Aliens are Green, Not

Margaret Turnbull makes The Case Against Litte Green Men. Excerpt:

[V]irtually 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.