Are we really alone?

The question of whether or not life exists anywhere else in the universe, especially so-called intelligent life, is one I have often pondered.  Ultimately, I have felt that the universe is so incredibly and unimaginably vast, that surely there has to be more than just us.  But what are the odds?

Well, you know what they say about guesswork and statistics, but nevertheless, I decided to do a bit of simple math to see how likely it might be that there is something else out there.

Here is what I came up with:

Estimating the odds of extra-terrestrial life in the universe

How many galaxies are there?

It is variously estimated that the universe contains somewhere between 100 billion and 70 trillion galaxies.  Most of the recent estimates are over 200 billion.

So lets say 200,000,000,000 galaxies in the universe.

How many stars are in an average galaxy?

There are as many as trillions of stars in the larger galaxies, of which there are many. Some of those larger galaxies have over 200 times the mass of our own modest galaxy, the Milky Way, which has an estimated 100 to 200 million stars.

So, for argument's sake let's be reasonably conservative and average number as 200 million stars in each galaxy.

How many stars are in the universe?

200 billion times 200 million gives us an estimated total of 40,000,000,000,000,000,000 or 40 thousand million billion stars - and that is a fairly conservative estimate.

How many stars have planetary systems?

Increasingly sophisticated planetary discovery technology is finding planets to be much more common than was once thought - and most cosmologists now believe that planetary formation around a star is commonplace. For the sake of argument let us say it's not and that star formation only ocurs in 1 out of 1 million stars - a hugely conservative estimate.  Now, let's also conservatively estimate that the average number of planets circulating stars which do have planets is only two.

That calculation results in:

40,000,000,000,000,000,000 divided by 1,000,000 times 2 =  80,000,000,000,000 planets in the universe. Eighty thousand billion.  Again, a very conservative figure.

How many planets are capable of supporting life?

Let's continue to be conservative and say that planets capable of supporting life are rare - perhaps only one in a 100,000 even.

That still gives us 800 million planets capable of having life!

Does anyone believe that the chances of producing life as advanced or more advanced than ours is only 1 in 800,000,000?

Now remember that most of the figures selected were quite conservative.  Using figures that were less conservative in the calculations could easily have produced numbers 1000 or more times greater.  In other words 800,000,000,000 or more planets with conditions favorable for producing life.

So, do you still think we are alone?