Sciency Words: Earth Similarity Index

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Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


As I’m sure you’ve heard by now, astronomers have discovered seven planets orbiting a nearby star called TRAPPIST-1. Even more exciting, most or all of these new worlds are being described as Earth-like planets. But what does that actually mean?

You’d be surprised by how many “Earth-like” planets/moons we have right here in our own Solar System.

Ag16 Earth-like Worlds
From left to right: Venus, Earth, Mars, and Titan (Saturn’s largest moon).

Earth-like is a rather vaguely defined term. So in 2011, a paper published in the journal Astrobiology attempted to establish an official mathematical system for calculating just how Earth-like an exoplanet is. It’s called the Earth Similarity Index or E.S.I.

Basically, the E.S.I. takes certain characteristics of a planet that can be quantified—such as a planet’s mass, radius, temperature, etc—and compares them to Earth’s. An E.S.I. score of zero indicates a planet that has absolutely nothing in common with Earth, while an E.S.I. of one means the planet is an exact match for Earth… at least with regard to the characteristics being measured and included in our calculations.

Of course even a planet with an E.S.I. of one is not necessarily habitable, so the same Astrobiology paper also proposes a Potential Habitability Index or P.H.I. But that, I think, is a Sciency Word for another day.

P.S.: If you want to dive into the math behind the E.S.I., click here.

5 thoughts on “Sciency Words: Earth Similarity Index

  1. So what’s the big deal about Trappist 1? I’ve read a couple articles, but not any actual papers, and I can’t find anything that would warrant the amount of attention it’s getting. We’ve been finding hundreds of exoplanets for years using the same technique, many of them “earth-like”. And given that Trappist 1 is an M-class star, there’s very little chance of it actually being what the average person would consider earth-like.

    Liked by 1 person

    1. I’m working on a post about this for next week. There’s a lot of hype about one or more of these planets being habitable; I think that’s just hype.

      I think what’s really got astronomers and astrophysicists excited is that this is sort of like a miniature solar system, and it’s positioned in such a way that it’s easy for us to collect data about it. It’s sort of giving us an opportunity to test our current theories about multi-planet systems.


  2. This is a great post. In all I’ve come across, I’ve never heard about the ESI. That’s really interesting, and thanks. It makes perfect sense, but also kind of makes grin because of how geocentric we are. Of course, it’s mostly through necessity and familiarity, but lots of times I wonder what we’re overlooking, you know? Thanks again for this great post. Here’s to you.

    Liked by 1 person

    1. Thanks, Scott. I was really happy about finding this term. At least now whenever I hear news about Earth-like planets, I’ll have a clearer idea about what that means.


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