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?
Planet Pailly 
Why on a scale of 0-1? Couldn’t they just stick with the normal 1-10 like everyone else? hahaha… Just another reason science sails right past me. Thanks for keeping things on a “normal” level with your posts. 🙂
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Thanks! I don’t know why they went with zero to one, but there are lots of other astronomy scales that are presented that way. Albedo (a measure of how much light an object reflects) is the one that comes immediately to mind. So I guess with the E.S.I. they were just following a precedent that was set a long time ago. Personally I think percentage scales would make the most sense, whether for E.S.I., albedo, or lots of other things.
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I’m sensing a lot of decimals here.
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The numbers I’ve seen only went to one or two decimal places, which does seem a bit odd now that I’m thinking about it. That might reflect the amount of guesswork that goes into this. Usually the number of decimal places reflects how accurate of a measurement you could get.
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Cool! I write scifi about a Mars colony, so I had to look it up: 0.73 since it lacks plate tectonics and can’t hold atmospheric water. I also write about a colony on Saturn’s moon Titan. What have I done to my poor settlers! Surface ESI = 0.1
Check out your favorite at https://www.researchgate.net/figure/Earth-Similarity-Index-ESI-for-47-Solar-System-bodies-with-radius-greater-than-100-km_fig1_51739720
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I was kind of surprised when I went to look up Venus’s E.S.I. I’d looked it up before, and I remembered it being the highest score in the Solar System, besides Earth itself. It was probably that interior E.S.I. I was looking at. But Venus’s surface E.S.I. is obviously much, much lower.
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I love the comics in these posts.
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Thank you! A lot of love goes into those comics!
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i seriously LOVE the comics you create. Keep up the great work!
Joy at The Joyous Living
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Thank you, I’m so glad to hear that!
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Oh, those crazy equations….Or as Lt. Tom Paris once said on Star Trek Voyage…..” I like your creative use of the minus sign!”
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I didn’t remember that quote, but that’s a good one. There was a lot to love on Voyager. Under-appreciated show.
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