The Ultimate Measure of a Planet—Habitability Isn’t a Yes/No Question

By Seth Shostak | December 6, 2011 1:57 pm

Seth Shostak is Senior Astronomer at the SETI Institute in California, and the host of the weekly radio show and podcast, “Big Picture Science.”

Back in the early days of “Star Trek,” whenever the Enterprise would chance upon a novel planet, we’d hear a quick analysis from Science Officer Spock. Frequently he would opine, “It’s an M-class planet, Captain.” That was the tip-off that this world was not only suited for life, but undoubtedly housed some intelligent beings eager for a meet-and-greet with the Enterprise crew.

But what is an “M-class planet” (also referred to as “class M”)? Clearly, it referred to a world on which intelligent life could thrive, and made it easy for the crew (and viewers) to see where the episode was headed. A recent paper by Washington State University astrobiologist Dirk Schulze-Makuch and his colleagues has suggested a somewhat similar way to categorize real-world orbs that might be home to cosmic confreres. Rather than giving planets a Spockian alphabetic designation, Schulze-Makuch prefers a less obscure, and more precise, numerical specification: a value between 0 and 1. A world that scores a 1 is identical to Earth in those attributes thought necessary for life. A score of 0 means that it’s a planet only an astronomer could love—likely to be as sterile as an autoclaved mule.

Schulze-Makuch computes this index—which he calls an Earth Similarity Index, or ESI—by considering both the composition of a planet (is it rocky and roughly the size of Earth?) and some crude measures of how salubrious the surface might be (does it have a thick atmosphere, and are temperatures above freezing and below boiling?) He combines parameters that define these characteristics in a series of multiplicative terms that are reminiscent of the well-known Drake equation, used to estimate the number of technologically adept civilizations in the Milky Way.

At present the number of worlds thought to have an ESI of 0.8 or greater—near-cousins of Earth—is only one: Gliese 581g (though that planet’s existence is disputed). But as additional data from NASA’s Kepler mission continue to stream in, we can expect that more such “habitable” planets will turn up. In particular, Kepler scientists reported this week on a newsworthy object called Kepler-22b. This planet is 2.4 times Earth’s diameter and in an orbit around a Sun-like star that places it securely in the habitable zone—where temperatures might be similar to a summer day in San Francisco.

I’ve made a quick calculation of Kepler-22b’s ESI, and come up with 0.79. (Mind you, there’s still no reliable estimate of the mass of this planet, so I’ve assumed it to have the same average density as the Earth). This puts Kepler-22b right on the edge of Schulze-Makuch’s “Earth-like” regime, and in rarefied company indeed. Clearly, it’s a world deserving further scrutiny, and the SETI Institute has already begun listening for radio signals from its direction, just in case it houses technically minded inhabitants.


So will the ESI scale prove useful? In principle, it might eventually develop the sort of utility that stellar types have for astronomers. For instance, learning that a star is of type G5 instantly conveys information about its size, temperature, and brightness to those in the know. If the ESI is adopted by both scientists and science writers, it could help guide the public as to just how excited they should become when a new planet is reported.

In any case, it’s a heck of a lot better than Spock’s “M-class” naming convention. That just sounds like an expensive car.

Image: This chart shows plots the ESI for known planets based on two dimensions: surface (temperature) and interior (composition) similarity to Earth. If the newly discovered Kepler-22b were on the chart, it would be just outside the .8 line. Courtesy of Planetary Habitability Laboratory, University of Puerto Rico.

  • Charles

    I always thought that “M-class” stood for “Many Voluptuous Women.”

  • Wil

    This is good food for thought. A couple of nit-picky things:

    1. Do the equations attempt to estimate the levels of lethal radition at the exoplanets’ surfaces?

    2. Since the calculated average densities of various exoplanets vary by more than a factor of ten, do the equations compare their estimated gravities to Earth’s gravity, or do they merely consider the estimated diameters of the exoplanets?

