Sixty B Planets May Be Habitable in Milky Way

Mr.Rogers

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A new study that calculates the influence of cloud behavior on climate doubles the number of potentially habitable planets orbiting red dwarfs, the most common type of stars in the universe. This finding means that in the Milky Way galaxy alone, 60 billion planets may be orbiting red dwarf stars in the habitable zone.

Researchers at the Univ. of Chicago and Northwestern Univ. based their study, which appears in Astrophysical Journal Letters, on rigorous computer simulations of cloud behavior on alien planets. This cloud behavior dramatically expanded the estimated habitable zone of red dwarfs, which are much smaller and fainter than stars like the sun.

Current data from NASA’s Kepler Mission, a space observatory searching for Earth-like planets orbiting other stars, suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf. The UChicago-Northwestern study roughly doubles that estimate. It also suggests new ways for astronomers to test whether planets orbiting red dwarfs have cloud cover.

“Most of the planets in the Milky Way orbit red dwarfs,” says Nicolas Cowan, a postdoctoral fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics. “A thermostat that makes such planets more clement means we don’t have to look as far to find a habitable planet.”
Cowan joins UChicago’s Dorian Abbot and Jun Yang as co-authors on the study. The scholars also provide astronomers with a means of verifying their conclusions with the James Webb Space Telescope, scheduled for launch in 2018.

The habitable zone refers to the space around a star where orbiting planets can maintain liquid water at their surface. The formula for calculating that zone has remained much the same for decades. But that approach largely neglects clouds, which exert a major climatic influence.

“Clouds cause warming, and they cause cooling on Earth,” says Abbot, an assistant professor in geophysical sciences. “They reflect sunlight to cool things off, and they absorb infrared radiation from the surface to make a greenhouse effect. That’s part of what keeps the planet warm enough to sustain life.”

A planet orbiting a star like the sun would have to complete an orbit approximately once a year to be far enough away to maintain water on its surface. “If you’re orbiting around a low-mass or dwarf star, you have to orbit about once a month, once every two months to receive the same amount of sunlight that we receive from the sun,” Cowan says.

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