1) The number of galaxies. An estimated 50 billion galaxies are visible with modern telescopes and the total number in the universe must surely exceed this number by a huge factor, but we will be conservative and simply double it. That’s 100,000,000,000 galaxies in the universe.
2) The number of stars in an average galaxy. As many as hundreds of billions in each galaxy.
Let us call it just 100 billion.
That’s 100,000,000,000 stars per galaxy.
3)The number of stars in the universe.
So the total number of stars in the universe is roughly 100 billion x 100 billion.
That’s 10,000,000,000,000,000,000,000 stars, 10 thousand, billion, billion. Properly known as 10 sextillion. And that’s a very conservative estimate.
4) The number of stars that have planetary systems. The original extra-solar system planet hunting technology dictated that a star needed to be close to us for a planet to be detected, usually by the stars ‘wobble’. Better technology that allows us to measure the dimming of a stars brightness when a planet crosses its disk has now revolutionized planet hunting and new planets are being discovered at an ever increasing rate. So far around 100 have been discovered so we have very little data to work on for this estimate. Even so, most cosmologists believe that planetary formation around a star is quite common place. For the sake of argument let us say it’s not and rate it at only one in a million and only one planet in each system, as we want a conservative estimate, not an exaggerated one. That calculation results in:
10,000,000,000,000,000 planets in the universe. Ten million, billion, as a conservative estimate.
5) The number of planets capable of supporting life. Let’s assume that this is very rare among planets and rate it at only one in a million. Simple division results in:
10,000,000,000 planets in the universe capable of producing life. Ten billion!