Maybe we really are alone in the cosmos.

When faced with a mystery about the natural world, science calls for a review of the best available evidence, and the drawing of theories and conclusions supported by the evidence. Now consider a grand question about our cosmos: are planets like Earth common? That is, worlds capable of supporting life as we know it in a "habitable" zone around a star where water in its liquid form? Worlds not too large so the gravity isn't crushing, and not too small so they can support an atmosphere? Until the last 15 years or so we had scant evidence to go on: the eight planets in our solar system (one of which met the above criteria) and the fact that there are billions of stars in our galaxy. That's it. One could make guesses, but one could not say with any confidence whether Earth-like worlds are common or rare. Since the mid-1990s... [read full story]