From Air & Space Magazine:
Astronomers have devised ingenious methods for finding objects that were, as late as 2010, thought to be rare: Earth-size planets orbiting other stars. So far, more than 4,000 exoplanets of all sizes have been discovered, leading scientists to estimate that, in the Milky Way alone, there are 100 million habitable planets. And yet, despite the recent discoveries, one of the greatest goals of the scientific community remains elusive: an image of a habitable, Earth-like planet orbiting in another solar system. Capturing an image is the only way to confidently identify a small rocky planet among the thousands discovered and to study its atmosphere, assess its orbit, and determine whether it might be capable of supporting life. The conundrum for astronomers is that—unlike stars, superheated gases, and other bright objects—Earth-like planets are too dim to be seen by telescopes.