I actually went through this myself not long ago. I estimated something similar. I was guessing about 2 or 3 planets that may have had or has intelligent life capable of developing civilizations and advanced technology in the average galaxy of 500 billion stars.<br><br>The question is not just what is the probability, but also the chance that they exist at the same time as when the others in the galaxy or universe exist as well. Intelligent life could die out due to any number of reasons given enough time and chance.<br><br>It's important to realize that dinosaurs lived in earth for 165 million years. And humans are far better at adapting and surving than they were. However, for the time being we are our own worst enemy.