Exomoon
An exomoon is a natural satellite orbiting a planet or other planetary-mass object beyond the Solar System. No exomoon has yet been confirmed.
Hill sphere: The region around an astronomical body where its gravity can retain orbiting satellites against a larger body's tidal influence. An exomoon must orbit within its planet's region of gravitational dominance.
Transit method: A method for finding exoplanets by measuring the dimming of a star as a planet passes in front of it. Most proposed exomoon searches begin with the planet's repeated stellar transits.
Kepler-1625b I: A proposed large moon candidate orbiting the gas giant Kepler-1625b, based on transit observations. Its reported transit and timing signals sparked a major debate over exomoon detection.
False positive: A result that appears to indicate a phenomenon but is actually produced by another cause or measurement error. Stellar variability and instrumental effects can imitate an exomoon signal.
Moon: Earth's natural satellite, which orbits the planet at an average distance of about 384,400 kilometers. It is the familiar benchmark for comparing size, orbit, and detectability beyond the Solar System.
Roche limit: The distance inside which tidal forces can disrupt a body held together primarily by gravity. A moon orbiting too close to its planet may be torn apart.
Kepler space telescope: A NASA space telescope that measured stellar brightness to discover and characterize exoplanets. Its precise, long-duration light curves enabled prominent exomoon searches.
Kepler-1708b I: A proposed large moon candidate orbiting Kepler-1708b, identified through analysis of transit observations. It became a second prominent candidate, though its interpretation remains disputed.
Circumplanetary disk: A disk of gas and dust surrounding a young planet, from which satellites may form. Its evolution may determine whether large moons form around giant planets.
Natural satellite: A naturally occurring object that orbits a planet, dwarf planet, or other larger body. Exomoons are the subset of natural satellites located outside the Solar System.