n the search for extraterrestrial life, planets take all the glory. We yearn to see worlds that mirror our own. The idea that Earth-like planets are out there, potentially playing host to thriving forms of alien life, is attractive. But in this search for a foil of our own rocky planet, there may be plenty of life in other star systems sprinkled among the moons.
Our own moon, given its lack of surface water, active geology, or an atmosphere, might have given previous generations of scientists a mistaken sense of other satellites’ possibilities. The past few decades have opened our eyes. The discoveries of Titan’s nitrogen-rich atmosphere (the only one in the solar system apart from Earth’s) and its liquid ethane and methane oceans, and Europa’s water ocean lurking under just a few miles of ice, show that these moons have some of the key elements for life. Extrapolating them to slightly more favorable conditions, it appears that moons actually might represent the most robust habitats in the galaxy.
As usual, science fiction has beaten us to the punch. There are dozens of books and movies in which aliens (or humans) live on giant planet moons. Moon worlds include that of Kurt Vonnegut’s The Sirens of Titan (which hosts the Tralfamadorians, who also appear in Slaughterhouse-Five) and Europa from Arthur C. Clarke’s 2010: Odyssey Two, in which an advanced civilization broadcasts: “ALL THESE WORLDS ARE YOURS—EXCEPT EUROPA. ATTEMPT NO LANDING THERE.” It’s hard to imagine a stronger (fictional) endorsement of the suitability of giant planet moons for life.
Using some simple math, we can calculate how likely it is that life might arise on these faraway gas giants’ moons.
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