What are the chances of finding life in outer space?

In his book, The Improbability Principle, Professor David Hand makes the point that extremely unlikely events are often quite common.

At first, this sounds like a contradiction, but it’s not, and it has serious implications for SETI, the Search for Extra Terrestrial Intelligence.

We have a difficult time comprehending large numbers and recognizing their implications. For example, the chance of being struck by lightning in any given year is around 1 in 300,000. Not odds you’d normally worry too much about, right?

The chance of being struck by lightning is absurdly low. But when you consider that there are 7+ billion people on the planet it all of a sudden becomes obvious that being killed by lightning is actually quite common. Around 24,000 people are killed every year by lightning, and another ten times that number are seriously injured!

Our ability to intuitively understand probabilities involving numbers in the…