Does anyone know Carl Sagan's quote and the formula?

Question

Carl Sagan once said that it was hubris for us to believe the Earth was the only planet in the universe that was inhabited. He also put forth a mathematical formula regarding the probability of other inhabitable planets in the universe. Does anyone know his exact statement and the mathematical formula?

My error. I meant possibility of HABITABLE planets.

Answer 1

Its the Drake equation (as faithfully reported above), and he calculated the total number on inhabitable worlds to be something like 100 billion. His quote was "the improbability of being the only life among the billions of habitable plants is impossible to believe. Spock would agree that it is logical.

Answer 2

If for some reason we were to desire a two-way communication with the inhabitants of some nearby galaxy, we might try the transmission of electromagnetic signals, or perhaps even the launching of an automatic probe vehicle. With either method, the elapsed transit time to the galaxy would be several millions of years at least. By that time in our future, there may be no civilization left on Earth to continue the dialogue. But if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after about 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization. Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their own world, an adventure and a challenge certainly difficult to duplicate.


Pale Blue Dot (1994) was Sagan at his best as he ranged through the Solar System and looked back on a distant Earth. Here’s the memorable conclusion:

The Cosmos extends, for all practical purposes, forever. After a brief sedentary hiatus, we are resuming our ancient nomadic way of life. Our remote descendants, safely arrayed on many worlds through the Solar System and beyond, will be unified by their common heritage, by their regard for their home planet, and by the knowledge that, whatever other life may be, the only humans in all the Universe come from Earth.

They will gaze up and strain to find the pale blue dot in their skies. They will love it no less for its obscurity and fragility. They will marvel at how vulnerable the repository of all our potential once was, how perilous our infancy, how humble our beginnings, how many rivers we had to cross before we found our way.

Answer 3

You mean the Drake equation?

http://en.wikipedia.org/wiki/Drake_equation

One word: NONSENSE.

Sagan, of course, was absolutely right, but the Drake equation is not a scientific way to talk about it.

If you are looking for real science, the extrasolar planet searches are the best we have, right now.

http://en.wikipedia.org/wiki/Extrasolar_planet

And man, that is some great stuff! I never expected the field to progress this far, this fast. Very exciting! Unless I get hit by a car, I expect to see the first blurry images of the nearest habitable earth-like planet to be available in my lifetime.

And one more word to the wise: why hang around bad science and pseudo-science like the Drake equation, when you can have the real thing?

Answer 4

Its usually refered to as the Drake equation:

N = N* fp ne fl fi fc fL

The equation can really be looked at as a number of questions:

N* represents the number of stars in the Milky Way Galaxy

Question: How many stars are in the Milky Way Galaxy?
Answer: Current estimates are 100 billion.

fp is the fraction of stars that have planets around them

Question: What percentage of stars have planetary systems?
Answer: Current estimates range from 20% to 50%.

ne is the number of planets per star that are capable of sustaining life

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.

fl is the fraction of planets in ne where life evolves

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%.

fi is the fraction of fl where intelligent life evolves

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%.

fc is the fraction of fi that communicate

Question: What percentage of intelligent races have the means and the desire to communicate?
Answer: 10% to 20%

fL is fraction of the planet's life during which the communicating civilizations live

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.

When all of these variables are multiplied together when come up with:

N, the number of communicating civilizations in the galaxy.