The Kepler Mission is up and running, trailing Earth around the Sun, its eye focused on a wedge of space about 3000 light-years long reaching out along our arm of the MIlky Way Galaxy -- just a nick of our neighborhood really. For three-and-a-half years (at least) the telescope will keep a continuous watch on 100,000 Sunlike stars. It will be watching for the tiny dimming of a star's light as a planet passes in front of the star, an event called a transit. From the amount of dimming, the timing of transits, and some basic physics, astronomers can calculate the size, mass, orbital period, distance from star and surface temperature of the planet. They will be looking for Earth-sized planets in the sweet zone, just far enough from a Sunlike star to have a rocky surface and liquid water.
To have a transit, the plane of the planet's orbit has to lined up with the Earth. Only about one exoplanet system similar to ours out of 200 can be expected to offer an edge on view, which is why Kepler will be monitoring so many stars. If all goes well, within four years we should know just how unique or how common our watery blue planet is in the universe.
The safest bet is that we are common. Why? Because every time in the past we thought ourselves unique or central, we turned out to be wrong.
I think of Giordano Bruno, Kepler's contemporary, who taught the multiplicity of worlds and paid the ultimate price for suggesting (among other heretical things) that we might not be the singular lords of creation. Bruno was unfortunate to live in a society that defined itself by hierarchy -- a great chain of being that reached from the center of the Earth to the foot of God's throne, with humans at the top of the material heap, and even among humans an itinerant philosopher fell somewhere below pontiff, cardinals, bishops, and secular princes. The divinely-ordained hierarchy brooked no notion of cosmic ordinariness.
And now look. Without leaving my desk on this Bahamian island (thanks to Google Street View) I can walk through the streets of Rome to the corner of the Campo de Fiori, where Bruno was burned at the stake in 1600, and there looming over the market stalls is the statue of the man himself, on the site of his execution. Perhaps the Kepler Mission should have been named for him.