In Search for Alien Life, Scientists Suggest a New Scheme
Thanks largely to the Kepler space telescope, astronomers have discovered more than 2,000 planets orbiting distant stars — not half bad considering that until recently we knew of only eight planets in the entire universe, all of them in the immediate neighborhood. The point of Kepler isn’t simply to rack up numbers, though: the ultimate goal is to find worlds similar to Earth — places where there’s a chance that alien life might have taken hold. Those planets could then get a closer look as a new, more powerful generation of telescopes comes on line.
But the search for life across interstellar space will still not be easy, and even the most advanced telescope on the drawing boards will have to work hard to suss it out, so it will be key to choose the best possible targets. That’s the reasoning behind a new paper in the journal Astrobiology in which environmental scientist Dirk Schulze-Makuch, of Washington State University, along with nine other colleagues, has proposed a new planet-classification scheme to make the sifting process easier.
Actually, they’ve proposed two schemes, designed to let observers slice their searches in two different ways. The first and crudest of their methods is something they call the Earth Similarity Index, or ESI. That’s just what it sounds like: it’s a measure of how closely an alien world matches Earth in terms of size and temperature. The temperature is important because biologists say liquid water is an essential ingredient for life as we know it: nutrients can dissolve easily in water in order to circulate to every part of an organism. Blood, after all, is essentially just water with stuff dissolved in it.
The size of the planet, meanwhile, is important because … well, actually, it’s not clear why. It’s true that if a planet is too small, like Mars, it might not have enough gravity to hold on to its atmosphere. Mars itself once did have a blanket of air, but it was probably blasted away by barrages of incoming asteroids billions of years ago. And if the planet is too big, gravity might have pulled in too much of an atmosphere, which would create crushing pressure at the surface. A world four or five times as massive as Earth might well be habitable, but anything bigger than that could cause problems. Still, since Earth is the only place we know harbors life, the ESI favors planets as much like ours as possible.