Planets and moons
Planets A-Z
Adams, John Couch (1819–1892) British astronomer who predicted the existence and position of Neptune. Without knowing it, Adams made his predictions at almost exactly the same time as Le Verrier, who is today credited with Neptune's discovery.
Ariel The brightest of Uranus’s moons. Ariel measures on average 1160 km (721 miles) across. The scarcity of craters on Ariel shows that the surface is newer than those of most moons. Volcanic eruptions and movements of the moon’s crust may have erased them.
Astronaut Someone who is trained to travel into space in a spacecraft.
Axis An imaginary straight line that runs through the centre of a planet from pole to pole.
Beta Regio A major upland region on Venus.
Callisto Jupiter’s second largest moon. It measures on average 4800 km (2980 miles) across. For its size, Callisto has more craters than any other planet or moon in the Solar System.
Caloris Basin The largest crater on Mercury. It measures 1500 km (932 miles) across and is one of the largest craters in the Solar System.
Cassini Division The largest of the gaps between Saturn’s rings. The gap is about 4800 km (3000 miles) wide. Gaps between rings are caused by the gravitational pull of the planet’s moons.
Cassini-Huygens Mission A project that sent a spacecraft to study Saturn and its rings and moons. The Huygens lander landed on Titan in 2005. The mission came to an end in 2017 when the Cassini probe was steered deliberately into Saturn.
Cassini, Giovanni Domenico (1625–1712) Italian astronomer who first observed four of Saturn’s moons and also discovered the largest gap between Saturn’s rings.
Celestial body Any natural object located outside of the Earth’s atmosphere.
Charon Pluto’s largest moon. It measures about 1200 km (745 miles) across.
Core The innermost part of a planet, moon or star.
Crater A saucer- or bowl-shaped feature found on the surface of many moons and asteroids, and some planets. Craters are formed either by the impact of comets, asteroids or meteoroids crashing to ground from space, or by volcanic eruptions.
Cryovolcano A type of volcano that erupts volatiles (chemicals with low boiling points such as water, ammonia or methane) instead of molten rock, from beneath the surface. It is often called an ice volcano (some types are called geysers). After eruption, the liquids usually freeze in the very low surface temperatures. Cryovolcanoes may form on icy moons and dwarf planets such as Pluto, Titan, Triton, Enceladus and Ceres.
Deimos The smaller of Mars’s two tiny moons. It measures 16 km (10 miles) across at its widest. Astronomers believe that it may once have been an asteroid.
Density A measure of how compact something is. An object is denser than another if its atoms are larger or more closely packed together.
Dust storm A storm created when strong winds lift up loose dust and blow it and blow it through the atmosphere as clouds. Dust storms on Mars can sometimes cover the entire planet.
Dwarf planet A relatively large, round object that orbits the Sun. The orbits of dwarf planets are littered with rubble. True planets have sufficient mass, and therefore gravity, to sweep clear their orbits of rubble.
Eclipse The passage of one celestial body in front of another, totally or partially hiding it from an observer. For example, a solar eclipse or lunar eclipse.
Equator An imaginary line around a planet’s middle, halfway between its north and south poles.
Exoplanet Sometimes called an extrasolar planet, a planet outside the Solar System that orbits another star. Since the first exoplanet was detected in 1995, more than 6200, belonging to more than 4700 different solar systems, have been discovered. A number of exoplanets may host life-forms, but none has been discovered so far.
Flyby The flight of a space probe very close to a planet or moon, from where it can obtain detailed images.
Galileo Galilei (1564–1642) Italian astronomer who was the first person to use a telescope for astronomy. Galileo made many important observations and discoveries, including the discovery of Jupiter’s moons, now sometimes known as the Galilean Moons. By observing that Venus, like the Moon, had phases, he confirmed that it must orbit the Sun.
Galileo mission A mission that sent a space probe to orbit Jupiter in 1989. It was the first spacecraft to orbit the planet.
Galle, Johann (1812–1910) German astronomer who first observed Neptune and recognize it as a planet, following calculations made by Urbain Le Verrier in 1846.
