Avatar moons may become a reality
Habitable alien moons like the one depicted in the blockbuster movie Avatar may become science fact within the next few years, according to a leading astronomer.
In the 3D film, a race of 10ft blue-skinned giants inhabits an Earth-like moon called Pandora.
Their world orbits a gas giant planet similar to Jupiter that cannot support life.
US astronomer and planet-hunter Lisa Kaltenegger, from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, believes there is every chance a real-life version of Pandora exists and will soon be found.
She has conducted research showing that a planned new space telescope will be able to identify nearby "exomoons" and discover if they are habitable.
The American space agency Nasa's James Webb Space Telescope (JWST) is due to be launched in 2014.
Dr Kaltenegger said: "If Pandora existed, we potentially could detect it and study its atmosphere in the next decade."
Astronomers have already spotted hundreds of Jupiter-sized gas giants orbiting stars, but none have conditions suitable for Earth-type life.
However a rocky moon orbiting a gas giant could harbour life if it was in the parent star's "habitable zone" - the region where temperatures are just right for liquid water.
"All of the gas giant planets in our solar system have rocky and icy moons," said Dr 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."
A Pandora-type moon could be identified when its planet "transits" across the face of the parent star. If the moon has an atmosphere, this will absorb a tiny amount of light from the star, leaving a spectrographic fingerprint of its composition.
Dr Kaltenegger calculated that Alpha Centauri A, the star featured in Avatar, would provide an excellent target for astronomers hunting habitable moons.
Alpha Centauri is the closest star system to the Sun, being only 4.37 light years away. It consists of three stars, the largest being Alpha Centauri A, which is slightly brighter than the Sun.
"Alpha Centauri A is a bright, nearby star very similar to our Sun, so it gives us a strong signal," said Dr Kaltenegger. "You would only need a handful of transits to find water, oxygen, carbon dioxide and methane on an Earth-like moon such as Pandora.
"If the Avatar movie is right in its vision, we could characterise that moon with JWST in the near future."
She said small, dim red dwarf stars may provide the best evidence of habitable planets or moons. This is because the habitable zone for a red dwarf is closer to the star, which increases the likelihood of a transit as seen from the Earth. Red dwarfs are also the most common kind of star in our galaxy, the Milky Way.
A planet close enough to a red dwarf to be in its habitable zone may also be tidally "locked" by gravity so that the same face always points towards the star. This could be a problem for life, since one side of the planet would bake in constant sunlight, while the other would freeze in constant darkness.
An exomoon orbiting the planet, on the other hand, would have regular day-night cycles and moderate temperatures.
"Alien moons orbiting gas giant planets may be more likely to be habitable than tidally locked Earth-sized planets or super-Earths," said Dr Kaltenegger. "We should certainly keep them in mind as we work toward the ultimate goal of finding alien life."
Dr Kaltenegger's research is published online in Astrophysical Journal Letters.