Interstellar: The Space Between Stars — Medium, Phenomena, and Travel Interstellar Interstellar denotes the space between stars inside a galaxy. This article summarizes the interstellar medium, its phases and chemistry, observational signatures, roles in star for…
Interstellar cloud A concentration of gas, plasma and dust in the space between stars; types include neutral (H I), ionized (H II) and molecular clouds, and they are key sites of star formation and I…
Interstellar medium The interstellar medium (ISM) is the gas and dust between stars in a galaxy, varying widely in density, temperature and ionisation; it fuels star formation and shapes galactic evol…
Interstellar travel: concepts, challenges and prospects Interstellar travel Comprehensive overview of human and robotic travel between stars: distances, propulsion concepts, mission architectures, technical and biological challenges, historical studies and…
Invariable plane of a planetary system Invariable plane The invariable plane is the plane through a system's barycenter perpendicular to its total angular momentum; in the Solar System it is dominated by the giant planets and is used as…
Io (moon): Jupiter's volcanically active Galilean satellite Io (moon) Io is the innermost of Jupiter's major moons and the most volcanically active world in the Solar System, driven by tidal heating and notable for its sulfur-rich, constantly changin…
Iocaste (Jupiter XXIV) — a small irregular moon of Jupiter Iocaste (moon) Iocaste is a roughly 5 km, retrograde irregular satellite of Jupiter discovered in 2000. It belongs to the Ananke group and follows a distant, inclined, and eccentric orbit, likely…
Iris Nebula (NGC 7023): a blue reflection nebula in Cepheus Iris nebula The Iris Nebula (NGC 7023) is a striking blue reflection nebula in the constellation Cepheus, illuminated by a young star and located roughly 1,300 light‑years away.
Irregular galaxy A galaxy lacking a regular shape or symmetry; often rich in gas and dust, frequently shaped by interactions, and important for studies of star formation and low-metallicity environ…
Isonoe: Jupiter's irregular retrograde moon in the Carme group Isonoe (moon) Isonoe is a 3.8 km irregular, retrograde moon of Jupiter in the Carme group, discovered in 2000; it orbits ~23.8 million km every ~752 days with a high-inclination, eccentric path.
James Webb Space Telescope (JWST) James Webb Space Telescope Overview of the James Webb Space Telescope: design, instruments, mission history, orbit and key scientific goals, with links to authoritative resources and explanations of its diff…
Jan Oort: Dutch astronomer who mapped the Galaxy and proposed the Oort cloud Jan Oort Jan H. Oort (1900–1992), Dutch astronomer who established galactic rotation, pioneered radio astronomy, proposed the Oort cloud and early evidence for dark matter, and shaped postw…
Janus (moon) Janus is a small, irregular moon of Saturn notable for sharing its orbit with Epimetheus and for its unusual co-orbital dynamics confirmed by spacecraft observations.
Jarnsaxa (Saturnian moon) Jarnsaxa (moon) Jarnsaxa is a small, distant irregular moon of Saturn discovered in 2006; roughly 6 km across, retrograde and part of the Norse group with a highly inclined, eccentric orbit.
Jikiken — Japanese scientific satellite (launched 16 September 1978) Jikiken Jikiken (じきけん) was a Japanese scientific satellite launched on 16 September 1978 by the Institute of Space and Astronautical Science at the University of Tokyo, part of Japan’s ear…
Johannes Kepler German mathematician and astronomer who formulated the three laws of planetary motion and made foundational contributions to optics, celestial mechanics and the Scientific Revoluti…
John Campbell Brown — Scottish Solar Physicist and Astronomer Royal John Campbell Brown Scottish astronomer John Campbell Brown (1947–2019) was a leading figure in solar physics, serving as Astronomer Royal for Scotland and Regius Professor of Astronomy at the Univers…
Jōkyō calendar (Jōkyō-reki): Japan's Edo-period lunisolar calendar Jōkyō calendar Jōkyō-reki (Teikyō-reki) was a Japanese lunisolar calendar adopted in the Edo period (official from 1685). Developed by Shibukawa Shunkai, it corrected earlier timing errors and gu…
Julian day: continuous day count used in astronomy Julian day A continuous count of days and fractions used by astronomers. Explains the Julian day number (integer days since the epoch), the Julian date (including fractional days), history, u…
Julian year (astronomical unit of time) Julian year A Julian year is an astronomical unit of duration equal to exactly 365.25 days of 86,400 seconds each, used to express long timescales such as orbital periods and proper motions.
Juliet (Uranian moon) Juliet (moon) Juliet is a small inner satellite of Uranus discovered by Voyager 2 in 1986. A member of the Portia group, it is an elongated, low-albedo moon about 53 km in radius with limited ob…
July 4 (date) July 4 July 4 is the 185th day of the Gregorian calendar (186th in leap years). Widely known for U.S. Independence Day, it also coincides roughly with Earth's aphelion and a range of seas…
Juno (NASA spacecraft) — Jupiter orbiter mission Juno (spacecraft) Juno is a NASA New Frontiers mission studying Jupiter from a polar orbit. Launched in 2011 and arriving in 2016, it probes the planet's interior, atmosphere, magnetic field and pol…
Jupiter — the largest planet in the Solar System Jupiter Overview of Jupiter: physical characteristics, atmosphere, moons (including the Galilean satellites), magnetosphere, rings, exploration history, and its significance.
