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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.

Albedo   A measure of how strongly an object reflects light. A completely dark, non-reflective object has an albedo of 0, while an object that reflects all light that hits it has an albedo of 1. The Earth has an albedo of 0.36 and Venus has an albedo of 0.65.

Ammonia   One of the most common chemicals found on Neptune and Uranus.

Aphelion   The point in an object’s orbit when it is farthest away from the Sun.

Ariel   The brightest of Uranus’s moons. Ariel measures 1160 km (721 miles) across. The scarcity of craters on Ariel suggests that 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

Astronomical Unit (AU)   The average distance between the Earth and the Sun. One AU is 149,597,871 km (92,917,931 miles).

Atmosphere   The envelope of gases surrounding a planet, moon or star.

Axis   An imaginary straight line that runs through the centre of a planet from pole to pole.

Belts   The darker of the striped cloud bands that surround Jupiter.

Beta Regio   A major upland region on Venus.

Borealis Basin   A large flat dip in the surface of Mars. The Borealis Basin may have been created by an impact with an asteroid.

Callisto   Jupiter’s second largest moon. It measures 4800 km (2980 miles) across. For its size, it 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 Probe landed on Titan in 2005. Cassini is still orbiting Saturn and returning data.

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.

Conjunction   The point when two celestial bodies appear to be in line with each other in the sky, for example when the Moon appears very close to a planet.

Core   The innermost part of a planet, moon or star.

  A saucer-shaped feature found on the surface of many moons and asteroids, and some planets. Craters are formed by the impact of meteorites, lumps of rock crashing to ground from space.

Crust   The outer layer of a planet or moon.

Cryovolcano   An ice volcano that forms on cold moons and other objects far out in space. Triton is covered in cryovolcanoes. Rather than molten rock, they erupt nitrogen gas and snow.

Day   The time that it takes a moon or planet to rotate once on its axis.

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 into clouds. Dust storms on Mars can sometimes cover the entire planet.

Dwarf planet   A relatively large, round object that orbits the Sun. Dwarf planets share their orbit with other objects of a similar size to themselves.

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.

Enceladus   One of Saturn’s moons. It measures 498 km (309 miles) across. Enceladus’s surface is mostly made up of water ice.

Equator   An imaginary line around a planet’s middle, halfway between its north and south poles.

Europa   Jupiter’s fourth largest moon. It measures 3130 km (1944 miles) across. Europa has an icy surface, which may have a water ocean beneath it.

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. Galileo Galilei made many important observations and discoveries, including the discovery of Jupiter’s moons, known as the Galilean Moons. By observing that Venus, like the Moon, had phases, he confirmed that the planets all 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 discovered Neptune in 1846. He was the first person to view Neptune and recognize it as a planet.

Ganymede   The largest of Jupiter’s moons. Bigger than the planet Mercury, it measures 5268 km (3272 miles) across 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 four large planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. They mostly consist of gases with no solid surface.

Gravity   The force that attracts all objects to each other. The larger an object’s mass, or the greater its density, 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 and been replaced with another smaller storm.

Great Red Spot   A giant storm in Jupiter’s atmosphere. The storm has been raging for at least 300 years.

Great White Spot   A large storm that occurs in Saturn’s atmosphere about every 30 years.

Greenhouse effect   Warming caused when heat from the Sun becomes trapped by a planet’s atmosphere. The intense temperatures on the surface of Venus are the result of heat being trapped by the planet’s thick atmosphere.

Heliocentric   Centred around 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.

Io   Jupiter’s third largest moon. It measures 3643 km (2263 miles) across. Io is covered with active volcanoes and pools of molten rock. It is the most volcanically active place in the Solar System.

Iron oxide   Otherwise known 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 the size of Australia. The highest of these mountains is slightly taller than Mount Everest on Earth.

Jovian planets   Another name for the gas giants. The word “Jovian” means “belonging to Jupiter”.

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.

Lander   A spacecraft designed to land on a planet, moon or asteroid in order to gather information from its surface.

Lava   Hot molten, or melted, rock.

Le Verrier, Urbain   (1811–1877) French astronomer who predicted the location and existence of Neptune, independently of John Couch Adams.

Light year   The distance a ray of light travels in one year. 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 detects objects by sending out radio waves and receiving their “echoes”.

Magnetic field   The region surrounding a magnet, an object which has two ends, called poles, and a force of attraction between them. The Sun and some planets, including Earth, have magnetic fields.

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   The fourth planet from the Sun, known as the Red Planet. Like Earth, Mars has volcanoes, mountains, canyons and polar ice caps.

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.

Methane   A chemical found in layers of ice and liquid on Neptune and Uranus.

Mimas   One of Saturn’s moons. Mimas has an enormous crater that measures 130 km (80 miles) across—a diameter one third that of the whole moon.

Miranda   The smallest of Uranus’s major moons, measuring 470 km (292 miles) across. Miranda has a jumbled-up surface, covered with deep grooves, canyons and mountain ranges. Astronomers think the unusual surface was formed by the moon being blasted apart and reassembled again.

