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Some scientists believe that 3.6 billion years ago Earth might have been purple, and that theory is giving us some clues in our search for life in the universe.

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Sources:

https://doi.org/10.1088/2041-8205/728/1/L6
https://www.ncbi.nlm.nih.gov/books/NBK22535/
https://doi.org/10.3390/sym6030781
https://doi.org/10.1038/35059215
https://doi.org/10.1128/microbe.1.120.1
https://doi.org/10.1017/S1473550418000423
https://pubmed.ncbi.nlm.nih.gov/27630250/
https://doi.org/10.1117/12.2530036
https://ecampusontario.pressbooks.pub/microbio/chapter/photosynthesis/
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In 2013, astronomers figured out an exoplanet's color for the first time. Its name is HD189733B, and, as far as we can tell, it's blue. From outside our solar system, Earth would look blue, too, with some green mixed in if you could look a little more closely, but that might not have always been the case. Some scientists think that about 3.5 billion years ago, all that green and possibly some of that blue might have been purple, and the idea is giving us new clues in our search for life elsewhere in the universe.

These days, almost all life that gets its energy from the sun does so by using the pigment chlorophyll. It's famously green, hence all the green plants you see around you. When sunlight hits chlorophyll some of the light gets absorbed, and that extra energy pushes an electron to a higher energy level. If a nearby molecule captures that electron, suddenly you can do all sorts of chemical reactions and produce other molecules that cells can use as energy. That's called phototrophy, which literally means "light eating," and it evolved about 2.7 billion years ago. Life that uses chlorophyll for this process can also capture carbon dioxide and produce oxygen, which we call photosynthesis.

Considering just how green the earth is, it's clearly a successful strategy, but it also doesn't really make sense. Chlorophyll looks green because it reflects green light while absorbing red and blue, but the sun actually emits more green light. In other words, chlorophyll is ignoring the part of the sunlight spectrum where there's more energy available, which seems kind of silly.

Back in 2006, a group of American researchers proposed an explanation for this, which they've since expanded on in a few papers. Maybe the green light was already in use. See, there are some organisms that don't use chlorophyll to get energy from light. Instead, they use another type of pigment called retinal, which is basically the opposite of chlorophyll when it comes to light absorption.

Retinal is purple, which means it absorbs that more abundant green light and reflects red and blue, and there are some good reasons to think that microbes evolved to use retinal before chlorophyll. On a molecular level, the process of using retinal to capture energy from sunlight is relatively simple, and organisms could make retinal using systems they already had. So it's possible that, for a while, the Earth was dominated by purple organisms.

The idea is that near the surface of the oceans there was a layer of retinal-containing microbes that were absorbing basically all the green wavelengths of sunlight, so in the waters below, where there was still some space, the light would have been mostly red and blue. In which case, it would make total sense for the life that evolved to live there to absorb those wavelengths. They'd have worked with what light was available.

But, in the end, chlorophyll took over because, sure, there's less light available in the red and blue parts of the spectrum, but overall it turns out chlorophyll is much more energy-efficient. Among other things, the more complex process that it uses can capture carbon dioxide and produce oxygen, which is super useful for all kinds of chemical reactions. It's also worth noting that oxygen produced by the organisms that used chlorophyll led to an environment where more complex life could evolve, including us. So, that's nice.

There are still certain types of microbes that use retinal, like halobacteria that live in very salty environments, which makes for some awesome purple lakes. But, overall, green is where it's at.

Now we have no idea if the Earth actually used to be purple. The thinking behind it makes sense, but it's mainly been proposed by one group of researchers, and, so far, we don't really have any proof. There could be other explanations for why chlorophyll doesn't absorb green light, like we know too much light can sometimes be harmful.

But even if it's just a possibility, the idea that a purple planet could exist is important for astronomers searching for signs of extraterrestrial life. If you're looking for evidence of green plants like we see here on Earth, your best bet might be to scan for what's known as the vegetation red edge, an area of the spectrum where there's a sharp increase in the light being reflected. That's because red light, which chlorophyll absorbs, is right next to the infrared part of the spectrum, which cells reflect.

But purple life would look different. Instead of a red edge, it would create a green edge, a sudden change in how much light it's reflecting as light transitions from red, which retinal reflects, to green, which retinal absorbs.

Our telescopes still aren't very good at taking direct images of exoplanets, but there are plans for more powerful instruments that could do it, like a space telescope concept called the Habitable Exoplanet Observatory, or HabEx. In the meantime, by studying retinal-based life in different environments, we can learn more about how they evolved and whether they could have once dominated life on Earth.

And someday when we're finally looking through those next-gen telescopes, it might be worth keeping an eye out for a purple planet.

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