Purple Plants Might Thrive Under Multiple Stars

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Pop culture was first introduced to the notion of life under two stars in the 1977 film “Star Wars: Episode IV – A New Hope.” In an introspective moment, hero Luke Skywalker watches a pair of sun-like stars set on the horizon.

His native world Tatooine is a desert planet. But two suns in the sky would be a big deal for vegetation on a tropical planet, reports Jack O’Malley-James of the University of St. Andrews in Scotland. He recently presented a study at a Royal Astronomical Society meeting in Llandudno, Wales.

Given the power of biological evolution, plant life would adapt to use energy from both suns. Or perhaps different forms of plants may evolve to use light from one specific sun, he reported.

Things could get complicated in multiple systems that have a yellow sun-like star with a red dwarf companion. Over 25 percent of sun-like stars and 50 percent of red dwarfs are found in multi-star systems. The plants would have access to a broader range of radiation than on Earth. Stellar radiation would stretch far into infrared wavelengths.

“Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth,” says O’Malley-James.

In 2008, astrobiologist Nancy Kiang of Columbia University predicted that alien plants living under a red sun could evolve other photosynthetic pigments that are colored purple, or even black (O’Malley-James speculates black or gray coloring).

Unlike Earth vegetation that reflects some green light, alien vegetation might absorb across the entire visible wavelength range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis.

Due to their interior structure, red dwarf stars can be more petulant than a solar-type star. A Hubble Space Telescope survey of 215,000 red dwarfs uncovered 100 powerful stellar flares over a seven-day period. Despite their puny mass, red dwarfs can pack a punch.

Titanic stellar flares erupt without warning and blast out lethal doses of ultraviolet radiation. Ocean life may be safe from the UV just a few feet under water, and still extract enough light for photosynthesis.

“Intense stellar flares could lead to plants that develop their own UV-blocking sun-screens, or photosynthesizing microorganisms that can move in response to a sudden flare,” reports O’Malley-James.

Astrobiologist Antigona Segura of the Universidad Nacional Autónoma de México in Mexico City simulated how a 1985 flare from the nearby red dwarf AD Leonis would have affected a hypothetical Earth-like planet.

Segura found that UV radiation actually split molecules of oxygen to create more ozone than it destroyed. This made a thicker ozone layer in the planetary atmosphere such that the surface experienced no more radiation than is typical on a sunny day on Earth.

O’Malley-James’ team ran simulations where an Earth-like planet either orbits two stars close together or orbits one of two widely separated stars (pictured above).

If the two suns tightly orbited each other the plant life would evolve to harvest the fully panchromatic spectrum of light available. But if the two suns were widely separated, such that they rose and set at different times, plants might evolve exotic energy gathering mechanisms.

One of the more extreme examples I can imagine is a wide binary system where a planet is bathed in continuous light when the stars are at opposition, in other words, on opposite side of the planet. For some period of time the planet would be in constant daylight.

But eventually the stars would be in conjunction — side by side — as they moved along their orbits. This would plunge one hemisphere of the planet into nightfall that would last for some fraction of the planet’s rotation (assuming it is not gravitationally tide-locked, which would bring even bigger problems!).

Vegetation would need to evolve to handle such environmental traumas. Not only would it have to adapt to the planet’s season cycle, but the cycle of stellar oppositions and conjunctions.

These collective studies paint a rich landscape for science fiction authors and sci-fi film artistic directors. We can imagine a “Technicolor planet” with a range of rich colors where plants that have fine-tuned their photosynthesis to the light from two or more stars.

In response to stellar flares plants may become mobile or deploy other types of protective shielding. In response to long nightfall, vegetation may go into extended hibernation.

The complex interplay of starlight might drive evolution into creating hybrid plant-animals, that collect energy through photosynthesis when light is available, but resort to mobility and foraging when things go dark. Or they might evolve to perpetually migrate across the planet to stay under their preferred sun.

An example on Earth is the eastern emerald Elysia (pictured here), a species of green sea slug. It has the uncanny appearance of a crawling leaf because it uses photosynthesis. The slug eats algae but then incorporates the algae’s chloroplasts — fundamental to planet cells — into its cell structure to conduct photosynthesis.

If the imagination goes far enough, you might conceive of a big-jawed plant like the creature from the original 1960 film “Little Shop of Horrors.” When nightfall comes the jungle is full of cries: “Feed me! Feed me!”

Image Credits: University of St. Andrews, ESO, University of Maine

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