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.
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.
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.
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-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.
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.
Equator An imaginary line around a planet’s middle, halfway between its north and south poles.
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.
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.
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.
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”.
Lava Hot molten, or melted, rock.
Mantle The rocky layer that lies between the crust and core of a planet.
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.
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.
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.
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.
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.
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.
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.
Titania The largest of Uranus’s moons, made up of ice and rock.
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.
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.
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.
Zones The lighter of the striped cloud bands that surround Jupiter.
Consultant: Dave Hawksett
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