Solar System

Co-orbital configuration

Co-orbital configuration: two or more bodies sharing a host's orbit in 1:1 resonance, in tadpole (Trojan), horseshoe, or quasi-satellite form; examples include Jupiter Trojans, Janus–Epimetheus, Cruithne, and Kamoʻoalewa.

Comet

A comet is a small solar system body of ice, rock, and dust that orbits the Sun and grows a coma and often one or more tails.

Comet Lovejoy

C/2011 W3 (Lovejoy) is a Kreutz sungrazing comet discovered by Australian amateur astronomer Terry Lovejoy in November 2011, which became the first sungrazer known to survive its perihelion passage.

Comet nucleus

A comet nucleus is the solid, icy central body of a comet, typically a few miles across, which sublimates near the Sun to feed the coma and tails.

Comet Swift–Tuttle

Comet Swift–Tuttle (109P) is a large periodic comet, discovered in 1862, that orbits the Sun every 133 years and sheds the dust behind the Perseid meteor shower each August.

Comet tail

A comet tail, also called a cometary tail, is a stream of gas and dust pushed away from a comet by the Sun's radiation and solar wind.

Contact binary (small Solar System body)

A contact binary is a small Solar System body made of two formerly separate lobes joined at a narrow neck, like the asteroids Itokawa and Arrokoth.

Convection zone

The convection zone is the Sun's outermost interior layer, where convection rather than radiation carries energy to the visible surface, spanning the outer third by radius.

Cordelia (moon)

Cordelia, also designated Uranus VI, is the innermost known moon of Uranus, a roughly 20 km satellite discovered by Voyager 2 in 1986 that shepherds the epsilon ring.

Core accretion model

The core accretion model is the leading theory of giant planet formation, in which a solid core of rock and ice grows until it triggers runaway gas accretion.

Core of Mars

The core of Mars is the planet's central region of liquid iron and nickel alloy, made unusually light-element-rich, and measured seismically by NASA's InSight lander between 2018 and 2022.

Coronal heating problem

The coronal heating problem is the unsolved question of why the Sun's outer atmosphere, the corona, reaches 1–2 million kelvin while the surface below is only about 6000 K.

Coronal hole

A coronal hole is a dark, cooler, less dense region of the Sun's corona with open magnetic field lines, the source of the fast solar wind.

Coronal loop

A coronal loop is an arch-shaped structure of hot plasma in the Sun's corona following closed magnetic field lines, best seen in X-ray and ultraviolet telescopes.

Coronal mass ejection

A coronal mass ejection (CME), also called a coronal transient, is a large eruption of magnetized plasma from the Sun's corona that drives the strongest geomagnetic storms at Earth.

Coronal rain

Coronal rain is cool, dense plasma condensing in the Sun's corona that falls along magnetic loops, formed by thermal instability and used to probe coronal heating.

Counter-Earth

Counter-Earth, or antichthon, is a hypothetical planet sharing Earth's orbit but always hidden behind the Sun, proposed by Philolaus in the fifth century BCE and later a common science fiction setting.

Crescent

A crescent is a curved shape with two narrow pointed ends, like the Moon when less than half its visible face is illuminated by the Sun.

Oct 21, 2026

Cressida

Cressida (Uranus IX, S/1986 U3) is a small inner moon of Uranus, 41 km in radius, discovered by Voyager 2 in January 1986 and named for Shakespeare's Troilus and Cressida; its 3:2 resonance with the eta ring allowed its mass to be measured.

Crucifixion darkness

The crucifixion darkness, also called the crucifixion eclipse, is the darkness from noon to three in the afternoon that three Gospels report at Jesus's crucifixion.

Cryovolcanism on Ceres

Cryovolcanism on Ceres is the eruption of cold, salt-rich brines onto the dwarf planet Ceres, which NASA's Dawn orbiter found shaped features like Ahuna Mons and Occator crater.

Cyllene (moon)

Cyllene, also designated Jupiter XLVIII and provisionally S/2003 J 13, is a small retrograde irregular moon of Jupiter about 2 km across, discovered in 2003 by Scott S. Sheppard's team.

Dalton Minimum

The Dalton Minimum was a period of reduced solar activity from 1790 to 1830, named after John Dalton, whose auroral observations first revealed it; it followed the Maunder minimum but was less severe.

Daphnis (moon)

Daphnis is a small moon of Saturn, about 8 kilometers across, discovered by Cassini in 2005. It orbits inside the Keeler Gap, sculpting its edges into waves.

Oct 21, 2026

Daura Chasma

Daura Chasma is a 729-km-long chasma, or extensional rift trough, on Venus, centered at 73.8°N, 53.8°E. IAU-approved in 1985, it is named for a Hausa great huntress of western Sudan.

Definition of planet

The definition of planet, also called the IAU definition of planet, is the 2006 International Astronomical Union resolution requiring a body to orbit the Sun, be nearly round, and clear its orbital neighborhood.

Deimos (moon)

Deimos, also known as Mars II, is the smaller and outer moon of Mars, discovered by Asaph Hall in 1877 and named for the Greek personification of flight.

Despina (moon)

Despina (Neptune V, provisional designation S/1989 N 3) is a small inner moon of Neptune about 148 kilometers across, discovered by Voyager 2 in 1989 as it orbits within the planet's faint ring system.

Destruction of the Moon

The destruction of the Moon is a hypothetical scenario and recurring science fiction premise in which an impact, tidal forces, or an unknown agent shatters Earth's satellite.