They might have several-hundred-year lifespans, like trees. This would make the notion of multi-year sublight voyages from world to world less off-putting.

If all of the above were true, they'd be a bit more likely than us to travel the void and visit new worlds.

If so, they might pass near earth, and in the process, they might observe us or our EM-frequency broadcasts.

In their initial observations, they'd likely see that we eat things. As chlorophyll-filled alien plants themselves, this would seem really shocking. They don't eat things, but we do. It's even possible that there home world might not have any animal-equivalents, or just have a relatively small number of animals, such as pollinating insects. Their only experience with predation might involve diseases. If so, we'd be about as creepy to them as flesh-eating bacteria is to us.

And so they might want to learn more about us before they introduce themselves. It'd be in their own best interests to figure out how much of a threat we pose. They might want to know if the plants and animals we feed upon are sentient. If any of our current food sources are self-aware and capable of independent thought, they might conclude that we would only see the alien plant people as food, too.

That'd be an interesting explanation for crop-circles, within the context of the thought experiment. In this hypothetical scenario, the circles are not a natural earthly phenomenon, and not a hoax, and not an attempt by aliens to communicate with humanity. Crop circles could be the attempts of the alien plants to communicate with terrestrial plant life. Or, at least the really intricate ones where the stalks are bent without being broken could be. (Or, they could be the earthly plants attempts to communicate back, but that's probably a greater stretch of the imagination.)

Cattle ranchers have a history of lynching those who rustle or poach their herd, so it's not exactly the best context to initiate diplomatic relations under.

Perhaps then our space-plants also abduct humans. Those who claim to have experienced alien abduction often describe various probing and sexual extractions. While this is a huge affront to most humans, it wouldn't necessarily be distressing to a plant/alien. If you're used to being pollinated by insects, the idea of a stranger (of another species) milking your pistil might be comforting, not shocking.

THE COLOR OF PLANTS ON OTHER WORLDS

SCIENTIFIC AMERICAN
By Nancy Y. Kiang

On other worlds, plants could be red, blue, and even black.

RED EARTH, GREEN EARTH, BLUE EARTH: Type M stars (red dwarfs) are feeble, so plants on an orbiting Earth-like world might need to be black to absorb all the available light (first panel). Young M stars fry planetary surfaces with ultraviolet flares, so any organisms must be aquatic (second). Our sun is type G (third). Around F stars, plants might get too much light and need to reflect much of it (fourth).

* What color will alien plants be? The question matters scientifically because the surface color of a planet can reveal whether anything lives there specifically, whether organisms collect energy from the parent star by the process of photosynthesis.

* Photosynthesis is adapted to the spectrum of light that reaches organisms. This spectrum is the result of the parent stars radiation spectrum, combined with the filtering effects of the planet's atmosphere and, for aquatic creatures, of liquid water.

* Light of any color from deep violet through the near-infrared could power photosynthesis. Around stars hotter and bluer than our sun, plants would tend to absorb blue light and could look green to yellow to red. Around cooler stars such as red dwarfs, planets receive less visible light, so plants might try to absorb as much of it as possible, making them look black.

The prospect of finding extraterrestrial life is no longer the domain of science fiction or UFO hunters. Rather than waiting for aliens to come to us, we are looking for them. We may not find technologically advanced civilizations, but we can look for the physical and chemical signs of fundamental life processes: 'biosignatures.' Beyond the solar system, astronomers have discovered more than 200 worlds orbiting other stars, so-called extrasolar planets. Although we have not been able to tell whether these planets harbor life, it is only a matter of time now. Last July astronomers confirmed the presence of water vapor on an extrasolar planet by observing the passage of starlight through the planet's atmosphere. The world's space agencies are now developing telescopes that will search for signs of life on Earth-size planets by observing the planets' light spectra.

Photosynthesis, in particular, could produce very conspicuous biosignatures. How plausible is it for photosynthesis to arise on another planet? Very. On Earth, the process is so successful that it is the foundation for nearly all life. Although some organisms live off the heat and methane of oceanic hydrothermal vents, the rich ecosystems on the planet's surface all depend on sunlight.

AN ARTIST'S CONCEPTION OF COLORFUL PLANTS ON DISTANT WORLDS

SPACE.COM
By Ker Than
 
A new study finds otherworldly vegetation could be red, orange or yellow. Credit: Doug Cummings, Caltech.

If trees grow on other planets, their leaves might be red, orange or yellow, and not only in autumn, scientists say.

Two new studies detailed in the March issue of the journal Astrobiology find that the color of a planet's photosynthetic organisms depend on the type of star the world orbits and the makeup of its atmosphere.

'You have a particular spectrum which is affected by the star's surface temperature, but once that light comes down through the atmosphere, the atmosphere filters that radiation,' said study team member Victoria Meadows of the Virtual Planet Laboratory at Caltech.

For example, our Sun radiates most of its energy in the green part of the visible spectrum. But ozone molecules in the Earth's atmosphere absorb much of this green light energy, allowing other colors, especially red, to filter through to the ground.

Why plants are green

This could explain why chlorophyll absorbs mostly red and blue light and reflects green light, the researchers say.

'Ozone filters out some of the blue-green radiation, so there's less of that available at the surface of the planet,' Meadows told SPACE.com.

Alternative explanations have also been proposed for the greenness of plants. One idea, called the purple Earth hypothesis, states that chlorophyll doesn't absorb green light because it appeared after another pigment, called retinal, was already present and it had to settle for the 'leftover' wavelengths that were not being absorbed.

The researchers reached their conclusions after analyzing 12 different kinds of light-sensitive pigments, including chlorophyll, that organisms on Earth use to harness the sun's energy.

Plant biosignatures

The researchers want to use their findings to guide the search for plant life on other worlds. To that end, Meadows' team at VPL entered the results of the pigment analysis study into a computer simulation that predicts what the light from a distant planet containing photosynthetic organisms will look like to space telescopes.

ALIEN CACTUS

For example, in addition to reflecting back visible green light, organisms on Earth that use chlorophyll for photosynthesis also reflect near infrared light. This reflected light can be seen from space and is called the 'red edge.'

The new findings suggest photosynthetic organisms on other planets might not produce a red edge, but some other biosignature instead. The researchers want to figure out what those alternative biosignatures might be.

'We're coming up with rules so that we can say more confidently what is photosynthesis when looking at spectra from these planets,' said study leader Nancy Kiang, a biometeorologist at NASA's Goddard Institute for Space Studies in New York.

Search for life

Whether or not scientists find life on distant worlds could depend on these rules.

'When we look at these faraway planets, we're not going to be able to spatially resolve them. We won't be able to see continents and oceans,' Meadows said. 'Everything we must learn about that planet will be in a single dot of light.'

Researchers already think they can make certain generalizations about photosynthesis in the universe at large. It's unlikely, for example, that plants on alien worlds will be blue.

'It appears that harvesting blue light is very common across the board for photosynthetic organisms' on Earth, Kiang said in a telephone interview. 'I think it is unlikely that anything will be blue.'

DR SEUSS AND THE LORAX

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