Kale (Jupiter XXXVII): an irregular moon of Jupiter Kale (moon) Kale is a small, retrograde irregular moon of Jupiter discovered in 2001. Part of the Carme group, it has an eccentric, highly inclined orbit and is named after one of the Charites…
Kallichore (Jupiter XLIV) — irregular moon of Jupiter Kallichore (moon) Kallichore is a small, retrograde irregular moon of Jupiter discovered in 2003. About 2 km across, it orbits in the Carme group at ~23.1 million km and was named for a nymph in 200…
Kalyke (Jupiter XXIII): an irregular retrograde moon of Jupiter Kalyke (moon) Kalyke is a small, irregular, retrograde moon of Jupiter discovered in 2000. About 5.2 km across, it orbits in the Carme group on a distant, eccentric, high‑inclination orbit and i…
Kari (moon) — Saturn XLV, an irregular retrograde satellite Kari (moon) Kari (Saturn XLV, S/2006 S 2) is a small irregular, retrograde moon of Saturn discovered in 2006. It orbits far from Saturn and is named for Kári of Norse mythology.
Kármán line — boundary between Earth's atmosphere and outer space Kármán line The Kármán line is the conventional boundary of space, set at 100 km above sea level. It’s used for records, legal definitions and to distinguish aerodynamic flight from orbital fl…
W. M. Keck Observatory Keck Observatory A pair of 10‑meter optical/infrared telescopes on Mauna Kea, Hawaii, notable for their segmented mirrors, adaptive optics, and wide-ranging contributions to astronomy.
Kepler (spacecraft): NASA’s exoplanet-hunting space observatory Kepler (spacecraft) Kepler was a NASA space observatory launched in 2009 to find Earth-size planets using transit photometry. It surveyed a fixed star field, produced vast datasets of planet candidate…
Kepler-186f — an Earth-sized exoplanet in the habitable zone Kepler-186f Kepler-186f is an Earth-sized exoplanet orbiting the M-dwarf Kepler-186 in the Cygnus region, discovered by the Kepler mission in 2014 and notable for lying within its star's habit…
Kepler-22b: a transiting exoplanet in the habitable zone of a Sun-like star Kepler-22b Kepler-22b is an exoplanet orbiting a G‑type star in Cygnus about 600 light‑years away. Discovered by Kepler and confirmed in 2011, it lies in its star's habitable zone though its…
Kepler-438b Kepler-438b is an Earth-sized exoplanet orbiting a red dwarf in the constellation Lyra, discovered by the Kepler spacecraft; it is notable for a high Earth Similarity Index but has…
Kepler-452b — an Earth-size exoplanet in the habitable zone of a Sun-like star Kepler-452b Kepler-452b is a transiting exoplanet discovered by the Kepler telescope in 2015. It is larger than Earth and orbits a Sun-like star within its habitable zone, though its compositi…
Kepler-62e — Super-Earth in the Habitable Zone of Kepler-62 Kepler-62e Kepler-62e is a confirmed super-Earth exoplanet discovered by Kepler in 2013. It orbits in the habitable zone of the K-type star Kepler-62 about 1,200 light-years away toward Lyra.
Kepler-62f — potentially habitable super-Earth Kepler-62f Kepler-62f is a small exoplanet in the habitable zone of the K-type star Kepler-62, roughly 990 light-years away. Its size and orbit make it a target for studies of possible surfac…
Kepler's laws of planetary motion Kepler's laws Three empirical laws that describe how bodies move around the Sun: elliptical paths, equal areas in equal times, and the relation T² ∝ a³ connecting orbital period and size.
Kepler's Supernova (SN 1604) Kepler's Supernova SN 1604, called Kepler's Supernova, appeared in 1604 in Ophiuchus. Its remnant is a nearby Type Ia supernova remnant studied across wavelengths to test explosion models and superno…
Kerberos (moon) Kerberos is a small natural satellite of the dwarf planet Pluto, discovered in 2011. This article summarizes its discovery, orbit and physical traits, naming, origin theories, and…
Kirkwood gaps Kirkwood gap Regions of the main asteroid belt depleted of asteroids where Jupiter's orbital resonances destabilize orbits, shaping asteroid distribution and supplying near‑Earth objects.
Kitt Peak National Observatory A major U.S. astronomical facility on Kitt Peak, Arizona, hosting a range of optical and solar telescopes used for research, surveys, and public outreach since the mid‑20th century…
Kiviuq — irregular moon of Saturn Kiviuq (moon) Kiviuq is a small irregular, non-spherical satellite of Saturn discovered in 2000. It belongs to the Inuit group and shows spectral similarities with Siarnaq and Paaliaq, suggestin…
Kore (Jupiter XLIX): a small retrograde moon of Jupiter Kore (moon) Kore (Jupiter XLIX) is a tiny, retrograde irregular moon of Jupiter discovered in 2003. It is roughly 2 km across and orbits at ~23.24 million km with a long, inclined, eccentric o…
Kraken Mare — Titan's largest hydrocarbon sea Kraken Mare Kraken Mare is the biggest known body of surface liquid on Saturn's moon Titan. An extensive sea of methane and ethane discovered by Cassini, it is notable for its size, compositio…
Kuiper Belt: the distant trans-Neptunian region of icy bodies Kuiper belt A summary of the Kuiper Belt: its location beyond Neptune, composition of icy bodies and dwarf planets, discovery history, relationship to scattered disk and comets, and why it mat…
KW Sagittarii: a very large red supergiant in Sagittarius KW Sagittarii KW Sagittarii (KW Sgr) is a cool, luminous red supergiant in the constellation Sagittarius. With a radius around 1,009 times the Sun, it is among the largest known stars and a subj…
KY Cygni — a very large red supergiant in Cygnus KY Cygni KY Cygni is an extremely large, luminous red supergiant in the constellation Cygnus, notable for its vast radius and high luminosity; size estimates and distance carry significant…
