Form for calculating how many planets in the galaxy have intelligent life

This calculation is based on the following assumptions:

• In order for life to exist, it must have a planet to live on.
• The earth is 5 billion years old, and the earth, solar system and galaxy are halfway through their "lifetime" (i.e. the length of time available for all evolution and development to take place).
• All the stars and planets in our galaxy came into being and will end at the same time (5 billion years ago and 5 billion years from now).
• Once a planet has existed long enough that intelligent life could evolve, the chance of it actually evolving at any given time is equal for the rest of that planet's duration.
• The number of stars and planets in the galaxy is static.
• Conditions suitable for life can only exist on planets that orbit stars. "Rogue" planets, if they exist, will never support life and are, therefore, not considered in our calculations.

Enter what you believe to be reasonable values for each of the following factors. Then click "Calculate" to see how many planets would result from that combination of factors. Enter all numbers as numeric numbers, e.g., "1,000,000" or "1000000", instead of "1 million".