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.
Lets 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 to 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 revolutionised planet hunting and new planets are being discovered at an ever increasing rate. So far (August 2003) 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 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!
For another approach I recommend The Drake Equation. This states that the number of communicating civilisations in our galaxy (note, our Galaxy only, not the universe) likely depends on a number of factors which must combine to yield a habitable planet where life has the chance to develop to a certain level of technological know-how. These factors include the rate of formation of stars like the Sun, the fraction of those with planets, the fraction of Earth- like planets, the fraction of such planets where life develops, the fraction of those where life becomes intelligent, the fraction of intelligent species who can communicate in a way we could detect, and the lifetime of the communicating civilisations. As you may imagine, There is a lot of debate about reasonable values for most of these factors.
Frank Drake's own estimate puts the number of communicating civilisations in just our Galaxy alone at 10,000.
Even though the figures I have used cannot of course be considered to be accurate, at least the figure of 10 sextillion stars in the universe is most definitely an underestimate. The number of life supporting planets that may be orbiting those stars is impossible to say, but by any reasonable estimate must surely run into the millions, if not billions. This is easy to justify on the basis that following the Big Bang the most abundant material in the universe was hydrogen and helium, being the most simple atoms, and this material forms the bulk of the raw ingredients for star formation. All stars begin life in the same manner, by the gravitational drawing together of these basic elements that then gravitationally collapse to form a star. Apart from size, all stars begin pretty much the same, with the remnants of the hydrogen and helium clouds that are not absorbed into the stars forming an orbiting disc that goes on to form the protoplanets. With this same process repeated many billions of times it would be only statistically reasonable to expect that many planets would have similar characteristics, and would be capable of supporting life of one form or another, just as our planet does.
In order to answer the question of the existence of extraterrestrial life, it need exist on only ONE other planet. Given those odds, how can it not exist?
