The 2009 blockbuster film “Avatar” presented a fairytale view of a paradise-like lush green moon called Pandora, orbiting a monster Jupiter-like planet.
As far as I can tell, the idea of habitable moons was first introduced into modern cinema in the 1977 sci-fi classic “Star Wars.” The forest moon of Endor — hideout for the Rebel Alliance — was conveniently eclipsed by a bloated Jovian planet when the Death Star arrived to blow the moon apart. That’s pretty lousy navigation for the Evil Empire’s pilots.
I thought this idea was a novelty until reality caught up with science fiction and we discovered planets orbiting other stars. In particular, astronomers found gas giant worlds nestled within the habitable zone of their stars, where temperatures are moderate enough for water to remain in liquid form. Continents would not be expected to be found on these giants. But, possibly, any moons orbiting them might be large enough to hold onto atmospheres and nurture life.
The satellite systems of Jupiter and Saturn are two distinct examples of what should be common throughout the galaxy.
At a frigid distance of 500 million miles from the sun, Jupiter has four major moons that are lifeless on the surface but geologically quite diverse. Two of them, Europa and Ganymede, would defrost into water worlds if brought to Earth’s distance from the sun.
The bulk of the mass in Saturn’s satellite system is tied up in Titan, a chilly world that is bigger than the planet Mercury. Surface temperatures of minus 300 degrees aside, the world is amazingly Earth-like with lakes, streams, a thick atmosphere, and volcanism.
The Hunt for Exomoons with Kepler (HEK) project at Harvard University uses Kepler data from over 3,000 exoplanet candidates to look for indirect evidence of moons orbiting giant planets. Any unseen moons would gravitationally tug on a planet and this would slightly alter the time of when the planet is measured passing in front of its star (transiting). The planet’s “mass transit” (pun intended) schedule would be off slightly. What’s more, if a large moon passes in front of or behind a planet it would cause unusual variations in its brightness of the star.
In a recent paper, Rene Heller of the Leibniz-Institute for Astrophysics Potsdam, Germany and Rory Barnes of the University of Washington in Seattle, predict that two exoplanets are good candidates for possessing habitable exomoons.
One, cataloged KOI211.01, is about one-third Jupiter’s mass and is in an Earth-sized orbit about a sun-like star. The other is Kepler 22b; located 600 light-years away, this candidate can be dubbed either a mini-Uranus or super-Earth, weighing in at about six Earth masses. The planet could be a big ball of water surrounding a rocky core. The HEK project has not detected any moons above about half an Earth mass orbiting this planet. (Titan is only 1/100th of Earth’s mass)
Exomoons are likely to be tidally locked to their planet just as our moon keeps one hemisphere facing Earth. Days would last for half of an orbital period. Seasons would be possible depending on the moon’s axial tilt plus that of the host planet, and how elliptical the planet’s orbit is about the central star. Eclipses of the parent star would happen frequently and this would alter the weather abruptly on a moon. Life might have a biorhythm in synch with the eclipses, just as some organisms on Earth sense the lunar phase cycle.
Living alongside a gas giant planet is likely rough on moons and could inhibit the emergence of life, say the researchers. If the moon is too close to the planet, tidal forces can make it so hot that the surface is always erupting volcanically. Jupiter’s innermost major satellite Io is an example of this.
A water-laden moon with a thick atmosphere might suffer from a runaway greenhouse effect. The water vapor, combined with other greenhouse gasses such as methane and carbon dioxide might make the moon too warm, as is the case of Venus. The closer the moon is to its gas giant planet, the more radiation it will absorb, simply from starlight reflected off the planet.
But if the moons are farther out they are not protected from galactic cosmic rays by the planet’s magnetic field, and are too small to generate a substantial field of their own. This radiation might sterilize the surface of life.
By analogy with the circumstellar habitable zone, these constraints define a circum-planetary “habitable edge,” say the researchers. They conclude that if either exoplanet hosted a moon at a distance greater than 10 planetary radii, then it could be habitable.
Characterizing such moons is certainly an order of magnitude or more difficult that identifying inhabited planets. So although there might easily be more inhabited moons in out galaxy than planets, characterizing them is a daunting task for far future super-telescopes… or maybe starships.
Image credit: NASA