According to a new study reported in the Astrophysical Journal Letters, there may be as many as 60 billion planets in the so-called habitable zone around red dwarf stars in our galaxy Milky Way alone – double what was previously thought.

Scientists based their study on computer simulations of cloud behavior on extrasolar planets. This cloud behavior dramatically expanded the habitable zone of red dwarf stars, which are much smaller and fainter than stars like the Sun.

Data from NASA’s Kepler Space Telescope suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf. The new study now doubles that number.

“Most of the planets in the Milky Way orbit red dwarfs, a thermostat that makes such planets more clement means we don’t have to look as far to find a habitable planet,” said study co-author Dr Nicolas Cowan from the Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics.

The formula for calculating the habitable zone of alien planets 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. 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,” explained senior author Dr Dorian Abbot from the University of Chicago.

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,” Dr Cowan said.

Planets in such a tight orbit would eventually become tidally locked with their sun. They would always keep the same side facing the sun, like the moon does toward Earth. Calculations of the team indicate that the star-facing side of the planet would experience vigorous convection and highly reflective clouds at a point that astronomers call the sub-stellar region. At that location the sun always sits directly overhead, at high noon.

The team’s three-dimensional global calculations determined for the first time the effect of water clouds on the inner edge of the habitable zone. The simulations are similar to the global climate simulations that scientists use to predict Earth climate.

These new simulations show that if there is any surface water on the planet, water clouds result. The simulations further show that cloud behavior has a significant cooling effect on the inner portion of the habitable zone, enabling planets to sustain water on their surfaces much closer to their sun.

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Bibliographic information: Jun Yang et al. Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets. Astrophysical Journal Letters, vol. 771, no. 2; doi: 10.1088/2041-8205/771/2/L45