A tour of the planets

The solar system has eight planets. The four inner planets, Mercury, Venus, Earth and Mars, are small and rocky, with solid surfaces. The four outer planets are giants: Jupiter and Saturn are gas giants made mostly of hydrogen and helium, while Uranus and Neptune are ice giants rich in water, ammonia and methane.

This split reflects how the solar system formed about 4.6 billion years ago. Close to the young sun it was too hot for ices to survive, so only rock and metal clumped together. Farther out, ices were plentiful, allowing larger cores to grow and gather huge atmospheres of gas.

Mercury is the smallest planet and the closest to the sun. With almost no atmosphere to hold heat, its surface swings from about 430 degrees Celsius in daylight to around minus 180 degrees at night. It is heavily cratered, like our Moon.

Venus is nearly Earth's size but utterly hostile. Its thick carbon dioxide atmosphere creates a runaway greenhouse effect, making it the hottest planet, with surface temperatures around 465 degrees Celsius, hot enough to melt lead. Surface pressure is about 90 times Earth's, and clouds of sulphuric acid hide the surface. Venus also rotates backward, and a day there is longer than its year.

Earth is the only planet known to have liquid water oceans on its surface and life. Its magnetic field shields the surface from much harmful radiation, and its large Moon helps stabilise the tilt of its axis.

Mars is about half Earth's diameter, with a thin carbon dioxide atmosphere and a cold, dusty red surface coloured by iron oxide. It has the tallest volcano in the solar system, Olympus Mons, and a canyon system, Valles Marineris, that would stretch across the United States. Dried riverbeds, minerals formed in water and ice at the poles show Mars was once wetter, making it a prime target in the search for past life.

Jupiter is the largest planet, more massive than all the other planets combined. Its striped clouds are bands of wind, and its Great Red Spot is a storm larger than Earth that has been observed for well over a century. Jupiter has dozens of moons, including Ganymede, the largest moon in the solar system, and Europa, which likely hides a salty ocean beneath its icy crust.

Saturn is famous for its rings, made of countless pieces of ice and rock ranging from dust to boulders. It is so low in density that it would float in a large enough bathtub of water. Its moon Titan has a thick atmosphere and lakes of liquid methane and ethane, and its moon Enceladus sprays jets of water from an underground ocean.

• Uranus was the first planet discovered with a telescope, by William Herschel in 1781.
• Uranus is tipped on its side, with an axial tilt of about 98 degrees, probably due to an ancient collision, so its poles take turns facing the sun.
• Neptune was located in 1846 after mathematicians predicted its position from unexplained wobbles in the orbit of Uranus.
• Neptune has the fastest winds measured in the solar system, reaching over 2,000 kilometres per hour.
• Both planets have faint rings and appear blue because methane in their atmospheres absorbs red light.

Pluto was discovered in 1930 and counted as the ninth planet for 76 years. Then astronomers found other icy bodies beyond Neptune, in a region called the Kuiper Belt, including Eris, which is almost the same size as Pluto.

In 2006 the International Astronomical Union defined a planet as a body that orbits the sun, is round because of its own gravity, and has cleared its orbit of other material. Pluto fails the third test, so it was reclassified as a dwarf planet. The New Horizons spacecraft flew past it in 2015 and revealed a surprisingly active world with nitrogen ice plains and mountains of water ice.