Ganymede The largest of Jupiter’s moons. Bigger than the planet Mercury, it measures 5268 km (3272 miles) across on average, and is the largest moon in the Solar System. Ganymede has an icy surface with dark plains covered in grooved patterns and craters.
Gas giants The two largest planets in the Solar System: Jupiter and Saturn. They mostly consist of gases with no solid surface.
Geocentric model A description of the Universe that has Earth at the centre, with the Sun, Moon, stars and planets all orbiting around it. Many astronomers in ancient civilizations, such as Aristotle and Ptolemy, believed the geocentric model correctly described the heavens. It was gradually replaced by the heliocentric model (in which the Earth and other planets orbit the Sun) of Copernicus, Galileo and Kepler.
Gravity The force that attracts all objects to each other. The larger an object’s mass, the greater its gravitational pull. The greater the distance between objects, the smaller the force of gravity between them. Gravity is the force that keeps the planets in orbit around the Sun.
Great Dark Spot A huge storm system that was photographed on Neptune in 1989. Winds in the spot were recorded to be moving at 2400 km/h (1490 mph)—the fastest winds ever recorded in the Solar System. In 1994, the spot was discovered to have vanished. Other spots have since been observed.
Great Red Spot A giant storm in Jupiter’s atmosphere. The storm has been raging for at least 300 years.
Great White Spot A type of large storm that occurs in Saturn’s atmosphere about every 30 years.
Heliocentric Centred on the Sun.
Herschel, William (1738–1822) An amateur British astronomer who discovered Uranus in 1781. He was also the first astronomer to realize that the Solar System is, itself, travelling through space.
Huygens, Christiaan (1629–1695) Dutch astronomer who discovered the moon Titan and spent much of his career studying the rings of Saturn.
Ice giant The class of planet, including Uranus and Neptune, used to distinguish them from the gas giants Jupiter and Saturn. Surrounding their cores are oceans of water mixed with methane and ammonia (described by scientists as "ices"). Their thick atmospheres are composed mainly of hydrogen, with some helium and methane.
InSight The Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is a lander designed to study the the size and composition of the interior of Mars. Successfully landed at Elysium Planitia on the Martian surface in 2018, InSight was operational until December 2022 when the mission ended.
Io Jupiter’s third largest moon. It measures 3643 km (2263 miles) across on average. Io is covered with active volcanoes. It is the most volcanically active place in the Solar System.
Iron oxide Known in one form as rust, iron oxide is a reddish-brown chemical that gives Mars its distinctive colour.
Ishtar Terra A region of mountains on Venus that covers an area somewhat larger than Australia. The highest of these mountains is one-quarter taller than Mount Everest on Earth.
Juno A US space probe launched in 2011 to investigate Jupiter. In orbit around the planet since 2016, Juno is measuring Jupiter's composition, gravity and magnetic field. It is also searching for clues about how the planet was formed, whether it has a rocky core and how much water it has in its atmosphere.
Jupiter The fifth planet from the Sun. It is the largest planet in the Solar System. A gas giant, it is more massive than all the other planets combined.
Kuiper Belt A disc-shaped zone in the outer Solar System, extending outwards from the orbit of Neptune, at a distance of 30 AU, to approximately 50 AU from the Sun. It consists mainly of small objects, remnants from when the Solar System formed. Most are composed largely of frozen volatiles (known as "ices"), such as methane, ammonia and water. The Kuiper Belt is home to three dwarf planets: Pluto, Haumea and Makemake.
Lander A spacecraft designed to land on a planet, moon, asteroid or comet in order to gather information from its surface.
Lava Hot molten, or melted, rock.
Le Verrier, Urbain (1811–1877) French mathematician who calculated the precise location of Neptune, independently of John Couch Adams.
Light year The distance a ray of light travels in one year in a vacuum. One light year is approximately 9.46 million million km (5.88 million million miles).
Magellan A US space probe sent to Venus in 1989. The probe used radar to build up pictures of the planet. Radar measures the distances to objects by sending out radio waves and receiving their “echoes”.
Mantle The rocky layer that lies between the crust and core of a planet.
