The Rocky Planets

Pick up a stone from the ground. The Earth under your feet is made of rock like that, all the way down, with metal in its middle. Three of the other planets are made the same way.

The four closest to the Sun

The four planets closest to the Sun are Mercury, Venus, Earth and Mars. They are called the rocky planets: small balls of rock and metal, with solid ground you could stand on. But each one is a very different place.

The four rocky planets side by side at their real sizes compared with each other: small grey Mercury, cloudy yellow Venus almost as big as the Earth, the blue Earth, and red Mars about half as wide as the Earth

Mercury: the smallest

Mercury is the closest planet to the Sun, and the smallest. It is only a little bigger than our Moon, and it looks like our Moon too: grey, and covered in craters.

Mercury has almost no air. Air works like a blanket: it holds on to the Sun's heat at night. With no blanket, Mercury's days are hot enough to melt lead (about 430 degrees Celsius), and its nights are colder than anywhere on the Earth (about 180 degrees below zero).

Venus: the hottest

Venus is almost exactly the same size as the Earth. But you would not want to visit it.

Venus has very thick air, so thick that standing on its ground would feel like being almost a kilometre deep in the sea. That thick air traps the Sun's heat, the way a car left in the sunshine with its windows closed gets hotter and hotter inside. So Venus is even hotter than Mercury, about 465 degrees, day and night, although it is twice as far from the Sun.

Venus is covered in thick, bright clouds. They bounce back a lot of sunlight, which is why Venus shines so brightly in our sky.

Earth: just right

Our Earth is not too hot and not too cold. It has liquid water in its seas and rivers, and air we can breathe. It is the only place we know of where there is life.

Mars: the red planet

Mars is about half as wide as the Earth. In our sky it shines a little orange-red, and up close it is red all over: its ground is covered in dust that has rusted, like an old iron nail.

A robot car on the red, dusty ground of Mars, with rocks around it and a huge volcano far away

Mars has thin air and is very cold, usually far below freezing. It has the tallest volcano we know of, Olympus Mons, about two and a half times as tall as Mount Everest. Robot cars called rovers have been driving on Mars for years, taking photos and studying its rocks.

Moons

Mercury and Venus have no moons. The Earth has one, our Moon. Mars has two tiny ones, shaped like potatoes.

Try it at home

If the Earth were your orange (about 8 centimetres across), how big would the other rocky planets be?

Venus - an orange just a little smaller than yours.
Mars - a ping-pong ball.
Mercury - a large grape.

Put them in a row on a table and compare. Then put a grape next to the orange for our Moon: a little smaller than Mercury.

Words to know

rocky planets - Mercury, Venus, Earth and Mars: small planets made of rock and metal, with solid ground
rover - a robot car that drives on another world

Big idea

The four planets closest to the Sun - Mercury, Venus, Earth and Mars - are small and rocky. Mercury is airless, Venus is the hottest, Mars is red and cold, and the Earth is the only one we know with life.