An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported as of January 2024 have been observed directly, with even fewer being resolved from their host star.
Epsilon Eridani b, also known as AEgir [sic], is an exoplanet approximately 10.5 light-years away orbiting the star Epsilon Eridani, in the constellation of Eridanus. The planet was discovered in 2000, and as of 2023 remains the only confirmed planet in its planetary system. It orbits at around 3.5 AU with a period of around 7.6 years, and has a mass around 0.6 times that of Jupiter. As of 2023, both the Extrasolar Planets Encyclopaedia and the NASA Exoplanet Archive list the planet as 'confirmed'.
HD 189733 b is an exoplanet in the constellation of Vulpecula approximately 64.5 light-years away from the Solar System. Astronomers in France discovered the planet orbiting the star HD 189733 on October 5, 2005, by observing its transit across the star's face. With a mass 11.2% higher than that of Jupiter and a radius 11.4% greater, HD 189733 b orbits its host star once every 2.2 days at an orbital speed of 152.0 kilometers per second, making it a hot Jupiter with poor prospects for extraterrestrial life.
WASP-12b is a hot Jupiter orbiting the star WASP-12, discovered on April 1, 2008, by the SuperWASP planetary transit survey. The planet takes only a little over one Earth day to orbit its star, in contrast to about 365.25 days for the Earth to orbit the Sun. Its distance from the star is only the Earth's distance from the Sun, with an eccentricity the same as Jupiter's. Consequently, it has one of the lowest densities for exoplanets. On December 3, 2013, scientists working with the Hubble Space Telescope (HST) reported detecting water in the atmosphere of the exoplanet. In July 2014, NASA announced finding very dry atmospheres on three exoplanets orbiting sun-like stars.
A subsatellite, also known as a submoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.
The Hunt for Exomoons with Kepler (HEK) is a project whose aim is to search for exomoons, natural satellites of exoplanets, using data collected by the Kepler space telescope. Founded by British exomoonologist David Kipping and affiliated with the Center for Astrophysics | Harvard & Smithsonian, HEK submitted its first paper on June 30, 2011. HEK has since submitted five more papers, finding some evidence for an exomoon around a planet orbiting Kepler-1625b in July 2017.
Kepler-90g is a super-puff exoplanet orbiting the early G-type main sequence star Kepler-90, one of eight planets around this star discovered using NASA's Kepler space telescope. It is located about 2,840 light-years (870 pc) from Earth, in the constellation Draco. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. It orbits its parent star about every 210.5 days at a distance of 0.71 astronomical units.
Kepler-27 is a star in the northern constellation of Cygnus, the swan. It is located at the celestial coordinates: Right Ascension 19h 28m 56.81962s, Declination +41° 05′ 09.1405″. With an apparent visual magnitude of 15.855, this star is too faint to be seen with the naked eye.
Kepler-25c is an exoplanet orbiting the star Kepler-25, located in the constellation Lyra. The planet was first detected as a candidate extrasolar planet by the Kepler space telescope in 2011. It was confirmed, in 2012, by Jason Steffen and collaborators using transit-timing variations obtained by the Kepler Space Telescope. It orbits its parent star at only 0.110 astronomical units away, and at its distance it completes an orbit once every 12.7 days.
Kepler-1625 is a 14th-magnitude solar-mass star located in the constellation of Cygnus approximately 7,200 light-years away. Its mass is within 5% of that of the Sun, but its radius is approximately 70% larger reflecting its more evolved state. A candidate gas giant exoplanet was detected by the Kepler Mission around the star in 2015, which was later validated as a real planet to >99% confidence in 2016. In 2018, the Hunt for Exomoons with Kepler project reported evidence for a Neptune-sized exomoon around this planet, based on observations from NASA’s Kepler mission and the Hubble Space Telescope. Subsequently, the evidence for and reality of this exomoon candidate has been subject to debate.
Kepler-1625b is a super-Jupiter exoplanet orbiting the Sun-like star Kepler-1625 about 2,500 parsecs away in the constellation of Cygnus. The large gas giant is approximately the same radius as Jupiter, and orbits its star every 287.4 days. In 2017, hints of a Neptune-sized exomoon in orbit of the planet was found using photometric observations collected by the Kepler Mission. Further evidence for a Neptunian moon was found the following year using the Hubble Space Telescope, where two independent lines of evidence constrained the mass and radius to be Neptune-like. The mass-signature has been independently recovered by two other teams. However, the radius-signature was independently recovered by one of the teams but not the other. The original discovery team later showed that this latter study appears affected by systematic error sources that may influence their findings.
GJ 9827 is a star in the constellation of Pisces. It is a K-type main-sequence star with an apparent magnitude of 10.250. It is 97 light-years away, based on parallax.
K2-28 is a metal rich M4-type main sequence star. One confirmed transiting exoplanet is known to orbit this star. There is another star 5.2 arcseconds to the north–east of K2-28 however this star has a different proper motion and is therefore physically unrelated and probably a background star.
L 98-59 is a bright M dwarf star, located in the constellation of Volans, at a distance of 10.608 parsecs, as measured by Gaia.
Kepler-1708b is a Jupiter-sized exoplanet orbiting the Sun-like star Kepler-1708, located in the constellation of Cygnus approximately 5,600 light years away from Earth. It was first detected in 2011 by NASA's Kepler mission using the transit method, but was not identified as a candidate planet until 2019. In 2021, a candidate Neptune-sized exomoon in orbit around Kepler-1708b was found by astronomer David Kipping and colleagues in an analysis using Kepler transit data.
Kepler-1708b I is an exomoon candidate that may orbit around the exoplanet Kepler-1708b, which is located about 5500 light-years from the Solar System.
Kepler-1658b is a hot Jupiter, a type of gas giant exoplanet, that orbits an F-type star called Kepler 1658, located about 2629 light-years away from the Solar System. It is the first planet identified by the Kepler space telescope after its launch in 2009, but later ruled out as false alarm since its transit could not be confirmed. A study published in 2019 established it as a planet, describing it as "the closest known planet in terms of orbital period to an evolved star." Analysis of the Transiting Exoplanet Survey Satellite (TESS) data in 2022 showed that it is gradually spiraling into its star.
