The starting point for an estimate of extraterrestrial life in the universe is to know the number of stars.
Observations, conducted over the last decade, through powerful telescopes confirmed the homogeneous structure of the universe on a large scale.
Therefore, in brief, by calculating the number of galaxies present in a known volume of space, and knowing the average number of stars present in a galaxy, it is possible to estimate (within a specified margin of error) the number of stars in the universe ( for more information: ESA – how many stars there are in the universe?).
This value is currently estimated at between 10 22 to 10 24 (billions of billions).
Count the stars in the universe is not, in principle, very different from counting grains of sand of a beach. Let’s try …
Let’s go for example in Brazil, in Rio de Janeiro at the famous Copacabana beach and we take some grains of sand.
Under the microscope we can observe that, on average, grains are similar to cubes of side 0.3 mm.
Now we measure the beach. We observe which is about 4 kilometers long and 60 meters in width. The depth should be between 2 and 10 meters, we take, for this value, 3 meters.
The number of grains of sand present in the whole of Copacabana beach is given by the volume of the beach divided by the volume of a single grain, then:
(4000m x 60m x 3m) / (3 x 10-4m)3 ≅ 1016