Moon   A natural object that orbits a planet.

Neptune   The eighth planet from the Sun. Neptune is a bright blue globe, streaked by fast-moving clouds and the occasional storm.

Nereid   One of Neptune’s moons. It measures 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.

Oberon   The second largest of Uranus’s moons. It measures 1522 km (945 miles) across and is the farthest major moon from the planet. It is covered in deep craters.

Olympus Mons   An enormous volcano on Mars. At 27 km (17 miles) high and 600 km (375 miles) wide, it is more than twice as high as Mount Everest on Earth, and its base covers an area of land larger than England. 

Orbit   The circular or elliptical (oval-shaped) path that one object takes around another object. For example, the Moon orbits the Earth, while the Earth orbits the Sun.

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.

Perihelion   The point in an object’s orbit when it is closest to the Sun.

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.

Planet   A large, round object that orbits the Sun. A planet does not share its orbit with any other large objects.

Planetesimal   A large lump of rock that orbited the Sun during the formation of the Solar System. The planetesimals collided with each other, eventually building up to form the eight planets of the Solar System.

Pluto   Previously thought to be the ninth planet, Pluto is now classified as a dwarf planet. It measures 2274 km (1412 miles) across and is the largest object in the Kuiper Belt, a region in the Solar System consisting of icy objects.

Pressure   The amount of weight, or force, bearing down on an object.

Proteus   The second largest of Neptune’s moons. It measures about 400 km (250 miles) across. It reflects very little light, so it appears very dark against the night sky.

Revolve   To spin in a circle around a central point or another object. For example, the Earth revolves on its axis once per day and revolves around the Sun once per year.

Ring   A band of dust, rocks, ice or other materials surrounding a planet at its equator. Saturn’s rings are probably fragments of a moon that was smashed by a passing comet.

Ringlet   A very fine planetary ring. Saturn’s rings are made up of hundreds or thousands of ringlets.

Satellite   An object that orbits a larger object. The Moon is a natural satellite. Artificial satellites are man-made objects that have been launched into orbit. They can be used for communications, navigation and weather forecasting.

Saturn   The sixth planet from the Sun. A gas giant, it is the second largest planet. Saturn’s rings made up of billions of blocks of ice, rock and dust, are broad and bright. They reach out more than 420,000 km (260,000 miles) from the planet.

Scooter   A giant, fast-moving storm on the planet Neptune.

Shepherd satellite   A moon that orbits a planet close to its rings. Shepherd satellites exert a gravitational pull on a planet’s rings, holding them together in a narrow band. Uranus’s moons Ophelia and Cordelia are shepherd satellites.

Solar System   The Sun and all of the objects that orbit (go round) it: the eight planets, their moons, dwarf planets, asteroids, comets and meteoroids, and vast amounts of gas and dust. The Sun’s mass, compared to the rest of the Solar System, gives it the gravitational pull that keeps these objects in orbit around it. The planets all orbit the Sun in an anticlockwise direction, following elliptical (oval) paths, rather than circular ones.

Space probe   An unmanned spacecraft guided from Earth. Some space probes have passed close to, entered orbit around, or landed on other planets and moons.

Sulphuric acid   A very corrosive acid that makes other substances disintegrate. Venus is covered with thick clouds containing sulphuric acid.

Telescope   An instrument that makes distant objects appear closer.

Terrestrial planets   Mercury, Venus, Earth and Mars, known as the “inner planets” because they are the planets closest to the Sun, are also called the “terrestrial” planets, meaning “Earth-like”. This is because, like the Earth, they are mostly made up of rock and metal.

Tharsis Bulge   A large mound on the Martian equator, where several massive volcanoes are found, including Arsia Mons, Ascraeus Mons, and Pavonis Mons.

Titan   Saturn’s largest moon. It measures 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 -238°C (-396°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. Scientists do not yet understand what this material is.

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 gas. This layer absorbs red light from the Sun, so that it only reflects blue light. It is this that gives Uranus its blue colour.

Valles Marineris   A huge canyon on Mars. At 200 km (125 miles) wide and 7 km (4 miles) deep it is 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 another planet’s atmosphere. In 1970, Venera 4 was the first space probe to land successfully on the surface of another planet and send information back to the 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 lava plains and volcanoes. Because its cloud cover reflects light from the Sun, Venus is a very bright object in the night sky.

Viking programme   Two US space probes that were sent to Mars in 1975 to search for evidence of life, past or present.

Volcano   An opening in a planet or moon’s surface through which hot, molten rock and gases spurt out.

Voyager programme   A US space mission that sent space probes to Jupiter, Saturn, Uranus and Neptune in the 1980s. They sent back data and images of the planets and their moons and rings. The space probe Voyager carries an audio-visual disc. Should aliens come across it, 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.

Year   The time that it takes an object to complete one full orbit of the Sun.

Zones   The lighter of the striped cloud bands that surround Jupiter.

ConsultantDave Hawksett


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