Alex McColgan

If you think that any of my numbers seem unreasonable, let me know in the comments below.

So, with that, let's see how many alien civilizations we might reasonably expect to see out in the night sky.

To begin with, we can input our values with reasonable certainty.

Scientists looking at the Milky Way galaxy can accurately predict how many stars form every year, as we have many examples to draw from.

Depending on who you ask, the number ranges from between 3 and 7, let's say 5 at a conservative estimate.

Fp is easy to solve too.

Through recent astronomical observations by the Kepler Space Telescope, it's become apparent that planets are very common in the solar system, with each star on average having one.

So let's set this number high as well, let's say 90%.

However, the number that we currently predict is at a suitable distance from their stars, as well as having the ideal mix of elements that would produce life similar to ours is much lower.

Of the 100 billion planets in the solar system, perhaps as few as 300 million fit into this category.

Obviously, this does not account for alien life that's significantly different from us, but let's discount them for the moment as then this would be even harder to predict.

This gives us a percentage chance of 0.3%, quite a small chance that one of the planets in a solar system is suitable for life, so 0.003 for NE.

So far, so substantiated by evidence.

Here is where things get a little tricky.

For the number of times that life has arisen, we only have one example to draw from, life on Earth.

To date, we have not proved that life arose on Mars or Ganymede, for all the conjecture on that front.

So, we can take this estimate one of two ways.

As near as we can tell from the fossil record, as soon as the planet cooled down enough, life came into being, which might indicate a high value for F, perhaps as high as a certainty 1.

But on the other hand, from what we know, all life originated from a common ancestor, which is to say, life formed on this planet from non-biological matter exactly once, and has never risen up again since.

Scientists have looked for evidence of bacteria that might have independently come into being, but so far haven't found any.