exomoon

An exomoon is a natural satellite that orbits an exoplanet, a planet outside the solar system. The study of exomoons focuses on their potential abundance, formation, and role in planetary systems. Exomoons are expected to vary widely in size and composition, from small rocky bodies to large icy worlds, much like the moons found around planets in our own solar system. They typically orbit within the planet’s gravitational sphere of influence, the Hill sphere, and their dynamics are shaped by the planet’s gravity, the star’s gravity, and tidal interactions.

Formation theories for exomoons are similar to those for solar-system moons. They include in-situ formation in a circumplanetary disk around a young planet, capture of passing objects, and accretion from debris after planetary collisions. The resulting moons might experience tidal heating, influence climate and orbital evolution, and, if large and icy, could harbor subsurface oceans under certain conditions.

Detection methods are challenging and rely on indirect signals. The primary approaches include transit timing variations and transit duration variations in the light curve of the host star, caused by the moon’s gravitational influence on the planet’s orbit. Other techniques involve microlensing or, in rare cases, direct imaging, though these signals are weak and hard to confirm with current instruments. Notable exomoon candidates have been reported, such as signals around Kepler-1625 b, but none have achieved widely accepted confirmation, and researchers continue to scrutinize potential detections with new data from ongoing and future missions.

The existence of exomoons has implications for planetary system formation, dynamics, and habitability, expanding the scope of where life-supporting environments might arise beyond planets alone.