purple exoplanets
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Purple Exoplanets Could Be Teeming with Alien Life

A team of researchers from Cornell University has significantly expanded the range of potential surface biosignatures for Earth-like exoplanets beyond the traditional “green” chlorophyll-based photosynthesis.

The study, published in the journal Monthly Notices of the Royal Astronomical Society, suggests that purple bacteria could serve as alternative biosignatures, potentially thriving on a much wider range of “purple exoplanets.”

The search for life beyond Earth has long focused on the “red edge” of vegetation, a distinctive spectral signature caused by chlorophyll in plants. However, this study proposes that looking at the spectra of a diverse collection of purple bacteria, including purple sulfur bacteria (PSB), purple non-sulfur bacteria (PNSB), and a purple cyanobacterium, may also hold clues for astronomers seeking that first hit of alien life outside of Earth.

“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,” explained the study’s lead author, Lígia Fonseca Coelho, a postdoctoral associate at the Carl Sagan Institute, in a press statement.

To conduct the study, the researchers grew a diverse set of purple bacteria in the laboratory under various conditions. The samples originated from both oxygen-rich and oxygen-poor environments. Cultures were deposited on filters until saturation, and their reflectance was measured from 400-2500 nm using a spectrometer with an integrating sphere to mimic exoplanet observing conditions.

The measured reflectance spectra revealed distinct signatures for the purple cyanobacterium, PNSB, and PSB. Despite being referred to as “purple”, the bacteria exhibited a wide range of colors including yellow, orange, brown and red due to these diverse pigments. Using the measured spectra, the researchers modeled the reflectance of Earth-like exoplanets dominated by purple bacteria. They simulated frozen and snowball planets, ocean worlds, and Earth-analogues with different surface coverage of purple bacteria, both with and without clouds.

The models showed that a wide variety of terrestrial planets could show signs of purple bacteria surface biopigments.

“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,” said co-author Lisa Kaltenegger.

The study suggests that purple bacteria could potentially thrive on exoplanets orbiting red dwarf stars, also known as M-stars. The spectral energy distribution of M-stars overlaps with the optimal spectral range for absorption and reflectance of key pigments in purple bacteria. This finding significantly expands the range of potentially habitable worlds in our galaxy. This, combined with the distinct spectral features of purple bacteria biopigments, could serve as a good yardstick to look for surface biosignatures for a wide range of exoplanet conditions, from oxygen-rich to oxygen-poor environments.

As the search for life beyond Earth enters a new era with more than 5,500 confirmed exoplanets, this study underscores the importance of expanding our understanding of the diversity of life on Earth and its potential manifestations on other worlds. By looking beyond the traditional “green” biosignatures, we increase our chances of detecting life on exoplanets.

“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,” Coelho explained, “we now have the tools to search for them.”

MJ Banias covers security and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.