Exomoon

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

Exomoons are difficult to detect and confirm using current techniques, and to date there have been no confirmed exomoon detections. However, observations from missions such as Kepler have observed a number of candidates. Two potential exomoons that may orbit rogue planets have also been detected by microlensing. In September 2019, astronomers reported that the observed dimmings of Tabby's Star may have been produced by fragments resulting from the disruption of an orphaned exomoon. Some exomoons may be potential habitats for extraterrestrial life. A possible exomoon, which, if real, may be habitable, has been discovered around one of the components of the binary rogue planet 2MASS J11193254−1137466 AB.

Although traditional usage implies moons orbit a planet, the discovery of brown dwarfs with planet-sized satellites blurs the distinction between planets and moons, due to the low mass of brown dwarfs. This confusion is resolved by the International Astronomical Union (IAU) declaration that "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/M_central < 2/(25+√621) are planets."

The IAU definition does not address the naming convention for the satellites of free-floating objects that are less massive than brown dwarfs and below the deuterium limit (the objects are typically referred to as free-floating planets, rogue planets, low-mass brown dwarfs or isolated planetary-mass objects). The satellites of these objects are typically referred to as exomoons in the literature.

Exomoons take their designation from that of their parent body plus a capital Roman numeral; thus, Kepler-1625b orbits Kepler-1625 (synonymous with Kepler-1625a) and itself may be orbited by Kepler-1625b I (no Kepler-1625b II is known, nor is I known to have a submoon).

Characteristics of any extrasolar satellite are likely to vary, as do the Solar System's moons. For extrasolar giant planets orbiting within their stellar habitable zone, there is the prospect that terrestrial planet-sized satellite may be capable of supporting life.^{[clarification needed]}

In August 2019, astronomers reported that an exomoon in the WASP-49b exoplanet system may be volcanically active.

For impact-generated moons of terrestrial planets not too far from their star, with a large planet–moon distance, it is expected that the orbital planes of moons will tend to be aligned with the planet's orbit around the star due to tides from the star, but if the planet–moon distance is small it may be inclined. For gas giants, the orbits of moons will tend to be aligned with the giant planet's equator because these formed in circumplanetary disks.