If planets are a dime a dozen, moons are less than a penny each. There are at least 139 moons in our solar system, and most of these are the property of the gas giant planets beyond Mars. More than just a nice accompaniment to planets, moons may frequently have habitats in which liquid water could ebb and flow – and possibly be home to life. Planetary scientist Paul Estrada investigates how moons around gas giants are formed, an important question since its answer would give us insight into the nature of moons around the myriad gas giant planets we know orbit other stars.
The birth of moons around gas giant planets is superficially similar to planetary formation; however, as Estrada points out, there are some very important differences. To begin with, the “environment” (pressure, density, temperature) of satellite birth is different from that of the planets. Perhaps most important, satellite systems are more compact, which means things tend to happen much faster than on the planetary scale. Consider that the giant planet Jupiter takes a dozen years to orbit the Sun, a lumbering pace compared to the days required for its moons to orbit once. As a result, once a moon forms, it has much less time to find a way to “survive.” This is because, just like the planets, there is the problem that the leftover gas which eventually dissipates over time (a time much longer than required to form the moons or planets) will slow down a newly forming satellite, causing it to spiral into its host planet. Clearly, this doesn’t always happen, and Estrada’s research elucidates exactly how such a catastrophic fate can be avoided. The incentive to understand satellite formation is strong, as these small worlds might be the most plentiful locations for life in the universe.