For scientists, few questions inspire as heated a debate as, What is life? You’d think we’d have that one down by now. But it turns out that defining life is really difficult. Go too narrow and it’s arbitrary, like you’re drawing a line in the sand. Go too broad and it’s meaningless, like there’s no line at all. A lot of scientists say they would prefer to leave the business of a precise definition up to philosophers.

Instead, researchers tend to focus on a few characteristics indicative of life, namely metabolism, growth, reaction to stimuli, and reproduction. Among those, metabolism is often viewed as the first among equals. After all, without some way to transfer energy for storage or use, organisms couldn’t grow, react, or reproduce.

The nice thing about metabolism is that it’s pretty universal. Every known organism harnesses energy in one of only a few ways. Or at least, that’s what we thought.

In this month’s cover story, writer Corey S. Powell investigates the discovery of a new form of life. The microbes, found—of all places—in the mud off Catalina Island, California, do not metabolize energy like any organism scientists have ever seen. They eat and breathe electricity directly. That’s the equivalent of sticking your finger into a light socket for a midmorning snack. From the perspective of known science, these creatures are weird—alien, even—and they’re not alone. Now that scientists know what to look for, they are finding these strange electricity-eating organisms all over.

Every time we increase our powers of perception, we find the universe is more complex than we imagined.

That poses an interesting question: If life so contrary to scientific understanding can exist on our own planet—a short ferry ride from Los Angeles, no less—what range of forms could it take on others? Perhaps we should have sent the equivalent of a lightning rod along to Mars with the Curiosity rover.

To some, this kind of force-you-to-question-everything discovery can be frustrating. It’s like the more you learn, the less you understand. To me, it’s inspiring. We used to think we were the center of the universe. Then Galileo and the telescope happened. We used to think that only Earth had conditions suitable for life. Then probes found water and organic molecules on Mars, and the Kepler observatory spotted nearly a thousand exoplanets, at least 22 of which are potentially habitable.

And that’s the rub with science. Every time we increase our powers of perception, we find the universe is more complex than we imagined and life more tenacious. In that sense, finding bizarre and seemingly alien life on this planet simply increases the odds that we’ll find it elsewhere. And when that happens, well, we’ll have a lot more to debate than a simple definition.

Enjoy the February 2015 issue of Popular Science.