Intelligent Life: Numbers For The Real World

In the movie "Contact," Jodie Foster says, "There are 400 billion stars out there, just in our galaxy alone. If just one out of a million of those had planets, and just one in a million of those had life, and just one out of a million of those had intelligent life, there would be literally millions of civilizations out there."

If there are 400 billion stars out there, and one out of a million of those has planets, and one out of a million of those has life, and one out of a million of those has intelligent life, then the chances of there being intelligent life here on Earth are a million to one. We'd better start looking.

Here's how the Mathemagician explains it:

If there are 400 billion stars out there and one out of a million of those has planets, then 400 billion divided by 1 million have planets. A billion divided by a million is a thousand (or a million in Britain, but it's an American movie). Out of those 400 thousand planets, if one out of a million has life, then 400,000 divided by 1 million (1,000,000) has life. That's 0.4 planets with life, or a 4 out of ten (or one out of 2.5) chance of finding any planet with life at all. Now, if one out of a million of those have intelligent life, then there's about a one in 2.5 million (continuing to divide by a million) chance that there's intelligent life anywhere. The unfortunate part is that the character who makes the statement is an astronomer who has spent her life on this search!

Of course, she's only considering one galaxy. As my friend Dave Dyson (of the Un-Scripted Theater Company) points out, there are about 250 billion galaxies out there, and we now know that there is intelligent life in every 2.5 million of those. That would mean about 100,000 planets with intelligent life. Still not millions, but if Jodie had considered all galaxies, she wouldn't be that far off.

Dave also says "If there are other civilizations out there, what are the chances they exist either a) currently, b) at the time in the past when light would have left them on its way to us, so they could potentially be observable by us, or c) at such time in the future when we can visit them?" So even considering other galaxies, we have to limit ourselves to civilizations that exist close enough to our own time.

From this page you can also find out about:

Today's fact: You are more likely to be killed by a snake than to be killed in an airplane crash. Animal Trivia at says that you have a 1 in 3,000,000 chance of being killed by a snake. You only have about a 1 in 6,000,000 chance of being killed in a plane crash, according to the cheerful people at

The Mathemagician has also heard that you are more likely to be kicked to death by a donkey than killed in an airplane crash, but some donkey lovers have written to him claiming that it's just anti-donkey propaganda. This quote also seems to be somewhat out-of-date. Here is confirmation from a reader (thanks, Mike!):

"The original quotation (ex USAF flight safety briefing) goes:

'In the year 1947 in the United States of America more people were kicked to death by donkeys than were killed in civilian flying accidents'

Trying to get the (London) American Embassy to confirm this from census figures used to be an initiative test!"

- Some guidelines for 'quick check' calculations.
- Some useful number facts for quick checks.
- Wordlore and wordplay.
- Return to the front gate of Esmerel.

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