Sciency Words A to Z: Goldilocks Zone

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, G is for:


Once upon a time, there was a little girl from outer space who came to visit the Solar System.  Her name was Goldilocks.  First, she landed on Mars, but she didn’t like it there.  It was too cold.  Then she tried to land on Venus, but she didn’t like it there either. It was too hot—way too hot.  And then finally, Goldilocks landed her spaceship on Earth.  When she came out of the airlock and walked down the landing ramp, she said to the astonished Earthlings, “Ah yes, this planet is just right!”

At least that’s how my version of the Goldilocks story goes.

Anyway, the concept of a Goldilocks zone (also known as a habitable zone, continuously habitable zone, or circumstellar habitable zone) is pretty simple.  Fairy tale simple, you might say.  The Goldilocks zone is the region of space around a star where liquid water can exist on a planet’s surface.  And as you know, if a planet has liquid water on its surface, then it could have life!

For a long time, our search for alien life has focused almost exclusively on Goldilocks planets.  But there are problems with limiting our search in that way.

In my post on carbon chauvinism, I told you there are other chauvinisms that astrobiologists have to deal with. One of them is water chauvinism, the presumption that water is necessary for life.  Another is surface chauvinism, the presumption that life can only exist on a planet’s surface.  Our obsession with Goldilocks zones is largely based on those two chauvinisms.

But looking to the moons of Jupiter and Saturn, we’ve already learned that there is more liquid water outside the Goldilocks zone than in it!  Several of those moons have vast oceans of liquid water beneath their surface, with only a relatively thin crust of ice overtop.  These subsurface oceans might be ideal environments for alien life.  So much for our surface chauvinism.

And then there’s Titan, a moon of Saturn, which has lakes of liquid methane and ethane on its surface.  Could those liquid hydrocarbons serve as a substitute for water in an alien biochemistry?  We don’t know.  It’s possible.  We certainly shouldn’t rule that possibility out.  And thus, so much for our water chauvinism.

To quote from Exoplanets by Michael Summers and James Trefil, “[…] the current focus on finding a Goldilocks planet amounts to a search for the least likely location of water and, presumably, life.”  I think there’s a bit of hyperbole in that statement, but I agree with the general point.  There are probably far more worlds in our galaxy like Europa, Enceladus, or Titan than there are like Earth.

Next time on Sciency Words A to Z, we’ll crack the surface of one of those icy moons and see what might be hidden in those dark, extraterrestrial depths.