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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 )


“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. Such gi­ant gas plan­ets, 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. 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 moon 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.”

Scott Fleming of the University of Florida has also argued that a single habitable-zone gas giant could serve as a “signpost” for perhaps several habitable moons.

Kalteneg­ger’s pa­per is posted on­line at the arXiv database of Cor­nell Un­ivers­ity in New York.


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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.