Drake's Equation

How many alien civilizations are out there? Let's try to guess. Our galaxy, the Milky Way, has about 100 billion stars. Suppose that:
A: The average number of planets per star is B: Of all planets, the conditions needed for life exist on C: Of planets where the right conditions exist, intelligent life actually forms on

0.1
1
10
(PICK ONE)
 one out of
1000
100
10
(PICK ONE)
 one out of
100,000
1000
10
(PICK ONE)

Then the number of planets with intelligent life in the Milky Way is

A * B * C * 100 billion =



This is a simplified version of a formula called Drake's Equation. The real Drake's Equation has seven terms instead of three.

If they all sent us radio signals at once, it might sound like this.