Exoplanet Cloud Behaviour Expands Habitable Zone

A new study that calculates the influence of cloud behaviour 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 University of Chicago and Northwestern University based their study, which appears in Astrophysical Journal Letters, on rigorous computer simulations of cloud behaviour on alien planets.

This cloud behaviour dramatically expanded the 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 now doubles that number.

"Most of the planets in the Milky Way orbit red dwarfs," said 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 is one of three co-authors of the study, as are UChicago's Dorian Abbot and Jun Yang. The trio also provide astronomers with a means of verifying their conclusions with the James Webb Space Telescope, scheduled for launch in 2018.

The formula for calculating the habitable zone of alien planets -- where they can orbit their star while still maintaining liquid water at their surface -- has remained much the same for decades. But the formula largely neglects clouds, which exert a major climatic influence.

"Clouds cause warming, and they cause cooling on Earth," said Abbot, an assistant professor in geophysical sciences at UChicago.

"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 said.