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, E is for:

EARTH SIMILARITY INDEX

From time to time, you might hear on the news that scientists have discovered a new Earth-like planet.  You’d think that would be huge news, but it’s rarely presented that way.  It’s more like a fluff story, the kind of thing news anchors can banter about before tossing to weather.  It’s enough to make you wonder what, exactly, the term Earth-like planet really means.

In 2011, this paper appeared in the journal Astrobiology.  The authors of that paper proposed a new system for quantifying how Earth-like another planet is.  They called their system the Earth Similarity Index or E.S.I.  The basic idea is you take four measurable properties—a planet’s mass, density, surface gravity (represented by escape velocity), and surface temperature—plug that information into an equation, and get a number between zero and one.

Numbers close to zero represent planets that are about as un-Earth-like as possible.  Numbers close to one represent planets that are almost exact matches for Earth. So in most cases, when people talk about Earth-like planets, what they mean are planets that scored highly on the E.S.I.

Unfortunately, because of the limits of current technology, a lot of guesswork has to go into our E.S.I. calculations.  Most of the time, we just can’t get the precise measurements we need.  Measuring a distant exoplanet’s surface temperature seems to be especially problematic.  But even if that weren’t the case, the E.S.I. still wouldn’t account for things like a planet’s atmosphere or the presence of liquid water, or many other key things that make Earth the planet that it is.

That same 2011 paper also proposes another system called the Potential Habitability Index or P.H.I.  Taken together, the E.S.I. and P.H.I. should give you a clear idea of just how Earth-like another planet really is.  A very clear idea.  But the stuff you have to measure for the P.H.I.—we’re not even close to being able to measure that stuff.  Not yet.

Someday in the future, as we continue to refine out observational techniques, maybe we’ll be able to put the E.S.I. and P.H.I. to good use.  Until then, any news you hear about newly discovered Earth-like planets is probably not as exciting as it sounds.  Unless, of course, this is the newscast you’re watching:

Next time on Sciency Words A to Z, nuclear physicist Enrico Fermi said it first: where is everybody?