What happens to a moon when its planet is stolen?

A recent study reveals that when a planet is ejected from its solar system, its moons may not be left behind. Researchers at Leiden Observatory simulated thousands of scenarios involving passing stars and planetary systems, finding that moons often remain attached to their planets during such dramatic events. This challenges the traditional view of rogue planets as solitary wanderers in space. The simulation process involved tracking the fates of both planets and their moons during close stellar encounters. Nearly 34,000 interactions were modeled, considering varying angles and distances of approaching stars. These simulations showed that the survival of a moon depends largely on its position relative to its host planet’s Hill radius, a critical measure defining the area where a planet's gravity dominates over the star's. The Hill radius acts as a kind of gravitational leash. Any moon within approximately 40% of this radius remains bound to its planet even during ejection. Beyond that point, the moon loses its grip and separates from the planet. In some cases, the separation occurs mid-flight, leaving the moon adrift independently. The findings suggest that moons like those of Jupiter, Io, Europa, Ganymede, and Callisto, would stay with their planet if it were suddenly expelled from the solar system. Io, for instance, orbits at less than 1% of Jupiter’s Hill radius, meaning it would remain in orbit even under extreme conditions. The study also uncovered subtle differences in the orbital characteristics of surviving moons. Those that remained close to their planet emerged with relatively stable, circular orbits, whereas those that barely clung on displayed signs of disturbance, such as stretching and tilting. These changes reflect the nature of the ejection event itself, whether caused by a nearby star or a gravitational interaction with another planet. One particularly intriguing aspect of the research involves the potential for continued geological activity on moons after ejection. The internal heating generated by tidal forces between a planet and its moon could persist even in the absence of sunlight. Europa, for example, maintains a subsurface ocean due to tidal flexing, and similar processes might continue if such a moon were cast into interstellar space. The researchers applied their findings to a real-world case: a microlensing event cataloged as MOA-2011-BLG-262L. While the object could be a rogue planet with a small moon, the data allow for multiple interpretations. However, if the object does contain a moon, the model suggests it likely originated near the orbit of Jupiter, reinforcing the idea that moons can travel far beyond their parent planets. The implications extend beyond our solar system. The study highlights the possibility that exomoons, moons orbiting planets outside our solar system, might accompany their planets during ejections, potentially leading to unique environments where life could exist. The ongoing gravitational influence of the planet could sustain internal heat and possibly even support biological processes in these distant, dark regions of space. As the research continues, scientists will look for further evidence supporting these conclusions. Future observations of rogue planets and their possible moons could provide insights into the dynamics of planetary systems and the resilience of celestial bodies in extreme conditions. For now, the study offers a compelling glimpse into the complex relationships that govern the movements of planets and their satellites across the cosmos.