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 hundreds of thousands, millions, billions or trillions is poor. We just don’t deal with these numbers in every day life so we have no comprehension of what they mean in practice.

When you consider statistical probabilities, the chance that NO ONE will be killed in the next year by lightning is a stunning 10^-10133 that’s 10 with over ten thousand zeros following it!!

When we consider the possibility of life in outer space, there’s two very important aspects to consider.

First, there is unequivocal evidence for life in outer space. Earth!

Second, knowing life arose at least once, we know life can arise. What we need to do is to determine what the probability of life arising elsewhere actually is. That’s no easy feat, but as science continues to learn more about the origins of life on Earth, the picture will become clearer.

There have been a number of attempts at quantifying the probability of life arising elsewhere in the universe, most notably, the Drake Equation, but recently an alternative has been suggested called the Seager Equation that seeks to narrow the focus from the generalized question “What’s the chance of life arising ANYWHERE?” to a very pointed and specific “Based on what we know, what are the chances we will be able to find life elsewhere in the next decade by detecting biosignature gases?

Seager Equation


  • N is the number of planets with detectable biosignature gases
  • N* is the number of stars within the sample
  • FQ is the fraction of quiet stars
  • FHZ is the fraction with rocky planets in the habitable zone
  • FO is the fraction of observable systems
  • FL is the fraction with life
  • FS is the fraction with detectable spectroscopic signatures

If we inject realistic figures based on our current scientific understanding and the ability of a new generation of satellite telescopes like the James Webb to search for exoplanets, this is what we get…



That’s two planets with signs of life!!!

Now, some might question the assumptions in this equation, pointing out that FL (Fraction with Life) is high being arbitrarily set at one, and they’d be right. But there are other factors that are deliberately understated. The sample size is 30,000 stars. The James Webb Space Telescope is capable of scanning 500,000 stars in the immediate neighborhood, each with potentially dozens of planets, not just a paltry 30,000.

What probability do you want to assign? At this point, we really have no way of really knowing what some of these values are, but we do know that there are at least 70 billion trillion (7 x 10^22) stars in the known universe.  Other estimates are several orders of magnitude higher again, at an octillion or 10^29


Picture credit. Sloan Digital Sky Survey map of the known universe. At this scale, no stars are visible. Individual galaxies like the Milky Way are also invisible. Only clusters of galaxies, super clusters and filaments containing millions of galaxies can be seen.

Remember our lightning example. Even highly improbable events can be common place given big enough numbers. If the chances of life arising around a given star are stupendously small, at one in a trillion or 10^11, there would still be trillions upon trillions of planets with life in our universe.

Interestingly, though, if the odds were that low, then the chances of life arising in our galaxy are less than one as there’s less than a trillion stars in the Milky Way, which could explain why we haven’t found anything. But for the odds to be that low, there would have to be something “wrong” with the majority of galaxies (as the majority of galaxies have less than a trillion stars). Point being, even if we take a worst case scenario, it seems highly unlikely we are entirely alone. The odds are against it.

Personally, I think the search is only just beginning. We’ve looked at so little of the universe, it’s no surprise we haven’t found life elsewhere. In the words of Neil deGrasse Tyson, “Life doesn’t exist anywhere but Earth? That’s like filling a cup with ocean water and saying there aren’t any whales.”


4 thoughts on “What are the chances of finding life in outer space?

  1. “Extremely unlikely events are often quite common” – a few miles from me, a 42-yr-old mother was struck by lightning while camping with the family. A h.s. biology teacher (fiance of my son’s friend) died when a tree fell on the tent she was sleeping in. I’m still waiting for a ship of aliens to come abduct me for my gardening skills to cultivate a lookalike planet Earth in some distant cosmos. 😉 Funny thing: I can’t get much of anything to grow in my sand and clay, and I can’t even get the compost heap to produce mulch. The husband and kids may never lose me to those aliens after all.

    • Yeah, although the odds are stacked against participants, someone always wins the lottery. Sad to hear about the fiancé of your son’s friend. Heartbreaking. As for being abducted for gardening skills, now there’s a story I could write 😉

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