Mariner Programme A US project that sent space probes to observe Mercury, Venus and Mars between 1964 and 1973.
Mars Orbiter Mission A space probe, also known as Mangalyaan, launched by the Indian Space Research Organisation that successfully entered Mars in 2014. It carried five scientific instruments as payload. Communications with the probe were lost in October 2022.
Mars Science Laboratory mission The landing of a rover named Curiosity on Mars in 2012, with the intention of exploring the Gale Crater and assess whether the area could once have been inhabited by microbial life. Curiosity is also designed to investigate Mars' climate and geology. As of May 2025, the rover is still operational.
Martian polar caps Two large areas of frozen carbon dioxide and water that cover the land at Mars’s north and south poles.
Mass A measure of the amount of matter an object contains.
Maxwell Montes A mountain range on Venus. It contains the highest summit on the planet.
Mercury The nearest planet to the Sun, and the smallest true planet in the Solar System. Mercury is a small planet covered with craters and lava plains.
Messenger A US space probe launched in 2004 to observe Mercury, sending back detailed images between 2011 and 2015.
Methane A chemical found in, for example, the atmospheres and layers of ice and liquid on Neptune and Uranus. Methane is a compound of carbon and hydrogen, and is the main constituent of natural gas.
Miranda The smallest of Uranus’s major moons, measuring on average 470 km (292 miles) across. Miranda has a jumbled-up surface, covered with deep grooves, canyons and mountain ranges. Some astronomers think the unusual surface was formed by the moon being blasted apart and reassembled again.
Moon A natural object that orbits a planet.
Nereid One of Neptune’s moons. It measures on average 340 km (211 miles) across and has a very unusual orbit. It takes it as close as 1.4 million km (860,000 miles) from Neptune and then as far as 9.6 million km (6 million miles) away from it.
New Horizons US space probe which became, on 14th July 2015, the first spacecraft to reach Pluto. Launched in 2006, it took detailed images and recorded measurements of the the dwarf planet as it zoomed past just 12,500 kilometres (7800 miles) from Pluto's surface. On 1st January 2019, New Horizons passed about 3500 kilometres (2000 miles) from Arrokoth, a tiny, potato-shaped world lying within what is known as the Kuiper Belt.
Oberon The second largest of Uranus’s moons. It measures 1522 km (945 miles) across on average and is the farthest major moon from the planet. It is covered in deep craters.
Pathfinder Probe A US space probe that was sent to Mars in 2007. The probe dispatched the small Sojourner Rover, which was the first wheeled vehicle to travel across the Martian surface.
Perseverance A rover designed and built to explore Jezero Crater on Mars for NASA's Mars 2020 mission. As of May 2025 it is still operational. The central aim of its mission, which began with a successful landing on Mars in February 2021, is to determine whether life once existed on Mars. Perseverance carried on board a mini-helicopter, named Ingenuity, which made the first powered flight ever to take place on another world when it took to the Martian skies on 19th April 2021.
Phobos The larger of Mars’s two tiny moons. It measures 26 km (16 miles) across at its widest. Astronomers believe that it may once have been an asteroid. Phobos will probably collide with Mars in about 50 million years.
Pioneer missions A series of US missions that sent space probes to Jupiter, Saturn and Venus in the 1970s. Pioneer 10 later became the first man-made object to leave the boundaries of the Solar System.
Pioneer Venus Project A US space probe that was launched in 1978 to carry out experiments on Venus’s atmosphere.
Planetesimal A large lump of rock that orbited the Sun during the formation of the Solar System. Some planetesimals collided with each other, eventually building up to form the eight planets of the Solar System, together with other planets and worlds, many of which have since been destroyed.
Pluto Once described as the ninth planet, Pluto is now classified as a dwarf planet. It measures 2274 km (1412 miles) across on average and is probably the largest object in the Kuiper Belt, a region in the Solar System consisting of icy objects.
Pressure The amount of force per unit area, such as pounds per square inch or kilograms per square metre.
Proteus The second largest of Neptune’s moons. It measures about 400 kilometres (250 miles) across. It reflects very little light, so it appears very dark.
