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.

Sciency Words: Exotheology

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


I was raised Catholic.  I went to Catholic school, and when I was very young I remember a religious instructor telling a group of us Catholic boys about how God made “all life on the earth.”  Naturally, I raised my hand and asked about life on other planets.  The instructor assured me (rather impatiently, as I recall) that there is no such thing.

This was not a satisfying answer for little me. It’s even more annoying to adult me.  So it was with great delight that I recently became acquainted with a new term: exotheology.

Appropriately, my first encounter with this word was in a book called Exoplanets, by Michael Summers and James Trefil. The “exo-” prefix in both “exoplanet” and “exotheology” means pretty much the same thing: something external to Earth or to our Solar System.  To quote from Exoplanets:

If there are living beings on other planets, questions—debated today in the relatively new field called exotheology—must be asked.  For example, did the Fall occur on every planet and for every race?  If it didn’t, was the Redemption needed for beings who had never experienced the Fall?  If the Fall is universal, did Jesus have to go to every world to die and be resurrected, or were the events on Earth enough to cover everyone?  If so, why is Earth so central?  Are there other paths to redemption on other worlds?  It’s not hard to see how this sort of theological questioning could go on forever.

Indeed it could.  No wonder that religion teacher didn’t want to get into this discussion with me when I was little!

Based on my subsequent research, it seems the term exotheology isn’t that new.  And lest you think this is a Christian only thing, it’s actually a Jewish rabbi who gets credit for the first usage of the term (or at least the first usage in print).

In 1965, Rabbi Norman Lamm wrote this essay entitled “The Religious Implications of Extraterrestrial Life: A Jewish Exotheology.”  Based on what I could read of Lamm’s essay, it sounds like the term “exotheology” was formed by analogy with “exobiology,” the scientific search for and study of alien life.  The word exobiology has since been changed to astrobiology, but it doesn’t seem like exotheology has been updated to astrotheology yet.

As for the big questions raised by exotheologists, I guess we’ll have to wait and see.  If we ever do encounter an alien civilization, what sort of religious beliefs might they have?  Would those beliefs match up at all with a Judeo-Christian worldview, or with any of the many other religious worldviews we have here on Earth?  I don’t know (but that won’t stop me from writing a Sci-Fi story about it!).