Society's key to finding the next Earth: The Ars guide to exoplanets

Are we alone in the Universe? For years, people have been making predictions, many using the Drake equation. That involves the use of various educated guesses about the frequency of planets, how many are habitable, and so on. Until about a decade ago, most of the values in the equation remained just that, however: guesses.

In the last dozen years, we've witnessed an amazing transformation in science and appear to be on the verge of several more. The existence of planets orbiting other stars—exoplanets—has gone from a hypothetical to a reality. We've now got a catalog of thousands of potential planets. In many cases, we even have an idea about their size, composition, and temperature. Some of them orbit stars that are, in galactic terms, right next door.

The result has been an incredible buzz of information—over the course of this winter, there were a series of updated estimates on the number of planets in the galaxy (answer: lots) along with various ways of slicing and dicing the numbers. How many Earth-like planets? How many orbiting stars like our Sun? In every case, the numbers were staggeringly large, with the possibility Earth could be one of millions, if not billions, of similar planets in our galaxy alone.