Exomoons

An exomoon is a natural satellite that orbits an exoplanet, a planet outside the solar system. Exomoons are of interest because they can influence the dynamics of planetary systems and, in some cases, may be detectable even when the planet itself is difficult to observe directly. They may range in size from small rocky bodies to large companions comparable to gas giant moons in our solar system.

Detecting exomoons is challenging. Most proposed methods rely on indirect effects in the observed light from the host star or the planet. Transit timing variations (TTV) and transit duration variations (TDV) occur when a moon's gravity perturbs a transiting planet, altering the timing and length of the transit. Gravitational microlensing, direct imaging, and, less commonly, pulsar timing are also used. As of now, there are no confirmed exomoons; several candidates have been reported, notably a proposed large moon around Kepler-1625b, but none have achieved broad confirmation.

Exomoons are thought to form in the same circumplanetary disk that gives birth to their planets, through accretion around gas giants, or to be captured from elsewhere in the system. Their orbital stability depends on the planet’s mass and distance from the star, and they must remain within the planet’s Hill sphere. Large moons tend to have relatively wide orbits, but overly distant ones can become unstable due to stellar tides and perturbations.

The study of exomoons informs planetary formation and the potential for habitable environments. In principle, sizable, rocky exomoons in temperate orbits could host atmospheres and liquid water, aided by tidal heating. Ongoing and future observations with missions such as Kepler data reanalyses, TESS, and the James Webb Space Telescope, along with advances in microlensing surveys, may provide stronger evidence or detections in the coming years.