Just in case you've forgotten since last week's hoopla about the newly discovered exoplanet:
Gosh, it's just like home! You can't swing a dead cat around the media coverage of Gliese 581C without hitting the word Earth. Earth-like. Super-Earth. Earth-like life. Earth twin. Sister planet. Earth-like conditions. Earth 2. The climate in exoplanet science is positively Gaiacentric.
It's not surprising, though, that people would use Earth as the ultimate yardstick for measuring the worth of a new planet. (And that science journalists would exploit it to attract eyes to their coverage.) Maybe it is a reflection of a deep-seated anxiety about whether we are alone in the universe.
In other words, when we find an "Earth-like" planet such as Gliese 581C with a rocky foundation and temperature conditions favorable to liquid water, then it seems more likely we might someday find a form of life there that seems familiar. You know, something with water-filled cells, containing something like DNA, doing things you would expect terrestrial cells and DNA to do. And we would not be alone.
Just for the record, the limits of what "life" means is really limited only by our imaginations and the rules of physics and chemistry. But even exobiologists — scientists who spend time thinking about what life could be like on other planets and what interplanetary explorers would have to do to find such life-forms — seem obsessed with Earth-like life. But, after all, there is an important advantage of focusing on Earth-like alien life: As we stumble around in the darkness of the solar system, at least we have a clear picture of what it is we are looking for. And if we find it, we might actually notice.
Obsessing about the Earth-likeness of life has a downside, however. Explorers could some dayland on Gliese 581C looking for Earth-like life — plants swaying in the breeze, soft-bodied wormy things at the shore of a lake, furry-leggy things skittering around the highlands — and remain utterly oblivious to the silicon-based life-forms groaning quietly beneath their feet as they walk on what they think is merely the rock-strewn shore of an exo-ocean.