1 July 2013

Lots more habitable planets out there, thanks to clouds

by Will Parker

An intriguing 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.

Data from NASA's Kepler Mission previously suggested there is approximately one Earth-size planet in the habitable zone of each red dwarf. The authors of the new paper say that in the Milky Way alone, up to 60 billion planets may be orbiting red dwarf stars in the habitable zone.

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 that formula largely neglects clouds, which exert a major climatic influence.

"Clouds cause warming and cooling on Earth," said study author Dorian Abbot, from the University of Chicago. "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."

Co-author Nicolas Cowan, from Northwestern University, explained that a planet orbiting a star like our 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."

Planets in such a tight orbit would eventually become tidally locked with their sun (always keeping the same side facing the sun, as the Moon does toward Earth). The new paper shows 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 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 required several months of processing, running mostly on a cluster of 216 networked computers at the University of Chicago. Previous attempts to simulate the inner edge of exoplanet habitable zones were one-dimensional. They mostly neglected clouds, 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."

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.

Astronomers using the soon-to-be-launched James Webb Telescope will be able to test the validity of these findings by measuring the temperature of the exoplanet at different points in its orbit. If a tidally locked exoplanet lacks significant cloud cover, astronomers 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. Once the planet comes back around to show its dark side to the telescope, temperatures would reach their lowest point.

But 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," team member Jun Yang explained.

Earth-observing satellites have documented the same effect. "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 the James Webb Telescope detects this signal from an exoplanet, Abbot noted, "it's almost definitely from clouds, and it's a confirmation that you do have surface liquid water."

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Source: University of Chicago