Are We Alone in the Universe After All?

In the movie Contact (which is based on the novel with the same title by Carl Sagan), Ellie Arroway (played by Jodie Foster) was speaking to a group of children: “I’ll tell you one thing about the universe, though. The universe is a pretty big place. It’s bigger than anything anyone has ever dreamed of before. So if it’s just us… seems like an awful waste of space. Right?” For us scientists, perhaps I should say most of us, it is almost like faith that there should be an intelligent life other than us somewhere out there in the universe. In 1961, an astronomer Frank Drake proposed the Drake equation (which is not really a mathematical equation but a probabilistic argument) $$N=R_\ast\cdot f_p\cdot n_e\cdot f_\ell\cdot f_i\cdot f_c\cdot L,$$ where

1.  $N$ is the number of active, communicative extraterrestrial civilizations.
2. $R_\ast$ is the average rate of star formation in our galaxy.
3. $f_p$ is the fraction of formed stars that have planets.
4. $n_e$ is the average number of planets per star that has planets.
5. $f_\ell$ is the fraction of those planets that actually develop life.
6. $f_i$ is the fraction of planets bearing life on which intelligent, civilized life has developed.
7. $f_c$ is the fraction of these civilizations that have developed communications, i.e., technologies that release detectable signs into space.
8. $L$ is the length of time over which such civilizations release detectable signals.

The original estimate for $N$ given by Frank Drake in 1961 is somewhere between 1000 and 100,000,000 civilizations in the Milky Way galaxy alone. (See here for more details on Drake’s educated guesses on the above quantities 1-8.) I remember back in the 80’s the Drake equation was a great talking point for astronomers to argue why we should support SETI (Search for Extraterrestrial Intelligence).