"Long before it's in the papers"
August 03, 2010

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Moon like that in “Avatar” could be real, astronomers say

Dec. 20, 2009
Courtesy Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics
and World Science staff

In the new block­bust­er film Av­a­tar, hu­mans vis­it the hab­it­a­ble—and in­hab­it­ed—al­ien moon Pan­do­ra. Life-bearing moons like Pan­do­ra or the Star Wars for­est moon of En­dor are a sta­ple of sci­ence fic­tion. 

But hab­it­a­ble moons may soon be­come sci­ence fact, and could per­haps even ex­ist around the same star that il­lu­mi­nates the fic­tional Pan­do­ra, as­tro­no­mers say. 

This artist's con­cep­tion shows a hy­po­thet­i­cal gas gi­ant plan­et with an Earth-like moon si­m­i­lar to the moon Pan­do­ra in the mov­ie Av­a­tar. (Cred­it: Da­vid A. Aguilar, CfA )

In the new blockbuster film Avatar, humans visit the habitable—and inhabited—alien moon Pandora. Life-bearing moons like Pandora or the Star Wars forest moon of Endor are a staple of science fiction. But habitable moons may soon become science fact, and could perhaps even exist around the same star shown illuminating the fictional Pandora, astronomers say. “If Pandora existed, we potentially could detect it and study its atmosphere in the next decade,” said Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. A new paper by Kaltenegger argues that NASA’s new James Webb Space Telescope, to be launched in 2014, will be able to study their atmospheres and detect key gases like carbon dioxide, oxygen, and water vapor. So far, planet searches have spotted hundreds of Jupiter-sized objects in a range of orbits. Gas giants, while easier to detect, could not serve as homes for life as we know it. However, scientists have speculated whether a rocky moon orbiting a gas giant could be life-friendly, if that planet orbited within the star’s habitable zone, the region warm enough for liquid water to exist. “All of the gas giant planets in our solar system have rocky and icy moons,” said Kaltenegger. “That raises the possibility that alien Jupiters will also have moons. Some of those may be Earth-sized and able to hold onto an atmosphere.” NASA’s space-based Kepler telescope looks for planets that cross in front of their host stars, which creates a mini-eclipse and dims the star by a small but detectable amount. Such a transit lasts only hours and requires exact alignment of star and planet along our line of sight. Once they have found an alien Jupiter, astronomers can look for orbiting moons, or exomoons. A moon’s gravity would tug on the planet and either speed or slow its transit, depending on whether the moon leads or trails the planet. The resulting transit duration variations would indicate the moon’s existence. Once a moon is found, the next obvious question would be: Does it have an atmosphere? If it does, those gases will absorb a fraction of the star’s light during the transit, leaving a tiny, telltale fingerprint to the atmosphere’s composition. The signal is strongest for large worlds with hot, puffy atmospheres, but an Earth-sized moon could be studied if conditions are just right. For example, the separation of moon and planet needs to be large enough that we could catch just the moon in transit, while its planet is off to one side of the star. Kaltenegger calculated what conditions are best for examining the atmospheres of alien moons. She found that Alpha Centauri A, the system featured in Avatar, would be an excellent target. “Alpha Centauri A is a bright, nearby star very similar to our Sun, so it gives us a strong signal” Kaltenegger explained. “You would only need a handful of transits to find water, oxygen, carbon dioxide, and methane on an Earth-like moon such as Pandora.” While Alpha Centauri A offers tantalizing possibilities, small, dim, red dwarf stars are better targets in the hunt for habitable planets or moons, she added. The habitable zone for a red dwarf is closer to the star, which increases the probability of a transit. Astronomers have debated whether tidal locking could be a problem for red dwarfs. A planet close enough to be in the habitable zone would also be close enough for the star’s gravity to slow it until one side always faces the star. (The same process keeps one side of the Moon always facing Earth.) One side of the planet then would be baked in constant sunlight, while the other side would freeze in constant darkness. An exomoon in the habitable zone wouldn’t face this dilemma. The moon would be tidally locked to its planet, not to the star, and therefore would have regular day-night cycles just like Earth. Its atmosphere would moderate temperatures, and plant life would have a source of energy moon-wide. “Alien moons orbiting gas giant planets may be more likely to be habitable than tidally locked Earth-sized planets or super-Earths,” said Kaltenegger. “We should certainly keep them in mind as we work toward the ultimate goal of finding alien life.” Kaltenegger’s paper is posted online at the arXiv database of Cornell University in New York.