Planet Earth

The Ultimate Measure of a Planet---Habitability Isn't a Yes/No Question

The CruxBy Seth ShostakDec 7, 2011 12:57 AM


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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.

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