Solar System bodies

Ganymede (moon)

Ganymede, also known as Jupiter III, is Jupiter's largest moon and the largest in the solar system, discovered by Galileo in 1610, with evidence of a subsurface ocean.

Gas giant

A gas giant, or gas planet, is a large planet composed mostly of hydrogen and helium with no true surface; Jupiter and Saturn are examples.

Geometric albedo

Geometric albedo is the ratio of a celestial body's brightness at full phase to that of a perfectly reflecting, diffusely scattering disk of the same apparent size.

Gerd (moon)

Gerd (Saturn LVII) is a small, retrograde irregular moon of Saturn's Norse group, about 3.5 kilometers across, first imaged in 2004 and named in 2022.

Giant planet

A giant planet, or Jovian planet, is a massive planet made mainly of gases and fluids with no solid surface, such as Jupiter, Saturn, Uranus, and Neptune.

Gleissberg cycle

The Gleissberg cycle is an 80- to 100-year modulation of the Sun's 11-year sunspot cycle, named for German astronomer Wolfgang Gleissberg, who described it around 1938.

Gravitation of the Moon

The gravitation of the Moon, or lunar gravity, is the field generated by the Moon's mass, which accelerates surface objects at about one sixth of Earth's rate.

Great comet

A great comet is a comet that becomes exceptionally bright to the naked eye, such as Hale-Bopp in 1997, usually by passing close to the Sun or Earth.

Great Dark Spot

The Great Dark Spot (GDS-89) was an Earth-sized anticyclonic storm on Neptune, discovered by Voyager 2 during its 1989 flyby and named for Jupiter's Great Red Spot; it had vanished by 1994.

Great Red Spot

The Great Red Spot (GRS) is a giant anticyclonic storm in Jupiter's southern hemisphere, the largest and longest-lived known vortex of any planet in the solar system.

Great White Spot

The Great White Spot (GWS) is a large, bright planet-encircling storm that erupts on Saturn roughly every 29.5 Earth years, with six recorded since 1876.

Hale cycle

The Hale cycle is the Sun's approximately 22-year magnetic cycle, named for George Ellery Hale, who discovered sunspot magnetic fields in 1908; its polarity reverses at each 11-year sunspot cycle.

Harrison Schmitt

Harrison Hagan "Jack" Schmitt is an American geologist, astronaut, and senator who flew on Apollo 17 in 1972 as the twelfth and last person to walk on the Moon.

Haumea

Haumea is a dwarf planet in the Kuiper Belt beyond Neptune, named for a Hawaiian goddess, with a fast 4-hour spin and two moons, Hi'iaka and Namaka.

HD 149026b

HD 149026b, officially named Smertrios, is a hot Saturn-mass exoplanet orbiting the metal-rich subgiant HD 149026 every 2.88 days. Discovered in 2005, it is the most metal-rich giant planet known.

Helene (moon)

Helene is a small Trojan moon of Saturn that shares Dione's orbit, staying 60 degrees ahead of it at a stable Lagrange point. It was discovered in 1980.

Helike (moon)

Helike, also designated Jupiter XLV and provisionally S/2003 J 6, is a small retrograde irregular moon of Jupiter in the Ananke group, discovered in 2003.

Heliotail

The heliotail is the trailing region of the heliosphere, extending opposite the Sun's motion through the galaxy like a wake, first imaged by NASA's IBEX mission in 2013.

Oct 21, 2026

Herse (Jupiter L, S/2003 J 17) is a small retrograde irregular moon of Jupiter in the Carme group, discovered February 27, 2003 by Gladman, Kavelaars, Petit, and Allen; mean radius about 1 km, orbital period over 734 days.

Hiʻiaka (moon)

Hiʻiaka is the outer and larger of Haumea's two known moons, discovered in 2005 and named for the Hawaiian goddess of Hawaii island and hula dancers.

Oct 21, 2026

Himalia

Himalia, Jupiter VI, is the fifth largest moon of Jupiter, discovered by Charles Perrine in 1904; a dark, irregular body about 85 km in radius.

Hippocamp (moon)

Hippocamp, also designated S/2004 N 1 and Neptune XIV, is the smallest known inner moon of Neptune, a roughly 34-kilometer-wide body discovered by Mark Showalter in 2013.

Hoba meteorite

The Hoba meteorite is the largest known intact meteorite on Earth, an iron of about 60 metric tons lying where it fell near Grootfontein in northern Namibia.

Iapetus (moon)

Iapetus is the third-largest moon of Saturn, a cratered icy body with one hemisphere dark and one bright and an equatorial ridge up to 20 km high.

Ice giant

An ice giant is a giant planet composed mainly of water, ammonia, and methane rather than hydrogen and helium; Uranus and Neptune are the Solar System's two members.

Ilmarë (moon)

Ilmarë is the large satellite of the trans-Neptunian object Varda, discovered in 2009 in Hubble images and named in 2014 after a handmaid of Varda in Tolkien's mythology.

Impact event

An impact event, also called an Earth impact, is a collision between an asteroid or comet and Earth or another body, leaving a crater or causing an airburst.

Inner main belt

The inner main belt is the innermost zone of the main asteroid belt, between about 2.05 and 2.5 au, dominated by Vesta and a major source of meteorites.

Internal structure of Jupiter

The internal structure of Jupiter is the layered arrangement of hydrogen, helium, and heavier elements inside the largest planet, inferred indirectly from gravity and magnetic-field measurements.