Is earth a lonely planet? Is there any other civilization in our milky way galaxy ? These questions are answered by Drake equation. First let us see the variables in the equation.
1. N# - There are roughly 100 billion stars in milky way galaxy.
2. FP - 20% to 50% have planetary system.
3. NE- 1 to 5 planets of each star have the capability to sustain life.
4. FL - 0% to 100% of the planets have the capacity to support and evolve life.
5. FI - In 0 % to 100 % of planets, the intelligent life evolves.
6. FC - 10% to 20% of intelligent life try to communicate.
7. FL - How long the intelligent life survive and communicate? It is tough to guess. We are communicating using radio waves for last 100 years only.
N - If we multiply all the above factors , we may get the probable number of intelligent and communicating civilization in our galaxy.
N = N#*FP*NE*FL*FI*FC*FL
Let us guess the minimum values, substitute in the equation and find the answer.
= 100 BILLION*50%*1*50%*20%*20%*10000 YEARS
N = 1000 (minimum)
The answer is 1000 civilizations . This answer gives some hope for the search of extra terrestrial life. To find the correct value for the variables in the equation, we have to still learn a lot.
Post script: conditions for life to evolve in a planet.
1. A comfortable temperature from -50 degree c to + 50 degree c
2. Water
3. Oxygen and carbon di oxide.cycle by plants ( photosynthesis) and animals (respiration)
4. Proper atmosphere.