Habitable-zone earthlike exoplanets are like cockroaches: if you see one on the counter, there are probably twenty more under the fridge. It would have been unreasonable to expect to find a second earthlike planet at this stage of the exoplanet hunt, but NASA's Kepler Space Telescope just did (and the finding has been corroborated by two telescopes here on Earth). And this tells us that earthlike habitable-zone exoplanets are probably very, very common.
You can look at a mathematical simulation of the planet (Kepler 186f) and its galaxy here:
What does this mean for exobiology? Lots. When it comes to discovering life in the universe, we have a sample size of one to work from—and that one precious sample, our own home, is a habitable-zone earthlike (very earthlike) planet. Absent evidence that life has evolved on any other kind of planet, we can reasonably assume that looking for planets like ours isn't a bad way to find life like ours.
But there's another exciting, and potentially very humbling, implication of this discovery: in 2018, we will very likely have the capacity to check other planets for photosynthesis. And when we say a habitable-zone earthlike exoplanet is a good place to look for life, the kind of life we're looking for can rely on photosynthesis for energy. (Chemosynthesis can theoretically happen almost anywhere.) So what will it mean if, in 2018, we point the James Webb Space Telescope at Kepler-186f or another habitable-zone earthlike planet, snap a chemical profile, and find evidence of photosynthesis—something that only living organisms (or machines built by living organisms) can produce?
It'll mean we've really and truly discovered extraterrestrial life.
And what will it mean if we point the JWST and find photosynthesis on dozens or hundreds of candidate habitable-zone earthlike planets?
It'll mean we'll be looking outward on a universe teeming with undiscovered life—more vast, abundant, and diverse than we could ever explore. And if our understanding of the universe is changed in this profound and deeply humbling way, I have a feeling we'll all remember where we were, and what we were doing, when it happened.