An artist's depiction of a habitable planet with two moons orbiting… (David A. Aguilar (CfA) )
Scientists at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said Wednesday morning that an "Earth-like" planet -- that is, a small rocky planet warm enough to have liquid water on its surface and potentially capable of hosting life -- could be as close as 13 light-years away.
It's hardly "next door" (as a press release touting the announcement put it), in any traditional sense: 13 light-years is something like 76 trillion miles away. But across the vast distances of the Milky Way, said Harvard astronomer Courtney Dressing, 13 light-years amounts to "a stroll in the park."
To arrive at the estimate, Dressing conducted an analysis of public data from NASA's Kepler telescope, which stares at about 150,000 target stars in one swatch of the Milky Way, searching for telltale fluctuations in their light that suggest a planet may orbit around them.
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Concentrating on stars in a sample of small red dwarfs -- about a quarter the mass of our sun, and about 0.2% as bright -- she calculated that 60% had planets smaller than Neptune orbiting them, and 6% had planets that could be considered Earth-like because of their size and their distance from their host star. Since red dwarfs are about 10 times more common than sun-like stars, the discovery suggests that there should be many, many Earth-like planets lurking within telescopes' reach.
"Now that we know there are going to be Earth-like planets nearby, we don't have to look so far away," said Harvard astronomer David Charbonneau.
But they will still have to look. Dressing's analysis identified three Kepler planet candidates that might, upon further analysis, be confirmed to be Earth-like planets. To find any of the statistically probable Earth-like planets orbiting red dwarfs closer by, scientists will need to conduct additional searches.
Speaking to reporters Wednesday morning, Dressing and Charbonneau noted that while it seems red dwarfs do host many planets that are similarly sized to Earth and warm enough to have liquid water on their surface -- two factors thought essential for a planet to harbor life -- the planets would be very different from Earth in many ways. Because red dwarfs are small and dim, the habitable zones around them form a tight circle, and any Earth-like planet would have to orbit "tucked in close," Charbonneau said.
That might mean that such a planet would be subjected to intense radiation, or might be "tidally locked" to its star, with one side always in the dark and the other always in the light. But while such conditions aren't what we see here on Earth, it doesn't mean that life couldn't necessarily thrive in them.
"It could have interesting effects for the evolution of life," Charbonneau said.
Caltech astronomer John Johnson, who was not involved in the work but also spoke with reporters on Wednesday, added that scientists don't fully understand conditions on habitability on Earth.
"Is it important to have a moon? Do we need a Jupiter-sized planet nearby? Do we need plate tectonics? This study will drive forward the study of habitability on our own planet and gives us a target to aim for," he said. "This puts us hot on the trail of finding planets that host life."