How many planets exists which might support life? Indeed, what is required for life to exist? How does life start? How does it evolve, and what fabulous creatures can evolution produce? How often do intelligent creatures appear in the giant tapestry of life? It is exactly these questions, and all of them, which are being addressed by the scientists of the Carl Sagan Center for the Study of Life in the Universe.

Directed by Dr. Frank Drake, the center brings together leading researchers in a field often called "astrobiology," the study of life in the universe.

Our team focuses on a wide set of disciplines ranging from observing and modeling the precursors of life in the depths of outer space to studies of Earth, where we are attempting to learn more about how life began and how its many diverse forms have survived and evolved.

Appropriate to the sweeping scope of this research, we have many partners in our work including NASA, the National Science Foundation, and major universities.

The Drake Equation was developed by Frank Drake in 1961 as a way to focus on the factors which determine how many intelligent, communicating civilizations there are in our galaxy. The equation can really be looked at as a number of questions:

Answer: Current estimates are 100 billion.

Question: What percentage of stars have planetary systems?

Answer: Current estimates range from 20% to 50%.

Question: For each star that does have a planetary system, how many planets are capable of sustaining life?

Answer: Current estimates range from 1 to 5.

Question: On what percentage of the planets that are capable of sustaining life does life actually evolve?

Answer: Current estimates range from 100% (where life can evolve it will) down to close to 0%.

Question: On the planets where life does evolve, what percentage evolves intelligent life?

Answer: Estimates range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%.

Question: What percentage of intelligent races have the means and the desire to communicate?

Answer: 10% to 20%

Question: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive?

Answer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th.

From Seti.org