Revolve To move spin in a circle (or ellipse) around a central point or another object. For example, the Earth revolves around the Sun once per year.
Ringlet A narrow planetary ring. Saturn’s rings are made up of hundreds or thousands of ringlets.
Rotate To turn around an axis. For example, the Earth rotates on its axis once per day.
Satellite An object that orbits a larger object. The Moon is a natural satellite. Artificial satellites—normally referred to simply as "satellites"—are man-made objects that have been launched into orbit. They can be used for communications, navigation and weather forecasting among other things.
Scooter A giant, fast-moving, short-lived storm on the planet Neptune.
Shepherd moon (satellite) A moon that orbits a planet close to its rings. Shepherd moons exert a gravitational pull on a planet’s rings, holding them together in a narrow band. Uranus’s moons Ophelia and Cordelia are shepherd moons.
Solar System The Sun and all of the objects that orbit it. The eight planets, their moons, dwarf planets, asteroids, comets and meteoroids, and vast amounts of gas and dust together make up the Solar System. The Sun’s mass gives it the gravitational pull that keeps these objects in orbit around it. The planets all orbit the Sun in an anticlockwise direction (when viewed from above its north pole), following elliptical (oval) paths, rather than circular ones.
Space probe An unmanned spacecraft from Earth. Some space probes have passed close to, entered orbit around, or landed on other planets and moons.
Sulphuric acid A highly corrosive acid. Venus is covered with thick clouds containing sulphuric acid.
Telescope An instrument that makes distant objects appear closer.
Tianwen-1 An interplanetary mission launched by the China National Space Administration (CNSA) in which a space probe was sent to Mars, arriving in 2021. It included an orbiter, a lander and the Zhurong rover.
Titan Saturn’s largest moon. It measures on average 5150 km (3200 miles) across, and is the second largest moon in the Solar System. Titan is the only moon in the Solar System known to have a thick atmosphere.
Titania The largest of Uranus’s moons, made up of ice and rock.
Tombaugh, Clyde (1906–1997) American astronomer who discovered Pluto in 1930.
Triton The largest of Neptune’s moons and the only major moon in the Solar System to orbit its “parent planet” in the opposite direction to the planet’s rotation. At -235°C (-391°F), Triton is the coldest known object in the Solar System.
Umbriel One of Uranus’s five large moons. It is covered in large craters. One of these, the Wunda Crater, has a ring of bright material at its base, which may be carbon dioxide.
Uranus The seventh planet from the Sun. Uranus is a featureless blue globe of gas. It orbits the Sun at a very unusual angle. It is tilted at 98° from the vertical, meaning that it orbits the Sun almost on its side. Uranus’s upper layer of clouds is made up of methane. This layer absorbs some red light from the Sun, giving Uranus its blue colour.
Valles Marineris A huge canyon on Mars. At up to 200 km (125 miles) wide and 7 km (4 miles) deep it is, in places, more than four times deeper than the Grand Canyon on Earth and as long as the United States is wide.
Venera missions A series of Soviet space probes sent to observe Venus. In 1967, Venera 4 was the first space probe to enter and study another planet’s atmosphere. In 1970, Venera 7 became the first space probe to soft land on the surface of another planet and send information back to Earth.
Venus The second planet from the Sun. About the same size as the Earth, shrouded in clouds of sulphuric acid, beneath which it is covered with old, solidified lava plains and volcanoes. Because its cloud cover reflects light from the Sun, Venus is a very bright object in the night sky—the next brightest after the Moon.
Viking programme Two US space probes that were sent to Mars in 1975 to search for evidence of life, past or present.
Voyager programme A US space mission that launched space probes to Jupiter, Saturn, Uranus and Neptune in 1977. They sent back data and images of the planets and their moons and rings. The twin Voyager space probes carry audio-visual discs. Should aliens come across them, they would hear the sounds of whales, a baby crying and greetings in 55 languages.
Wrinkle ridges Enormous cliffs on the surface of Mercury that were created as the planet’s surface cooled down and shrank over time. Some of Mercury’s “wrinkle ridges” are more than 4 km (2.5 miles) high.
Consultant: Mike Goldsmith