Clouds May Help Reveal More Habitable Planets
July 2, 2013

Computer Simulations Reveal There May Be 60 Billion Planets That Could Sustain Life

April Flowers for - Your Universe Online

According to a new study from the University of Chicago and Northwestern University, the influence of cloud behavior on climate doubles the number of potentially habitable planets orbiting the most common type of stars in the universe, red dwarfs. In the Milky Way alone, the findings suggest, 60 billion planets may be orbiting red dwarf stars in the habitable zone.

The study, published in Astrophysical Journal Letters, is based on rigorous computer simulations of cloud behavior on alien planets. Such behavior dramatically expanded the habitable zone of red dwarf stars - smaller and fainter than other Sun-like stars.

Previous data obtained from NASA's Kepler space observatory, which is searching for Earth-like planets orbiting other stars, suggested there is approximately one Earth-sized planet in the habitable zone of each red dwarf. The new study findings double 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."

The team provides other astronomers with a means of verifying their conclusions using the James Webb Space Telescope, which is scheduled to launch in 2018.

The habitable zone for alien planets is the area where they are able to orbit their star while still maintaining liquid water on the surface. The formula for calculating this zone has remained mostly constant for decades. According to the new study, it fails to take into consideration clouds, which exert a major climatic influence.

"Clouds cause warming, and they cause cooling on Earth," said Dorian 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."

Like the Earth, a planet orbiting a star like our Sun would need to complete an orbit approximately once a year to maintain surface water. "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.

Such a small orbit would eventually tidally lock the planet to the sun, forcing the planet to always keep the same side facing the star much like our moon does the Earth. The team's calculations indicate the side of the planet facing the star would experience vigorous convection and highly reflective clouds at a point that astronomers call the sub-stellar region. At the sub-stellar region, the sun always sits directly overhead at high noon.

The researchers used 3D global calculations to determine the effect of water clouds on the inner edge of the habitable zone, for the first time ever. Such simulations are similar in nature to those used to predict Earth climate. The new simulations required several months of processing, running on a cluster of 216 networked computers at UChicago. Previous studies attempting to simulate the inner edge of exoplanet habitable zones were one-dimensional, mostly neglecting the clouds and focusing instead on charting how temperature decreases with altitude.

"There's no way you can do clouds properly in one-dimension," Cowan said. "But in a three-dimensional model, you're actually simulating the way air moves and the way moisture moves through the entire atmosphere of the planet."

If there is any surface water on the planet, the new simulations reveal water clouds will result. Further, the simulations reveal that cloud behavior has a significant cooling effect on the inner portion of the habitable zone. This behavior enables planets to sustain water on their surfaces much closer to their host star.

Using the James Webb Space Telescope, astronomers will be able to test the validity of these findings by measuring the temperature of the planet at different points in its orbit. Scientists will measure the highest temperatures when the dayside of the exoplanet is facing the telescope, which occurs when the planet is on the far side of its star if a tidally locked exoplanet lacks significant cloud cover. Once the planet's orbit takes it far enough around to show its dark side to the telescope, temperatures would reach their lowest point.

Jun Yang, a postdoctoral scientist in geophysical sciences at UChicago, says that if highly reflective clouds dominate the dayside of the exoplanet, they will block a lot of infrared radiation from the surface. In that situation "you would measure the coldest temperatures when the planet is on the opposite side, and you would measure the warmest temperatures when you are looking at the night side, because there you are actually looking at the surface rather than these high clouds," Yang said.

This effect has been documented with Earth-observing satellites. "If you look at Brazil or Indonesia with an infrared telescope from space, it can look cold, and that's because you're seeing the cloud deck," Cowan said. "The cloud deck is at high altitude, and it's extremely cold up there."

If this signal from an exoplanet is detected by the James Webb Telescope, Abbot noted, "it's almost definitely from clouds, and it's a confirmation that you do have surface liquid water."