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Scientists are searching for extraterrestrial vegetation. Alien life can take various forms, but one possibility captivates scientists

In the Orion Arm of the Milky Way galaxy, approximately 93 million miles (150 million kilometers) from the yellow dwarf star around which it orbits, there is a rocky, medium-sized planet. On the edge of a vast southern ocean, gently lapping against the shores of a bright pink lake. With its snow-white crystal-clear shores and a salt concentration 38 times greater than in pickled olives, among its sole inhabitants, there exists a kind of enigmatic life with a striking purple hue. These primitive microorganisms clearly resemble extraterrestrials: they are not quite bacteria, but not quite like anything else, thriving in conditions that seem utterly inhospitable.

This planet, of course, is Earth, and the lake is located on the southern coast of Western Australia. But the purple "halobacteria" that help the waters of Lake Hillier remain perpetually pink, like Barbie, are also considered a key to the search for a certain type of life in more distant worlds: extraterrestrial vegetation.

Over the past three decades, astronomers have identified 5528 planets beyond our solar system— a mix of strange worlds ranging from those so dark that they are locked in eternal night (TrES-2 b) to a planet with a poltergeist, orbiting a undead star (PSR B1257+12 b). In the not-so-distant future, astronomers are confident that - if life exists beyond Earth - they will be able to detect a definitive sign of extraterrestrial photosynthesis.

It is expected that extraterrestrial plants will be as diverse as you can imagine: forests of black trees under a sky with multiple suns; thickets of alien shrubs leaning in one direction against a backdrop of perpetual sunset; carnivorous underbrush capable of consuming other extraterrestrial life forms entirely.

Planet of Vegetation

Regardless of whether extraterrestrials resemble complex jellyfish-like creatures, three-headed beetles, or are so unfathomably strange that they defy imagination, there are certain rules in their biology established by physics. And one thing can be said for sure: to withstand the relentless pull of entropy, which seeks to break living organisms apart, atom by atom, they will need an energy source. There are only three known ways to obtain this resource from the environment. Extraterrestrials could obtain it directly by capturing it from sunlight, as plants do, or by using inorganic chemicals, as bacteria do in hydrothermal vents. Alternatively, they could take a shortcut and simply consume another organism that has already done one of these actions, as animals do.

It is believed that three billion years ago, the entire Earth could have had a purple hue due to vegetation growing at that time (Photo: Getty Images).

Regardless of whether extraterrestrials resemble complex jellyfish-like creatures, three-headed beetles, or are so unfathomably strange that they defy imagination, there are certain rules in their biology established by physics. And one thing can be said for sure: to withstand the relentless pull of entropy, which seeks to break living organisms apart, atom by atom, they will need an energy source. There are only three known ways to obtain this resource from the environment. Extraterrestrials could obtain it directly by capturing it from sunlight, as plants do, or by using inorganic chemicals, as bacteria do in hydrothermal vents. Alternatively, they could take a shortcut and simply consume another organism that has already done one of these actions, as animals do.

It is believed that three billion years ago, the entire Earth could have had a purple hue due to vegetation growing at that time (Photo: Getty Images).

This article is part of a week-long special on extraterrestrial life - in celebration of the upcoming 60th anniversary of the BBC's most famous extraterrestrial life form, Doctor Who.

On Earth, the overwhelming majority of life - at least by weight - adheres to the first option. It is estimated that of the 550 gigatons of carbon contained in living organisms today, 450 gigatons are found in vegetation. What if most of life in the Universe is photosynthetic? Is it possible that our search for intelligent civilizations has been misguided all along - and that we should have been looking for flora, not fauna?

"We know that on our planet, photosynthesis was successful almost from the beginning of life," says Nancy Kiang, a biometeorologist at NASA's Goddard Institute for Space Studies in New York. "...and it is expected that this process could be successful on another planet."

However, finding this extraterrestrial vegetation may prove to be a challenge.

A Clever Experiment

On December 8, 1990, a brilliant metallic object passed by Earth and aimed its instruments at us. It was the spacecraft "Galileo," and it began a series of measurements with an extraordinary goal: to check if there is life on our planet. Then it disappeared into the depths of our solar system.

The experiment was conceived by American astronomer Carl Sagan, and the idea was simple: if humanity were to see Earth from space for the first time, could we detect unmistakable signs of life without actually visiting? Galileo sent back data from equipment designed to measure various wavelengths of electromagnetic radiation, such as ultraviolet and infrared radiation, and detected some promising signals. This, in turn, could teach us how to find life on more distant worlds.

While plants appear distinctly green to the human eye, they have another, more remarkable characteristic. First, Sagan found an abundance of oxygen — a possible sign of photosynthesis. Oxygen from Earth's atmosphere is constantly removed due to geological and chemical processes, so it was believed to be a definitive marker of biological plant activity: if they weren't constantly producing it, it would disappear. However, it now appears that this is not always the case; later research showed that barren planets can sometimes contain oxygen. So, to be sure that plants are producing oxygen, we will need something more.