Photograph by NASA

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An artist's conception of an exoplanet orbiting a star 30 light-years away from Earth.

Photograph by NASA

Explosion in Number of Potentially Habitable Worlds

Climate model predicts 60 billion water-friendly planets around red dwarf stars.

The number of potentially habitable worlds circling red dwarf stars—the most abundant type of star in our Milky Way galaxy—may have just doubled to 60 billion, a new study suggests.

Using global climate models originally created for studying global warming on Earth, a team of researchers from the University of Chicago and Northwestern University created 3-D models of how large-scale cloud patterns affect atmospheric temperatures on Earth-size planets orbiting stars smaller and cooler than our sun. (See also: "'Shocking' Superstorm Seen on Exoplanet—A First.")

So what's new? Researchers found that the atmospheric circulation and cloud cover on these exoplanets meant these worlds could orbit their stars more closely than previously thought—expanding the habitable zone around red dwarf stars.

Computer simulations developed by Dorian Abbot, a planetary scientist at the University of Chicago, show that we should be looking at orbits much closer to red dwarfs than we've done in the past for worlds that can support liquidwater and, possibly, life. (Related: "Think Outside the Box to Find Extraterrestrial Life.")

And since red dwarfs are the most common type of star populating the universe, future searches for habitable planets may want to focus on them.

Why is it important? "While we don't have an accurate estimate because they are hard to see, we believe that there are roughly 100 billion red dwarfs in just the Milky Way galaxy alone," said Abbot, co-author of the new study published this week in the The Astrophysical Journal Letters.

"So with these cool dwarf stars being the most common in our galaxy, the closest habitable-zone planet we may find will most likely be orbiting this type of star."

What also makes red dwarf systems such a cosmic catch is that the stars are so small. That means the relative size of any orbiting planet will be larger.

This is a key factor when using the transit method—where a star's brightness dims when a planet glides in front of its host star—to search for exoplanets. (Related: "Bumper Crop of Habitable Worlds Discovered?")

And since red dwarfs are cooler than the sun, their habitable zone—where water can exist in liquid form—will be much closer in than the habitable zones of other types of stars. The result would be exoplanets that experience a year lasting only 30 to 40 days instead of 365 days like on Earth.

Exoplanets orbiting in such habitable zones are so close to their star that they are tidally locked, meaning that they always present the same face to their star.

Since the same face of the planet always points to the star, that half heats upquickly and air rises, creating a global atmospheric circulation and large-scale cloud cover.

The computer models show that these clouds would reflect much of the incoming starlight, thus cooling the planet.

What does this mean? "What this means for planet hunters is that we get to see more orbits, and obtain more measurements, so in the end our hunting techniques just work better," explained Abbot. "We can look for planets hugging their host red dwarf stars much closer than we previously thought was possible.

"Even though the planet is being exposed to twice as much solar energy, we now think there could still be plenty of liquid water on its surface," he said.

Abbot and his team will probably have to wait a few more years to test their findings, when Hubble's more powerful successor, the James Webb Space Telescope, launches in 2018.