Why alien life might look purple

When inspecting the cosmos for potentially habitable worlds, scientists have long looked for the color green. Green is the fundamental color of life on Earth, after all. But what if life on faraway planets wasn’t green at all? In fact, what if it were purple?

In a study published in April in the Monthly Notices of the Royal Astronomical Society, scientists zoom in on purple bacteria—these violet and magenta hued microorganisms are found in some of the most extreme environments on our own planet. The researchers collected and grew samples of the bacteria and measured the wavelengths of light they reflect. The idea is to add to the database of possible signatures of life that future astronomers can look for on other worlds.

“There's such a 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 doesn't happen to be green.”

Why purple?

Long before we had the verdant forests and bright green algal blooms that color our world today, Earth was a difficult place to live. It had little oxygen. Temperatures were extreme. 

But these harsh conditions are also ones where organisms like purple bacteria can thrive. 

Instead of using chlorophyll, the green organelle that most plants have today used for photosynthesis, purple bacteria uses bacteriochlorophyll and carotenoids, which allows them to perform photosynthesis in low-light and low-oxygen environments. 

“So you can even imagine another Earth in another time, earlier for instance, could maybe be purple if these organisms were abundant, because it would have the conditions for them to actually survive and thrive,” explains study co-author Ligia Fonseca Coelho, a microbiologist at Cornell University. 

In other words, purple worlds could be possible.

In fact, scientists hypothesize that early Earth might have been purple. In a 2018 study, researchers concluded that purple archaea, another type of microorganism that uses a molecule called retinal to photosynthesize, could have dominated our planet before it was filled with oxygen. “What this new study does is expand potential lifeforms that might provide a purple signature,” says Shiladitya DasSarma, a molecular biologist at the University of Maryland and lead author of the 2018 paper.

Now, the scientists behind the most recent paper have added spectral data on 20 species of purple bacteria, collected from places like marshes and lakes. The researchers measured the wavelengths of light the bacteria reflected and modeled how those patterns could look when seen on a faraway planet. 

The result is a collection of light signatures that the team is adding to an ongoing database. These data are publicly available, says Kaltenegger, where scientists can use these signatures to inform their own projects. 

Signatures of habitable worlds

Astronomers look for life on other planets using markers called biosignatures.  The color of a planet’s surface can 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 that we have available today,” says Edward Schwieterman, an astronomer at University of California Riverside who was not involved with the study. For instance, the James Webb Space Telescope can only detect biosignatures in an exoplanet’s atmosphere, like whether it has oxygen, methane, or other gases. It’s unable to measure the reflected light from the planet’s surface. 

“The difficulty is to translate what we're studying in the laboratory to astronomical measurements,” agrees DasSarma.

But researchers hope that the new work will inform upcoming projects like Chile’s Extremely Large Telescope and NASA’s Habitable World Observatory, which both aim to capture images that can obtain these surface-level measurements. The observatories are slated to come online by the end of 2030. 

“It pushes us to make sure that this future mission has the capability of detecting the signatures,” says Schwieterman, who is also a part of a working group on biosignatures for the Habitable Worlds Observatory.

Extraterrestrial biodiversity

Understanding purple life on Earth also expands what scientists can consider life elsewhere. Many habitable rocky planets revolve around stars known as red suns, smaller dimmer versions of the yellow sun in our solar system. Purple organisms are able to use the low-energy rays that these red suns emit. 

“They're actually the most abundant types of stars,” says Coelho. “So this is why the purple model is also important, because it fills this void that exists for the type of life that could actually thrive on planets around these abundant stars,” she says.

In the meantime, the scientists at Cornell are continuing to expand their database of colors and signatures, looking into other lifeforms that can survive in different extreme conditions. 

“We have this life on Earth. And if you think about it like a huge puzzle, we're trying to identify the puzzles that are more likely to exist around the planets that we can identify,” explains Kaltenegger.

“All the amazing biodiversity we have, we have to study it to give us tools to search for life on other planets,” says Coelho. “Biodiversity is needed in astronomy.”