In the three years since film director James Cameron wrote the script for his new blockbuster Avatar, a lot has changed in the field of exoplanet research (the study of planets around other stars). Nobody knows this better than one of its leading practitioners, Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics, who marvels herself at how fast the young science is progressing. "We can actually overtake a sci-fi movie," she says with a laugh.
Cameron set his story on Pandora, a moon orbiting a giant gas planet circling Alpha Centauri A, one of the closest stars to our sun. Recent, real-life searches have all but ruled out that scenario: Jupiter-size worlds don't appear to exist around Alpha Centauri. But a new paper by Kaltenegger offers something perhaps more exciting: a near-term hope of finding signs of life around "exomoons" like Pandora.
One method used to find exoplanets is to watch how the light of their host stars dims and brightens as the dark planet crosses, or "transits," the star's disk. Kaltenegger realized that if a transiting planet had a large enough moon, and if the moon's orbit extended beyond the star's diameter as seen from our vantage point, the moon would also briefly appear to cross the stellar disk just before or after the planet finished its own transit. In that short interval when the moon partly eclipses the star, astronomers could analyze the starlight filtering through the moon's atmosphere (if it has one), looking for oxygen, water, carbon dioxide, and other chemicals suggestive of life.
NASA's Kepler telescope should soon tell us whether the technique is viable. Any exomoons discovered by Kepler will be too far away to inspect for bio-signatures, but if the method works, it could be applied to nearby stars like Alpha Centauri. Better still, we don't have to wait for some exotic new instrument to be built. The 6.5-meter James Webb Space Telescope, due to launch in 2014, should work nicely.
The hunt for habitable worlds beyond Earth just picked up pace.