What purple can tell us about life on other planets

Editor’s Note: This feature is part of Colorscope, an award-winning series exploring our perception of color and its use across cultures, one shade at a time. See more here.

The lush green planet that we call home may have actually looked purple in its earliest days, according to the “Purple Earth hypothesis.”

No, this doesn’t mean there were purple trees or purple grass or purple animals. It would have been before any multicellular organisms even evolved, when single-cell microorganisms dominated the planet and possibly created a purple hue that could be seen from space.

These purple organisms may have reigned supreme and existed in varying concentrations across the planet, said Shiladitya DasSarma, a microbiologist and professor at the University of Maryland. DasSarma has studied one of these microorganisms and created the Purple Earth hypothesis.

But what if Earth wasn’t the only planet that experienced this purple phase? What if this hypothesis could give us insight into potential life on exoplanets, the planets outside our solar system? Scientists are in the process of figuring that out.

“That’s where it might make a difference when we look at exoplanets. We would want to consider that the pigments on an alien planet might be different than what we have on modern Earth,” DasSarma said. “And if the Purple Earth hypothesis was correct and there was a dominance of purple organisms in the early Earth, then we might be able to find another planet that’s at an earlier stage of evolution of the planet, where the purple pigments might have dominated.”

Two theories

There are a few variations of the Purple Earth hypothesis among scientists.

DasSarma’s research has focused on halophiles, microorganisms that still exist today in environments with a high concentration of salt, such as the Great Salt Lake and the Dead Sea. They use their strong purple retinal pigment – the same visual pigment in the eyes of humans – to absorb light and generate growth, and he says it’s likely that they’ve been present on Earth for millions, if not billions, of years.

These organisms may have been dominant until the development of chlorophyll, the green pigment that is essential to the type of photosynthesis we see in plants, which reintroduces oxygen into the atmosphere.

“What happened I think is that the chlorophyll-based photosynthesis is more efficient, which means they generally have outcompeted the the purple organisms,” DasSarma said. “In evolution, you had progression from more primitive to more complex and capable systems.”

Another theory is that the planet was populated by ancient purple bacteria, whose descendants are found today in many lakes that do not require the same high salt concentration halophiles do.

These lakes contain thick mats of this purple bacteria, but they typically bloom so deep in the water column that they’re not visible from the surface, said Jennifer Glass, an assistant professor at Georgia Tech’s School of Earth and Atmospheric Sciences.

“However, in the deep past, before oxygen accumulated in the atmosphere, these purple bacteria might have proliferated to such an extent that they could have been detectable on the Earth’s surface,” she said

If the planet were to have a purple hue, the purple bacteria would have to be widely populated, she said. But there is an important caveat to this theory.

“The big question, and a topic of active scientific debate, is whether the purple bacteria living on the ancient Earth had the ability to protect themselves from UV radiation well enough to inhabit vast expanses of the land surface, like plants do on the modern Earth,” she said. “The spectral signature of the pigment would be much stronger if the pigmented bacteria covered the land surface.”

Frank Stewart, associate professor in the Georgia Tech School of Biology, said there’s strong evidence that the purple bacteria Glass mentioned were widespread on early Earth. They are adapted for life with low oxygen levels, which was a signature of ancient oceans, he said.

He knows less about the probability of DasSarma’s theory. “But, in all likelihood, both ideas are valid and both microbial types likely played an important role in creating bio-signatures distinct from what we see dominating the modern earth,” he wrote in an email.

Purple life beyond Earth

A new review, which DasSarma co-authored, explores what early signs of life on Earth may be able to tell us about exoplanets.

The microorganism that he studies reside in places where nothing else can grow. But he can imagine that on a distant planet, it might be a dominant life form.

“As the sophistication in the telescope increases, the prediction is that they will be able to determine the spectrum coming from these planets,” DasSarma said. “They’re so far away, and really there’s no light, it’s only the star they’d be able to see, but that’s the goal. Because its so important to be able to detect life elsewhere in the universe.”

Science is getting closer and closer to developing telescopes with better resolution that could give deeper insights into other planets, said Hilairy Hartnett, associate professor in the Arizona State University School of Life Sciences and co-author of the study. One day, they’ll be able to see not only the color of the planets but the chemistry of their atmospheres.

“The theory of the purple planet is really intriguing, and it’s intriguing because it’s important for us to think about the fact that we might not be looking for life that looks like life on the modern Earth,” Hartnett said. “We might be looking for life that looks like Earth in the Archean (era of early life), or we might be looking for life that looks like Earth life that lives way at the bottom of the ocean. And we have to think about what that’s going to look like. If it uses retinal pigments, it’ll be purple.”

See more from our Colorscope series here.