As James Cameron's animated sci-fi movie Avatar goes on general release, astronomers point out that the movie's habitable moon called "Pandora" may exist in reality.
Although none have been found to date, "exomoons" orbiting exoplanets are sure to exist. Could an exomoon be detected? If so, could that exomoon's atmosphere be probed? Yes and yes, according to today's announcement by the Harvard-Smithsonian Center for Astrophysics (CfA), Mass.
Pandora is a fictional alien world 5 light-years away from Earth orbiting an exoplanet in the system of Alpha Centauri A. As detailed in an exclusive Discovery News interview with the visionary director of Avatar, Cameron has been very careful to base his creation on science fact (unlike some movies I won't mention). The biology of Avatar's alien creatures are no doubt impressive, but what about the planetary physics? Could an exomoon be habitable... even inhabited?
"If Pandora existed, we potentially could detect it and study its atmosphere in the next decade," said CfA's Lisa Kaltenegger.
Kaltenegger and her colleagues point out that all the gas giant planets in our solar system (Jupiter, Saturn, Uranus and Neptune) have moons, and if these moons are large enough, they possess an atmosphere.
One example is Saturn's large moon Titan that has a very thick atmosphere and - as the 2005 ESA Huygens mission discovered - a lot of the surface features are very Earth-like. It's not a huge leap of the imagination to think that a larger rocky moon, orbiting a gas giant (a large Jupiter-like exoplanet) in another star system might have an exomoon with an atmosphere too.
"All of the gas giant planets in our solar system have rocky and icy moons," Kaltenegger added. "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."
However, seeking out exoplanets is hard, and although over 400 of these worlds have been detected, some hi-tech piece of kit would be needed to go on an exomoon hunt.
Looking for Wobbly Exoplanets
When searching for exoplanets, astronomers will often look for a slight dimming in star light as an exoplanet passes in front of its host star. The exoplanet-hunting Kepler space telescope is currently looking for these exoplanet "transits" and it is hoped that the presence of exomoons orbiting the exoplanets may be detected too.
As an exomoon orbits its host exoplanet (in the same way our moon orbits the Earth), the exomoon will "tug" on the exoplanet very slightly. From Kepler's point of view, the telescope will see the transiting exoplanet wobble very slightly as it passes in front of the star. This could be the signature of the presence of an exomoon.
Say if Kepler could detect the presence of an exomoon (not necessarily in the Alpha Centauri A system as no exoplanets have been spotted there, yet), how could astronomers work out if it's a real-life Pandora?
"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."
"If the Avatar movie is right in its vision, we could characterize that moon with JWST [the James Webb Space Telescope] in the near future," she added.
Exomoons Better than Exoplanets?
Interestingly, exomoons provide an exciting opportunity for the survivability of life. In the case of an exoplanet orbiting a red dwarf star, the exoplanet would have to orbit very close to the star to exist within the "habitable zone" (the region surrounding a star that's neither too far away or too close, but in just the right location for water to be maintained in a liquid state). This is because red dwarfs emit less light than a star like our sun; therefore an exoplanet needs to be closer to a red dwarf to receive a quantity of light for life as we know it to survive.
But there's a problem. If an exoplanet orbiting a red dwarf is within the habitable zone of the star, there's a high probability that the exoplanet will be "tidally locked" with the star. Tidal locking means that the same side of the exoplanet will always be facing the star, continuously getting heated. The far side will be forever cold. This kind of situation is very bad for life to survive as there will be no such thing as "days," just one eternal, boring "day."
However, put an exomoon in orbit around this tidally locked exoplanet and we'll have a situation where the exomoon is always orbiting (it will therefore have days). The exomoon would become the more likely location for life to survive, even thrive.
Whether or not Pandora is really out there, perhaps there's more chance of an Earth-sized exomoon (with an atmosphere) rather than an exoplanet nurturing life...
Source: CfA press release