Astronomy Professor, Writer, Emmy Award Winner
Some scientists working on the question of extraterrestrial intelligence, myself among them, have attempted to estimate the number of advanced technical civilizations in the Milky Way galaxy - that is, societies capable of radio astronomy. Such estimates are little better than guesses [but] when we do the arithmetic, the number that my colleagues and I come up with is around a million technical civilizations in our Galaxy alone.
Former Astronomy Professor
...I tried to estimate the number of planets in our galaxy with intelligent, technological civilizations. The result has come to be called the Drake equation. But I've recently realized that [we] underestimated the number of possible life-sustaining planets because we thought they had to be confined to a particular orbit: within the continuously habitable zone... We used to think N was about 10,000. Now I think it could be a great deal larger.