By SETI Institute
updated 5/20/2004 8:57:22 PM ET 2004-05-21T00:57:22

Not so long ago, putative extraterrestrials were the color of moss. Generic space aliens were inevitably described as ‘Little Green Men,’ probably because an avocado complexion is dramatically unlike any human skin tint.

Green was alien, in other words.

But that idea is more than passing strange, because any glance out the window will convince you that green is one of life’s most popular colors on our planet. Chlorophyll, the foundation of the terrestrial food chain, is green. Most biology on Earth runs on green power, including (indirectly) you.

However, virtually all the solar-powered life on our planet is plant life. Animals don’t seem to be interested in direct food production from sunlight. Are they missing a bet? Is there any reason why an alien with a green epidermis couldn’t produce its food by just hanging around in the sun?

There is. And the reason can be traced to energy efficiency. At the Earth’s surface, sunlight provides about 100 watts of power per square meter. If you were conscious during high school biology class, you’ll recall that when this light strikes a leaf, it encourages the combination of water and carbon dioxide into sugars. These sugars then allow the plant to produce pollen (for reproduction), blossoms and nectar (for pollinators), leaves (for intercepting more light), and roots (for sucking up more water).

What most people don’t know is that out of the original 100 watts striking a square meter of leaf, only about 35% is actually absorbed. (If the absorption were fully efficient, plants would be black.) Worse, the photosynthetic reactions that subsequently occur with the remaining 35 watts are so inefficient that only about one-fourth of that energy results in usable sugars. Hence for every 100 watts of perfectly good sunlight, only about 8 watts ends up as plant food.

That means that your typical backyard bush runs on only as much power as a bicycle headlamp, even during the day. If you’re a bush, that’s good enough: you can’t, and don’t, scour the countryside for food, water, or breeding partners.

But a human-sized animal can’t be happy with a bush-sized energy budget. As a typical adult, you need at least 2,000 Calories a day. Making the conversion to less arcane units, that works out to about 100 watts of power, 24 hours a day. But remember that if you got your energy through photosynthesis, you would absorb only 8 watts for each square meter of skin. Most of us have about 3 square meters of epidermis, roughly half of which is in shade at any given time (more, if you insist on wearing clothes). So that’s just over a dozen watts of daytime power, nearly 10 times less than our burn rate. To provide the energy for one day’s worth of your gusto-grabbing lifestyle, you’d need to bake on the back patio for three weeks.

Okay, so maybe it’s not reasonable for big, mobile animals to get food straight from the Sun. But what about smaller critters? A hummingbird, admittedly not necessarily a good model for ET, but still an active animal, uses about 8 Calories per day, or about a half watt. To get this energy from 5 hours of daily photosynthesizing, this little guy would need a collecting area of 0.3 square meters. For a creature that’s only 10 cm long, the required wingspan would be three meters. That would make for a hummer the length of a Hummer, and one unwieldy bird.

Bottom line? Animals are clever enough to let plants sit around in the Sun all day, building up stores of energy. The wily critters then spend a few minutes harvesting this slow work as a salad – or chowing down on other animals that have already dined on salads. It’s all a matter of energetics, and you can bet that many extraterrestrials will have the same strategy.

Frankly, if you’re an animal, it isn’t easy being green.

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