Number of Habitable Planets Within Galaxy Doubles With New Study

Planets with the ability to maintain liquid water at their surface may be much more common than previously believed, according to a new study published in the journal Astrophysical Journal Letters.

Led by researchers at the University of Chicago and Northwestern University, the report features new calculations regarding the influence of cloud behavior on climate that effectively double the number of potentially habitable planets orbiting red dwarfs, the most common type of stars in the universe.

Current data from NASA's Kepler Mission suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf. However, the new study roughly doubles that estimate, also suggesting new ways for astronomers to test whether planets orbiting red dwarfs have cloud cover.

If correct, the Milky Way alone may harbor as many as 60 billion planets conducive to alien life.

"Most of the planets in the Milky Way orbit red dwarfs," Nicolas Cowan, a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics, said in a press release. "A thermostat that makes such planets more clement means we don't have to look as far to find a habitable planet."

Cowan and his colleagues decided to examine the formula for calculating the habitable zone around a star because, while it's remained much the same for decades, it largely neglects 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 the University of Chicago and fellow researcher. "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, the researchers explain, would have to complete an orbit approximately once a year to be far enough away to maintain water on its surface. However, in the case of red dwarfs, which are much smaller and fainter than the Sun, this drops to approximately once every month or two months.

Planets in such a tight orbit would eventually become tidally locked with their star, meaning the same side of the planet's face would always point in its direction. As a result, calculations made by the researchers show 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.

Knowing this, the team's three-dimensional global calculations were able to determine 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's climate and require 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, the researchers explained, mostly 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. Furthermore, they demonstrate 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.

Come 2018, astronomers using the James Webb Telescope will be able to test the validity of these findings by measuring the temperature of a planet at different points in its orbit. Should a tidally locked exoplanet lack significant cloud cover, astronomers will simply measure the highest temperatures when the dayside of the exoplanet is facing the telescope and the lowest point when the planet comes back around to show its dark side.

But if highly reflective clouds dominate the dayside of the exoplanet, they will block a significant amount of infrared radiation from the surface, explained Ju Yang, a postdoctoral scientist in geophysical sciences at the University of Chicago.

Should this happen, "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.

Earth-observing satellites, however, have already documented this 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 said, "it's almost definitely from clouds, and it's a confirmation that you do have surface liquid water."