Sciency Words A to Z: The Zero-One-Infinity Rule

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, Z is for:


All month, we’ve been talking about astrobiology, SETI, and the possibility that we are not alone in the universe.  I’d like to end this series with a prediction for the future, and conveniently my prediction is related to a Z-word: the zero-one-infinity rule.

The zero-one-infinity rule was originally created by Dutch computer scientist Willem Louis Van Der Poel.  For the purposes of computer programming, the rule has to do with how many times a user is allowed to do a thing (whatever that thing might be).

It makes sense for a user to never be allowed to do a certain thing (zero), or it makes sense for a user to do a thing only once (one).  But if you’re going to allow a user to do a thing more than once, you may as well let the user do that thing as many times as the user wants.  As a rule of thumb, the zero-one-infinity rule means there’s no reason to impose arbitrary limits on what users can do.

The zero-one-infinity rule has been adapted to many other scientific fields, including the field of astrobiology.  How many places can life exist in the universe?

  • Zero: the universe might not allow life to exist at all.  Of course we already know this isn’t  true, otherwise we wouldn’t be here.
  • One: the universe might only allow life to develop once.  In this view, Earth is a crazy exception, a one-time fluke in a universe that otherwise does not allow life to exist.
  • Infinity: the universe allows life to exist anywhere and everywhere it can.  Life might still be rare in this view, but there are no arbitrary limits imposed on life.

I remember in the 80’s and early 90’s there were a lot of people (including one of my science teachers) who honestly believed our Solar System might be unique.  No other star except our Sun was known to have planets. Maybe that was because there were no other stars with planets.  In short, our Solar System was a “one” in the zero-one-infinity rule.

Then in 1992, astronomers announced the discovery of the first known exoplanets—planets orbiting a star other than our Sun. At the time, we still had no idea just how many exoplanets we might find, but if the universe had allowed two solar systems to form, why not three?  Why not a dozen, or a thousand, or a million?  As soon as the case for “one” crumbled, the possibilities were suddenly limitless.

I predict the same thing will happen when we finally discover alien life.  Maybe it will be microorganisms on Mars, or sea monsters on Europa, or ham radio enthusiasts in the constellation Sagittarius.  It won’t matter which kind of life we find, specifically. Any alien life will do.

In this special edition of Time Magazine, there’s a brief mention of the zero-one-infinity rule.  In that article, NASA scientist Chris McKay sums up the whole field of astrobiology by saying, “So what we’re searching for is two.”  Because once we know that life developed on not one but two worlds… why not three?  Why not a dozen, or a million?  The possibilities will be truly limitless.

Sciency Words A to Z: Rare Earth Hypothesis

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, R is for:


Once upon a time, it was believed that the Sun, Moon, planets, and all the stars revolved around the Earth.  This was known as the geocentric theory.

Copernicus, Galileo, Kepler, and others set us straight about our planet’s physical location in space.  However, it is still sometimes asserted that Earth is special or unique in other ways.  Such assertions are often referred to in a derogatory sense as “geocentrisms.”

It’s tempting to dismiss the Rare Earth Hypothesis as just another geocentrism.  The idea was first presented in 2000 in a book called Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Ward and Donald Brownlee.  In that book, Ward and Brownlee go through all the conditions they say were necessary for complex life to develop on this planet.  Crucially, they point out all the ways things could have gone wrong, all the ways complex life on Earth could have been prematurely snuffed out.

In other words, we are very, very, very lucky to be here, according to Ward and Brownlee, and the odds of finding another planet that was as lucky as Earth must be astronomically low.  Sure, there might be lots of planets where biology got started. Simple microorganisms may be quite common.  But complex, multicellular life like we have here on Earth—that’s rare.  And intelligent life forms like us are rarer still.  Perhaps intelligent life is so rare that we’re the only ones.

My favorite response to the Rare Earth Hypothesis comes from NASA astronomer Chris McKay.  In All These Worlds Are Yours, McKay’s argument is described as the Rare Titan Hypothsis.

Imagine intelligent life has developed on Titan (such a thing seems unlikely, I know, but there may be something living on Titan).  Titanian scientists look through their telescopes and soon realize that no other world in the Solar System is quite like their own.  Earth, for example, if too hot for life as the Titanians know it, and there’s far too much of that poisonous oxygen in the atmosphere anyway.  Furthermore, water would wreak havoc on what the Titanians would consider a biomolecule.

Perhaps a pair of Titanian scientists then decide to publish a book.  They list all the conditions required for complex life to develop on Titan, point out all the ways Titanian life could have been snuffed out prematurely, and argue that the odds of finding another Titan-like world must be astronomically low.

Personally, I think there’s some validity to the Rare Earth Hypothesis, but McKay’s point is worth bearing in mind.  There could be many different ways for life to develop in our universe.  Earth is but one example.  Planets that are just like Earth may indeed be rare—extremely rare—but there’s no reason to conclude that Earth-like life is the only kind of complex life out there.

Next time on Sciency Words A to Z… oh my gosh, we’ve finally made it to S!  It’s finally time to talk about SETI!