can use these signatures to guide their own projects.
Habitable worlds
Astronomers search for life on other planets using signs called biosignatures. The color of a planet’s surface could be one such biosignature. To see it, astronomers use a technique called reflected light spectroscopy.
But “this type of observation cannot be done with the types of telescopes we have today,” says Edward Schwieterman, an astronomer at the University of California Riverside who was not involved in the study. For example, the James Webb Space Telescope can only detect biosignatures in an exoplanet’s atmosphere, such as whether it contains oxygen, methane or other gases. It is unable to measure light reflected from the planet’s surface.
“The difficulty is translating what we study in the lab into astronomical measurements,” agrees DeSarma.
But researchers hope the new work will benefit upcoming projects like the Extremely Large Telescope in Chile and NASA’s Global Habitable Observatory, both of which aim to capture images that can get these surface-level measurements. The observatories are scheduled to become operational by the end of 2030.
“This pushes us to make sure that this future mission has the potential to detect signatures,” says Schwieterman, who is also part of a working group on biosignatures for the Observatory on Habitable Worlds.
Extraterrestrial biodiversity
Understanding purple life on Earth also expands what scientists can consider life elsewhere. Many habitable, rocky planets orbit stars known as red suns, which are smaller, fainter versions of the yellow sun in our solar system. Purple beings are able to use the low-energy rays that these red suns emit.
“It’s actually the most abundant type of star,” Coelho says. “That’s why the purple model is also important, because it fills this void that exists for the kind of life that can actually thrive on planets surrounding these abundant stars,” she says.
Meanwhile, scientists at Cornell University continue to expand their database of colors and signatures, and look into other life forms that can survive in different extreme conditions.
“We have this life on Earth. And if you think about it like it’s a huge mystery, we’re trying to determine what mysteries are most likely to exist around the planets that we can identify.
“All the amazing biodiversity we have, we need to study it to give us the tools to search for life on other planets,” Coelho says. “Biodiversity is needed in astronomy.”
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