@ Finkelstein:
"Astronomers estimate 100 billion habitable Earth-like planets in the Milky Way, 50 sextillion in the universe"
Unfortunately the majority of those planets would be in the core of our galaxy where stars are so close together that they could pull planets out of their orbit freezing them; increase cometary bombardment eliminating complex life from forming; and getting fried by novas/supernovas that are close by. Life might be very common but advanced life might not.
The book Rare Earth: Why Complex Life is Uncommon in the Universe has a good and very readable explanation of how this would play through. Highly recommended.