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An illustration of the Kepler-16 system.

An artist's depiction of the "Tatooine" planet Kepler-16b

Illustration courtesy Caltech/NASA

Victoria Jaggard in Austin, Texas

National Geographic News

Published January 9, 2012

A new planet found last fall may be orbiting two stars, but it's far from a real-life Tatooine. Dubbed Kepler-16b, the world is a cold, Saturn-size gas giant with little chance of hosting desert farmers like the fictional Star Wars world.

But according to new computer simulations, the Kepler-16 stars may still shine on a world fit for life—a hypothetical Earthlike moon orbiting Kepler-16b.

Kepler-16b was discovered by NASA's Kepler spacecraft, which looks for dips in starlight as a planet transits—or passes in front of—a star, as seen from Earth.

For the new study, Billy Quarles, a doctoral student at the University of Texas at Arlington, and colleagues simulated several possible configurations for a theoretical Earth-mass world in the Kepler-16 system.

The team started by drawing up a "laundry list of parameters" for defining the habitable zone—the region around a star where a planet gets enough heat to host liquid water, essential for life as we know it—Quarles said Monday during a meeting of the American Astronomical Society in Austin, Texas.

(Related: "Earthlike Planet Found Orbiting at Right Distance for Life.")

The researchers assumed that the brighter of the two Kepler-16 stars is the main source of heat and light for any orbiting worlds.

Based on that star's size and temperature, the team determined that the main habitable zone possible around the Kepler-16 stars would extend from about 34 million to 66 million miles (55 to 106 million kilometers) out.

Capturing a Habitable Moon

With a roughly circular orbit about 65 million miles from the stars, the Saturn-like planet is on the outer edge of this main habitable zone. And while this "Tatooine" is uninhabitable, an Earthlike moon in Kepler-16b's orbit could sustain life, the researchers said.

The group isn't yet ready to say whether a moon could have formed alongside the planet. But their simulations suggest a moon could have arrived, fully formed, later in Kepler-16b's life.

According to the new models, a planet closer to the brighter star, squarely in the habitable zone, could have long ago been ejected from its orbit due to gravitational interactions with the other objects in the system.

Kepler-16b's gravitational pull could have attracted the Earthlike planet during its journey outward, turning the world from planet to moon.

Such a moon would technically be in the main habitable zone of the Kepler-16 system and—unlike Mars, on the outer edge of the habitable zone in our solar system—the moon would be massive enough to retain an Earthlike atmosphere, the team said.

First to Find an Alien Moon?

If astronomers were to discover an Earthlike satellite orbiting Kepler-16b—a big if—it would be a major first.

More than 700 alien planets have been confirmed so far, and Kepler has identified more than 2,000 more potential planets. (Related: "Fifty New Planets Found—Largest Haul Yet.")

As of yet, though, no moons have been detected outside our solar system.

With the new study, "we can say there are exomoons possible around Kepler-16b, and what's important about this is that they are detectable ... down to 0.2 Earth masses," Quarles said.

To do so would require looking for subtle irregularities in the gas giant's orbit that could be caused by a moon's gravitational pull—something Kepler is equipped to do.

In fact, a new project using the Kepler telescope aims to make the the first systematic search for planets with moons. The new study suggests Kepler-16b may be an ideal target for the new initiative.

"For this system," Quarles said, "there should be a drive to determine if there's an exomoon there, because it could be the first one to be detected."

A "Drastic" Earth Also Possible

In addition to considering the possibility of an Earthlike exomoon of Kepler-16b, the team considered whether an as-yet undetected Earthlike planet could exist in what they call an extended habitable zone around the Kepler-16 stars.

The results suggest that a planet around 88 million miles from the stars—outside the orbit of the existing Saturn-like world—could maintain a stable orbit.

(Also see "New Planet May Be Among Most Earthlike—Weather Permitting.")

That theoretical, far-flung world could retain enough heat for liquid water, Quarles said, if it has "a very drastic atmosphere" of heat-trapping gases such as carbon dioxide and methane.

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