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:

THE RARE EARTH HYPOTHESIS

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!