What Will Plants Be Like on Alien Worlds?

Scientists know enough about exoplanets to speculate about how simple plants might arise on them. But don't count on them being green.

Consider the possibility of alien plants. After all, plenty of exoplanets likely have conditions friendly to the development of plants, even if evolution there never makes it as far as complex organisms and animals. But if moss, algae, and lichen envelop lush exoplanets in the faraway realms of the Milky Way, those worlds and the stars they circle could be completely different than our own. Extraterrestrial flora could be nothing like we’ve ever seen before.

Most of the rocky exoplanets discovered so far orbit red dwarf stars, the most abundant type of star in the galaxy. They give off fainter, redder light than the sun. “It’s natural to ask, if photosynthesis happens in a range of visible light— 400 to 700 nanometers—and you take a star that’s fainter, cooler, and redder, is there enough light to support photosynthesis?” says Thomas Haworth, a physicist at the Queen Mary University of London. His tentative answer to that question, recently published in the Monthly Notices of the Royal Astronomical Society, is a “yes, sometimes.” His team’s conclusion, that conditions around red dwarf stars aren’t a deal breaker for life, is encouraging. But life might have adapted very differently to the light of redder suns. 

Most plants on Earth, including leafy vegetation, mosses, and cyanobacteria, use photosynthesis to turn sunlight and carbon dioxide into energy and oxygen. Plants use chlorophyll pigments to transform that solar energy into chemical energy. Chlorophyll gives plants their green colour, and it’s tuned to absorb sunlight in the part of the spectrum that goes from violet-blue to orange-red. But astrobiologists have noted that there’s a “red edge” for vegetation, meaning that chlorophyll doesn’t absorb many photons at longer, redder wavelengths beyond 700 nanometers. Those are precisely the wavelengths at which these small red dwarf stars give off most of their light. That seems to pose a problem for photosynthetic species. 

So along with his colleague, biologist Christopher Duffy, Haworth tried to envision how extraterrestrial photosynthesis might work, even under unusual conditions. “We wanted to develop a general model of photosynthesis that wasn’t tied to any particular species,” Duffy says. In particular, they modeled light-harvesting antennae—pigment-protein complexes that all photosynthetic organisms have—which collect photons and channel the light energy down to a reaction center that carries out the photochemistry needed to turn it into chemical energy.

They concluded that organisms with extremely efficient antennae could indeed absorb dim light redder than 700 nm, but that oxygenic photosynthesis might be a struggle. In that scenario, organisms would have to invest lots of their energy just to keep the photosynthetic machinery running. Evolutionarily, this might limit them to remaining, say, pond-dwelling green-blue bacteria, not structures that could colonize land. 

And although green plants, with their reliance on chlorophyll and sunlight, dominate the Earth, neither biology nor physics requires it to work that way. We already know of species on our own planet that follow different rules. There are subterranean microbes that make “dark oxygen” in the absence of light. And there are purple bacteria and green sulfur bacteria that conduct photosynthesis without oxygen, using different pigments and gases, especially sulfur. They rely on infrared light for energy, between 800 to 1,000 nanometers. That’s well within the range of red dwarfs’ starlight.

Duffy and Haworth speculate that on remote planets, communities of purple bacteria could swell in black sulfurous oceans, or spread in films around local sources of hydrogen sulfide. If they evolved into plants that could survive on land, like Earth plants they would still angle their light-absorbing surfaces toward their star, but they might be purple, red, or orange, depending on the wavelengths of light they are attuned to. They’d still have clumps of cells that coax nutrients from the ground, but they would be seeking different nutrients. (For plants on Earth, nitrates and phosphates are critical.)

If these scientists are correct that botanical life could arise in red dwarf systems, astronomers then need to figure out where to point their telescopes to find it. To start, scientists typically focus on the habitable zone around each star, also sometimes called a “Goldilocks” region because it’s neither too hot nor too cold for liquid water on a planet’s surface. (Too hot and water will evaporate away. Too cold and it will permanently turn to ice.) Since water is likely necessary for most kinds of life, it’s an exciting development when astronomers find a rocky world in this zone—or in the case of the TRAPPIST-1 system, multiple worlds.

But University of Georgia astrophysicist Cassandra Hall says perhaps it’s time to rethink the habitable zone in a way that emphasizes not just water but also light. In a study earlier this year, Hall’s group focused on factors like starlight intensity, the planet’s surface temperature, the density of its atmosphere, and how much energy organisms would need to expend for mere survival, rather than growth. Considering these together, they estimated a “photosynthetic habitable zone” that lies a bit closer to a planet’s star than the traditional habitable zone for water. Think of an orbit more like Earth’s and less like Mars’.

Hall highlights five promising worlds that have already been discovered: Kepler-452 b, Kepler-1638 b, Kepler-1544 b, Kepler-62 e and Kepler-62 f. They’re rocky planets in the Milky Way, mostly a bit larger than Earth but not gas giants like “mini-Neptunes,” and they spend a significant fraction of their orbits, if not the entire orbit, within their star’s photosynthetic habitable zone. (Astronomers found them all within the past decade using NASA’s Kepler Space Telescope.) 

Of course, the hard part is trying to spot clear signs of life from more than 1,000 light-years away. Astrobiologists look for particular chemical signatures lurking in exoplanets’ atmospheres. “Generally, you’re looking for signs of chemical disequilibrium, large amounts of gases that are incompatible with each other because they react with each other to form different things,” Hall says. These could indicate life processes like respiration or decay. 

A combination of carbon dioxide and methane would be a prime example, since both can be given off by life forms, and methane doesn’t last long unless it’s constantly being produced, such as from the decomposition of plant matter by bacteria. But that’s no smoking gun: Carbon and methane could just as well be produced by a lifeless, volcanically active world. 

Other signatures could include oxygen, or its spin-off, ozone, which is generated when stellar radiation splits oxygen molecules. Or perhaps sulfide gases could indicate the presence of photosynthesis without the presence of oxygen. Yet all of these can come from abiotic sources, such as ozone from water vapor in the atmosphere, or sulfides from volcanoes.

While Earth is a natural reference point, scientists shouldn’t limit their perspective to only life as we know it, argues Nathalie Cabrol, an astrobiologist and director of the SETI Institute’s Carl Sagan Center. Seeking just the right conditions for oxygenic photosynthesis could mean narrowing the search too much. It’s possible life isn’t that rare in the universe. “Right now, we have no clue if we have the only biochemistry,” she says.

If alien plants can survive or even thrive without oxygenic photosynthesis, that ultimately could mean expanding, rather than tapering, the habitable zone, Cabrol says. “We need to keep our minds open.”

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