The climate influence of clouds could double the number of potentially habitable planets orbiting the most common type of stars, U.S. scientists say.

Astrophysicists at the University of Chicago say this finding means in the Milky Way galaxy alone, 60 billion planets may be orbiting in the habitable zone of red dwarf stars, the most common stars in the universe.

Working with colleagues from Northwestern University, the researchers based their study on rigorous computer simulations of cloud behavior on alien planets, behavior that dramatically expanded the habitable zone of red dwarfs, which are much smaller and fainter than our sun.

While current data from NASA's Kepler planet-hunting space telescope suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf, the new study has doubled that number.

Clouds can act like a "thermostat" to regulate potential climate conditions on alien planets, the researchers said.

"Most of the planets in the Milky Way orbit red dwarfs," Nicolas Cowan of Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics said.

"A thermostat that makes such planets more clement means we don't have to look as far to find a habitable planet."

The researchers have reported their study in Astrophysical Research Letters.