The Kepler telescope discovered more than 1,200 planets in just one tiny corner of the Milky Way. Crunching the numbers, a conservative estimate says there should be at least fifty billion planets in the entire galaxy, and about 500 million of those should be inside the habitable zone. But how many of those planets have life on them, let alone other intelligent beings? That's the question we still can't answer...but we're getting closer.
The latest estimates suggest there are roughly 300 billion stars in our galaxy. Assuming the Kepler sample is representative, there should be at least one planet for every six stars, and even the 50 billion figure might be far too low. After all, Kepler can still only detect planets that orbit relatively far away from their stars, and we're only going to see exoplanets at all if the observational conditions are right. That means there are likely even more planets that we're not seeing, and in particular we might be missing out on plenty of planets in the habitable zones.
Still, for the sake of argument, let's say the 500 million potentially habitable planets is accurate. How many of those planets have life on them? Unfortunately, there's still no way to know, because we still don't have any comparison point for life on Earth, anyway to sense just how unique (or commonplace) our planet really is. If the existence of life is a 1 in 500 million chance, then we're probably all alone in the Milky Way.
But what if it's 1 in 1,000? Then there might be half a million worlds teeming with life, and that's just in our own galaxy. Of course, there's still the question of how many of these theoretical life-supporting planets have given rise to intelligent life. There's some reason to be cautiously optimistic about simple life springing up on habitable worlds - after all, we've found some inconclusive but tantalizing evidence of very simple life (or at least its building blocks) on other worlds just within our own solar system.
As for intelligent life...well, I suppose UFO enthusiasts might argue there's lots of evidence for that as well, but that's decidedly outside the scientific mainstream. At the end of the day, 50 billion planets (at least!) is an almost impossibly massive figure, and it's hard to believe that we could be all alone in a galaxy that is so teeming with worlds, even if we're just sharing the Milky Way with a lot of alien microbes and bacteria. We can't know just how common or uncommon life is from this data alone, but at least we're starting to fill in some of the first few variables of the Drake equation.