Man is always interested to know about the life on other planets in universe. It is very difficult for man to find out what is present outside the solar system. Frank Drake and Karl Sagan stated a formula to find out the probability of presence of life on other planets.

Number of advanced lives in universe is given by formula,

n = a * b * c * d * e * f * g

where,

a = number of stars taking birth in a Milky Way.

b = ratio of number of stars having planets moving around them to the total number of stars.

c = number of stars having conditions which will help in initiation of life.

d = number of planets on which life exists in any form.

e = number of planets on which advanced technical life exists.

f = number of cultures using communication system.

g = average life of culture.

In above formula, a, b, c are astronomical, d, e are biological and f, g are social study related. Milky Way contains 400 billion stars. According to scientists, 10 new stars take birth in Milky Way per year.

So, a = 10 stars / year

Out of 10 stars, let's assume that only one star has planets moving around it. Therefore, b = 0.1

Only one of all planets has conditions favorable for initiation of life. Therefore, c = 0.1

If planet is favorable for initiation of life, then the life must flourish on the planet. Therefore, d = 0.01

Production of advance, intelligent, cultural life can take anytime. e_max = 1 and e_min = 0.01

Any culture will always try to adopt to communication system. Out of that, if only 10% efforts are successful then, f = 0.1

Value of 'g' depends on many factors such as natural extremes, wars, etc. So, we will not find value of 'g'.

Now, using moderate values; n comes out to be,

n = 10 * 0.1 * 0.1 *1 * 1 * 0.1 * g

n = 0.01 g

And using lowest values; n comes out to be,

n = 10 * 0.1 * 0.1 * 0.01 * 0.01 * 0.1 * g

n = 10 to the power -6 * g

n = 0.000001 * g

This is the probability of life in universe.