Carl Sagan re-did Drake's calculation, and estimated perhaps millions of planets may have intelligent life forms. Since then, new data indicate that the conditions for life are much rarer in our galaxy than previously thought. Life in the galaxy must meet a stringent set of conditions, so stringent that perhaps only a few hundred planets in the galaxy have intelligent life. Some physicists believe that we already have been visited by a type three civilization. The most efficient way to explore the galaxy is to send self-replicating robot probes to land on distant moons.
Moons have no erosion or rust and the probes can last for millions of years.
These probes, like a virus, create millions of copies of themselves to land on other moons. Therefore, within 100,000 years, you can probe an entire galaxy. On our own moon, there could be a probe left over from a passing type three civilization waiting to be discovered. This was the basis of the movie 2001. However, to exceed the speed of light, may take thousands of years until we become a type two civilization.