For the longest time, it was assumed that we could never know the answer to this question. After we started measuring the distances to the stars, we realized how mind-bogglingly vast the galaxy was, and it sort of crushed the hope for exploration of those stars. This was before we knew about other galaxies, which are even mind-bogglingly further. Hopes for interstellar exploration are dim, barring the discovery of exotic things like "Negative matter" or energy on the scale of Plank Energy. But Frank Drake realized that using probabilistic statements, he could estimate the number of alien civilizations in existence. It's very non-controversial, and just a bit conservative:
N = R* x fp * ne * fl * fi * fc * Lor the Number of civilizations in our galaxy is equal to the rate of star formation in our galaxy, the fraction of those stars that have planets, the number of planets that can potentially support life per star, the fraction that go on to _develop_ life, the fraction that go on to develop intelligent life, the fraction of intelligent life that develop technology, and all this times the length of time that civilizations will release detectable signals.
It's difficult to estimate fl, fi and fc, but thanks to exciting work on the moon Titan, as well as Mars, the percent of planets that develop life, or are capable of developing life might not be as low as we thought - that is fl. R* is considerably easier to measure, and is estimated to be around 7 stars per year, averaged over the life of our galaxy. This leaves fp and ne - fraction with planets, and number of earth like planets - as the last of the parameters we have an honest chance at getting a great estimation of.
That's where Kepler comes in. The Kepler satellite is a stellar observatory launched by NASA 2009, with the goal of finding earth-like planets. The problem with detecting planets in space is two-fold. A) they're tiny, and b) they don't really glow. Imagine you're on a boat, on a clear night, trying to look at another boat with a pair of binoculars. You can see the other boat - it has lights on - but you can't see the people because they don't glow, and they're too far away. Occasionally, one might pass under some light, and you could get a glimpse of them for a short moment they're fully illuminated but you have to be very lucky to see this, and it doesn't work from a greater distance away. Or they might pass in front of the light, and though you can't see them, you know they're there because they blocked the light. It's easier to see them from a greater distance, since you're inferring from the dip in light reaching you, not by directly imaging them.
The Kepler mission similarly uses this 'transit' method for looking at stars, looking for tiny dips on light as the planets pass in front of them, temporarily blocking some of the light. Previously, our methods were constrained to looking for planets so massive that their gravity causes their mother star to wobble. Unsurprisingly, most of the planets we've found are massive and close to their mother star - definitely not candidates for life supporting planets. The Kepler mission can spot much smaller planets using the transit method, and will let us (among other things) narrow down the window on how many earth like planets there are.
|A screen capture from Dimitar Sasselov's presentation at the TEDGlobal conference.|
I'm not an astronomer, so I can't generalize this to a rate and a number for the purposes of the Drake Equation, but it's pretty dang clear to me that the new values will be high. This, holding the rest of the estimated values of the Drake Equation steady, raises the number of other civilizations out there. As a biologist, I can't help but feel life is almost inevitable (although intelligent life rare). I have more than gut reasons for holding this view, and maybe I'll elaborate on them later. I strongly suspect within my life-time, we'll find proof of life on Titan (before Mars - I'll even gamble money on it), which will widen the niche that life is show to be able to cling to. In reality, I strongly suspect we will find that life, though not cheap, is abundant and resplendent.
You can read more at discovery news, and spaceref.com. And pretty soon, I suspect, everywhere else.