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An artist places us on the surface of Gliese 876 d, a planet in or near the habitable zone around a red dwarf star. (Inga Nielsen, Hamburg OBS., Gate To Nowhere)

Earth-Like Planets Could be Right Next Door

Astronomers estimate that billions of habitable planets are orbiting red dwarf stars. What would it be like to live there?

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Even in places where skies are dark and the Milky Way seems to fill our view, the night sky is teeming with stars we cannot see. These other stars, invisible to the naked eye, are red dwarfs—smaller, cooler, dimmer, and far, far more common in the galaxy than stars like our sun.

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Only visible with telescopes, red dwarfs constitute about three-fourths of the hundreds of billions of stars in the Milky Way. Until this year, those billions of stars were generally overlooked by astronomers searching for another class of objects that can’t be seen with the naked eye: exoplanets. Now two recent studies have looked exclusively at red dwarf stars, and the estimates are in: More than 50 percent could harbor potentially habitable planets. That enormous probability leads to one more exciting conclusion: Another planet with life could be just a few light-years away. In a galaxy that’s 100,000 light-years across, that’s just down the block from Earth.

Given the challenges of taking a detailed look at a habitable planet orbiting a star like our sun—it will take a new generation of powerful and expensive telescopes, almost certainly in space, to study a small, rocky planet amid a yellow star’s glare—this huge population of red dwarfs offers immense opportunities for discovery. And a chance to open our imaginations to wildly different kinds of life in the universe.

“I think we have a natural tendency to always talk about things that are like what we know,” says Courtney Dressing, a graduate student with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Dressing and David Charbonneau, who focuses on finding and characterizing exoplanets, published the first of the two papers that estimated the prevalence of habitable planets orbiting red dwarfs. Their calculation came in on the low end; they believe around 15 percent of the red dwarfs are likely to have habitable planets in their thrall, a percentage that amounts to tens of billions of stars. The second study was conducted by Ravi kumar Kopparapu, an exoplanet researcher at Pennsylvania State University. By calculating a larger habitable zone, Kopparapu estimated as many 48 to 61 percent could potentially harbor life.

“We’ve come to realize that the universe is a pretty unusual place, so we don’t always have to find the exact same conditions [as Earth for habitability to be possible],” says Dressing. “We realize now that it’s important to look for potentially habitable planets around smaller types of stars as well.”

Red dwarfs are, on average, one-third the size and 1,000 times dimmer than the sun. For astronomers looking for exoplanets, the smaller size and dimness of red dwarfs mean a lot less glare that could conceal an orbiting planet. The difficulty of resolving detail on a planet near a star is particularly acute in the search for planets around red dwarfs, because astronomers estimate that the habitable zone—the region where it’s neither too hot nor too cold for liquid water to exist—around these cooler stars is much closer to the star than even Mercury is to the sun.

Armed with nearly two decades of exoplanet discoveries, astronomers have started to assemble a picture of what a habitable planet might look like, but most have focused on what we know: a planet like Earth orbiting a yellow dwarf star like our sun, classified as a G-type, main-sequence star. But what about the smaller, red M-type stars? What might your home be like if it were in red dwarf orbit? The answer is kind of like Earth, and a lot unlike Earth.

From the surface of this imaginary habitable planet, one of the first things we’d notice is that the sun is very big in the sky. It would appear one and a half to three times bigger than Earth’s sun, given the close orbit, says John Johnson, an exoplanet researcher and assistant professor at the California Institute of Technology in Pasadena. And, as you might infer from the name, this sun is going to appear extremely red, due to its cooler temperature. Red dwarfs are about half as hot as our sun.

So far, this planet on which we stand is a bit foreign compared to Earth, but not shockingly different. This is where any sense of familiarity ends, however, because another consequence of orbiting so close to its star is that the planet could be tidally locked, so that one side always faces its sun. Astronomers call this synchronous rotation, when an orbiting body takes the same time to rotate around its axis as it does to revolve around another body, just like our moon does as it orbits Earth.

Or it could be like Mercury, says Jim Kasting, an exoplanet researcher at Pennsylvania State University and the author of How to Find a Habitable Planet. Mercury, he explains, is caught in a “spin- orbit resonance” and rotates three times for every two orbits around the sun. This stable relationship between orbital and rotational periods is caused by the long shape, or eccentricity, of Mercury’s orbital path, which in turn is caused by the pull of gravity from the many outer planets—a situation that could also exist for a close-orbiting exoplanet. In other words, it’s possible that on our imaginary planet, we would enjoy only three sunrises every two years.

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