The Milky Way has approximately 200 billion stars, at least half of which have orbiting planets. If you estimate that each solar system holds 5 planets, that means there are ~500 billion planets in our galaxy. Surely some of these planets have Earth-like conditions, allowing for evolution to occur. Perhaps civilizations have formed and technology has grown strong enough to emit signals.
Seti-scientists are listening in closely. 300 miles north of San Francisco, deep in the desert, the giant Allen Telescope Array is being expanded. Shown above, the array is a collective of computer-controlled radio antennas designed to work as one integrated unit. Eventually there will be 300 dishes searching for potential alien messages drifting through space.
Unfortunately, they've been searching for 50 years, and haven't heard a peep. In their defense, up until 1995, scientists didn't even know if planets existed beyond our solar system (exoplanets). This made it difficult for them to decide where exactly they should be looking.
Technology prevailed again, and a flurry of exoplanets have recently been discovered. More than 260 to be exact... but none of them anything like Earth. Then, in early 2007, an impressive discovery was made from an observatory in northern Chile. Astronomers found Gliese 581C: the smallest planet ever found orbiting a main sequence star. It was 5.5 times the mass of our planet, and appeared to be at the right distance from its star to support life. For now, other than its location, all other information about this planet is completely speculative.
In 2009, Nasa plans to launch Kepler, a space telescope looking for new worlds. It is designed to be sensitive enough to detect Earth-like planets, and will scan 100,000 stars, day + night, for four years. Nasa's calculations predict that at least 50 Earth-like planets should be found.
All of this information will be very useful for the Allen Telescope Array. Instead of aimlessly looking through the entire galaxy, they can focus their efforts only on planets capable of supporting life.