300 articles
Perdita (moon)
Perdita is a small inner moon of Uranus, about 30 km across, discovered in 1999 in archived Voyager 2 images and confirmed by Hubble in 2003.
Phobos (Φόβος) (moon)
Phobos is the larger and innermost of Mars's two moons, an irregular, dark body discovered in 1877 that orbits closer to its planet than any other moon.
Phosphine in the atmosphere of Venus
Phosphine in the atmosphere of Venus is a claimed 2020 detection of the gas phosphine in Venus's clouds, led by Jane Greaves, which remains debated.
Photosphere
The photosphere is the visible surface of the Sun, the lowest layer of the solar atmosphere, which emits most of the Sun's light at about 5,500 °C.
Planet
A planet, sometimes called a major planet, is a body orbiting the Sun that is nearly round and has cleared its orbital neighborhood, a definition adopted in 2006.
Planet Nine
Planet Nine is a hypothetical planet of five to ten Earth masses proposed in 2016 to orbit the Sun far beyond Neptune and explain the clustering of distant Kuiper Belt object orbits.
Planetary core
A planetary core is the iron-rich central region of a differentiated planet, formed when dense metal sank from the silicate mantle during the body's earliest evolution.
Planetary differentiation
Planetary differentiation is the process by which gravity separates a planet's molten interior into layers of different density: a dense metal core, a silicate mantle, and a crust.
Planetary habitability
Planetary habitability is the study of whether a planetary body can sustain life, extrapolated from Earth and centered on the habitable zone where liquid water can exist.
Planets beyond Neptune
Planets beyond Neptune are hypothetical massive bodies, including the proposed Planet Nine, inferred from the clustered orbits of distant icy objects; only Pluto was ever found, in 1930.
Pluto
Pluto is a dwarf planet in the Kuiper Belt beyond Neptune, reclassified from ninth planet in 2006 and explored by NASA's New Horizons flyby of 2015.
Puck
Puck (S/1985 U1, Uranus XV) is the largest small inner moon of Uranus, about 150 km across, discovered December 30, 1985 by Voyager 2, orbiting between the epsilon ring and Miranda, named for Shakespeare's sprite.
Radiative zone
The radiative zone is the layer of the Sun's interior between the core and the convection zone, where energy travels outward as photons over more than 170,000 years.
Retrograde rotation
Retrograde rotation is the rotation of a body opposite to its orbital direction, seen in Venus, which spins backward every 243.690 days, and Uranus, tilted 97.77 degrees.
Rhea (moon)
Rhea, designated Saturn V, is the second largest moon of Saturn, an airless, heavily cratered body of ice and rock discovered by Giovanni Cassini on December 23, 1672.
Ring seismology
Ring seismology, also known as kronoseismology, is a method of planetary seismology that reads a planet's internal oscillations from waves they raise in its rings.
Ring system
A ring system is a disk of small particles orbiting a planet in a common plane. All four giant planets of the solar system have known rings.
Rings of Chariklo and Haumea
The rings of Chariklo and Haumea are the first ring systems found around Solar System bodies other than the giant planets, discovered in 2013 and 2017.
Rings of Jupiter
The Rings of Jupiter, or Jupiter's rings, are faint dusty rings discovered by Voyager 1 in 1979, fed by dust from the planet's small inner moons.
Rings of Neptune
The rings of Neptune are five faint, narrow rings and four dusty ring arcs encircling Neptune, confirmed by Voyager 2 images in 1989 and named Galle, Le Verrier, Lassell, Arago, and Adams.
Rings of Saturn
Saturn's rings, also called the Saturnian rings, are the largest and brightest planetary ring system in the Solar System, made of water ice and spanning about 70,000 km.
Rosalind (moon)
Rosalind is a small inner moon of Uranus, about 36 km in radius, discovered by the Voyager 2 team in January 1986 and named for a Shakespeare character.
Ryugu samples
The Ryugu samples are about 5.4 grams of pristine material from the carbonaceous asteroid Ryugu, returned by JAXA's Hayabusa2 spacecraft in December 2020, the first from a carbonaceous asteroid.
S-type asteroid
An S-type asteroid is a stony asteroid of silicate minerals and nickel-iron, making up about 17 percent of asteroids and commonest in the inner main belt.
S/2015 (136472) 1
S/2015 (136472) 1, nicknamed MK 2, is the only known moon of the dwarf planet Makemake, spotted in Hubble Space Telescope images and announced on April 26, 2016.
Samundra Vallis
Samundra Vallis is a 110-km-wide valley (vallis) on Venus at 24.1S, 347.1E, IAU-approved in 1994 and named after Samundra, an Indian river goddess, per the USGS gazetteer.
Sao (moon)
Sao, formally designated Neptune XI, is a small prograde irregular satellite of Neptune about 20 km across, discovered in 2002 and named for one of the Nereids, the sea nymphs of Greek mythology.
Sati Vallis
Sati Vallis is a 225 km valley on Venus at 3.2°N, 334.4°E, in the V-31 Sif Mons quadrangle, named for an Egyptian river goddess and adopted by the IAU in 1994.
Saturn
Saturn is the sixth planet from the Sun and a gas giant, known for its rings, its density below that of water, and 274 confirmed moons as of 2025.
Saturn's fuzzy core
Saturn's fuzzy core is a diffuse central region where rock and ice mix gradually into the surrounding hydrogen and helium, extending to about 60 percent of Saturn's radius.