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Observations

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Twin Earth May Be Better Than Earth for Life

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Artist's vision of Kepler-186f

Pseudo-Earths are out there. That’s the message of today’s exciting announcement that a planet about the same size as Earth lives in its star’s habitable zone—the temperate region around a star where liquid water might flow. “For me, the impact is to prove that such planets really do exist,” said David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to Nature.

It’s an article of faith that the planets most likely to harbor life are the planets most like Earth. Our home is, after all, the only known place in the universe that gives life shelter. From our sample of size of one we peer out into the cosmos for places that might mirror back the essential features that make our planet so fertile.

Unfortunately, discovering distant planets is so difficult that we can only discern the barest information about any potential neighbor. In the case of Kepler-186f, the newly discovered Earth-like exoplanet, astronomers know its size (about 1.11 times the radius of Earth), the length of its year (130 Earth days), and how much solar radiation it receives (about 32 percent of what the Earth gets from the Sun). Other questions—what is it made of? does it have an atmosphere? how hot is its surface?—we can only answer indirectly, if at all. There’s no way to tell, in other words, if the surface of Kepler-186f supports swimmable temperatures between zero and 100 degrees Celsius.

But we can flip the question of habitability around. Instead of assuming that the most life-friendly planets are Earth-sized orbs circling a Sun-like star, we can ask what characteristics a planet might have if we were to build it from scratch with the express purpose of setting the stage for the genesis of life and evolutionary success. What simple beginnings would best brew life’s endless forms most beautiful?

The answer, it turns out, isn’t an exact copy of Earth. Astronomers have recently begun to ponder the possibility of a “superhabitable” planet—one that has all the life-giving features of Earth, but more so. What are the characteristic signatures of such a world? As the astronomers René Heller and John Armstrong describe in a recent paper, these planets will be slightly larger than Earth—up to about two or three times Earth’s radius. These chubby Earths would presumably have more magnetic shielding from solar radiation; greater tectonic activity, which means more volcanoes to belch carbon dioxide into the atmosphere; a thicker atmosphere, held in by the greater surface gravity, and simply more surface area for life to roam.

A moon wouldn’t hurt either. As Lee Billings described in our January story “The Search for Life on Faraway Moons,” a moon surrounding a distant planet could create a tidal heating effect—the orbit of the moon stretching and contracting the planet enough to generate frictional heating forces that would make a planet habitable even if it orbited outside of its star’s habitable zone. Discovering a moon is much trickier than finding a planet itself, however. (A few intrepid astronomers are on the case.) For now, the only special planetary attributes we can reliably search for are those that a spacecraft like Kepler can discern: size and distance from star. And, of course, what the star itself looks like.

In this, the planet Kepler-186f may be a winner. The star it orbits—Kepler 186—is a dwarf star, dimmer and smaller than our Sun. These stars burn a little less vigorously than our Sun does, which means their nuclear fuel can last for billions of years longer—additional billions of years for evolution to do its work.

With a radius 11 percent larger than Earth’s and an orbit around an extremely long-lived dwarf star, Kepler 186f may be undersold as a twin of Earth. It could be our first example of a superhabitable world.

Image courtesy NASA Ames/SETI Institute/JPL-Caltech

The views expressed are those of the author and are not necessarily those of Scientific American.

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