  • Chris

    @1 Charles
    The M stands for Minshara. Although they never defined what Minshara was. The other Star Trek Classifications are given at

    Although I wouldn’t be so quick to dismiss the letter designation. As you point out with stars we have the OBAFGKM. The numbers were added later to give more specific info. We could easily have a class M1 or M5 planet perhaps telling on the amount of water, temp, pressure, gravity… Hopefully soon that will be a problem scientists have to worry about.

  • J-Doug


    1. I don’t believe so. This is a useful but ultimately “quick and dirty” means of excluding planets that are not Earth-like in important characteristics.

    2. Measurements of diameter come quicker than measurements of mass. Once you have both, then you have density (of course). Kepler 22-b is unlisted because we don’t have mass estimates yet.

  • uneducated hillbilly

    All of this “can life survive on this planet” talk sounds cool and all, but riddle me this. Scientists haven’t even seen the “end” of the universe and we don’t even know how many galaxies there are. We don’t even know how many stars there are. If we really are just our little planet in outer space that has life on it, the fact is, the universe doesn’t need our tools to determine habitability. There could be other planets with chemicals and substances that we don’t even have and conditions we are unaware of which would make our viewing of those planets for signs of life vain. Not to mention the fact that if we were even able to get there it would take thousands if not millions of years for us to arrive. In that time, whatever people would be on the theoretical spaceship, would have theoretically evolved into something else that might not even care about life on other planets. With all of this in mind I have to ask: Why do we care so much about finding life on other planets? Why the search for intelligent life? I would have to say that there is Someone who placed that desire within us who is an Intelligent Designer and Creator. Call me crazy, that’s fine. Wouldn’t be the first time. I find us humans an interesting bunch I guess. We look out into the cosmos trying to see life, but when someone mentions that an intelligent life has actually made contact with us in the form of a deity, we write it off as something with no proof, even though we have, dare I say it, an entire book full of His words. Meanwhile all we can see is a blue planet 600 light years away with good temperatures and say, “Ahha! Look! There’s proof that maybe somewhere out there there could be something.”

  • Wil

    Mr. Hillbilly,

    Call me cynical, but I think the main reason NASA and the media constantly play up the possibility of extraterrestrial life, or even merely the possibility of extraterrestrial liquid water, is that NASA is entirely government funded. NASA must therefore keep the general public engaged and excited, or Congress will have free rein to cut their funding. Congress generally resists cutting the funding for popular departments or programs.

    As to your other point: I believe we know two things regarding the nature of extraterrestrial life. First, we humans have a long and deep track record of greatly underestimating the possibilities and the complexities of nature. We are inherently close-minded and lack even a trace of an imagination. In fact, any scientist who dares show even a little imagination, is usually attacked by fellow scientists for the rest of his life (and beyond). Therefore, if we do ever see evidence of intelligent life nearby in our galaxy, it will probably be far different than imagined by anybody so far.

    The second thing we know is that all extraterrestrial intelligent life must follow the same laws of physics and chemistry that reign here on Earth. That knowledge helps rule out a lot of alternatives, which in turn allows SETI projects to be more efficient and focused.

  • UnclXcntrix

    Wil and Mr. Hillbilly,
    I agree with Wil on most points, and would like to add that part of what we are looking for is not only life, but life similar to what is on earth. That would be more exciting to more people than life that is so alien we could hardly recognize it, though there may be a greater chance of finding the hardly recognizable kind. And, finding extraterrestrial life is not just a publicity stunt. It would be a huge stimulus to science, firing the imagination of all people, everywhere on earth.

    As for the bible being a book full of words from an intelligent life form, I have to think that it is really a book of myth (with a deeper truth) written by -presumably wise- men. If it really started out as the original words of God, it has been translated, re-organized, edited, and re-interpreted so many times through history – by men- that it hardly resembles what God originally said. Kinda like the whisper game with a whole book, a couple thousand years, and a whole lot of big egos and all-to-human agendas.


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