If 100% of the stars observed by Kepler had one Earth-like terrestrial planet in an orbit identical to that of the Earth, how many Earth-like planets would you expect to be discovered? How about 10%?
Suppose that 100 Earth-like planets are discovered by Kepler. From this result, estimate the percentage of stars that would have Earth-like planets.
The Kepler mission of NASA is designed to observe more than 100,000 stars every 30 minutes and check their brightness for presence Earth like planets. Let us assume, that Kepler observes 100,000 stars.
If 100% of these stars have Earth-like planets, then we will have a 100,000 Earth like planets. If only 10% of these stars have Earth-like planets, then we will have 10,000 (= 10/100 x 100,000) Earth-like planets.
If 100 Earth-like planets are discovered by Kepler, then the percentage of the stars that have Earth-like planets can be calculated as:
% of stars with Earth-like planets = (100/100,000) x 100 = 0.1%.
Kepler mission is only concentrating on two constellations, Cygnus and Lyra. There are billions of other constellations and in reality, there can be a very large number of Earth-like planets.
Hope this helps.