The rocky planets

This is our solar system, with eight planets. These four planets closest to the Sun are smaller in size than the rest. They are Mercury, Venus, Earth, and Mars. They are often described as rocky or terrestrial planets, because of their solid surface made of rock and metals. All four have a metal-rich core in the centre, mostly made of iron.

The core is surrounded by a mantle of rock, covered by a solid rocky crust on the surface. The crust of terrestrial planets forms geological structures on the surface, such as volcanoes, canyons, or mountains. Rocky planets normally have a layer of gases around them — an atmosphere. The atmospheres of rocky planets are created after the planet has formed. Gases in the atmosphere come from volcanic eruptions or are brought by comets impacting the surface.

Despite all the similarities, each of the planets is different and has unique characteristics. The planet closest to the Sun is Mercury. Mercury is the smallest terrestrial planet in the solar system. Its diameter is about three times smaller than the diameter of Earth. Mercury’s core makes up almost two thirds of its volume.

Mercury’s atmosphere is very thin, so it can’t trap enough heat close to the planet. Temperatures swing between more than 400 degrees Celsius during the day and nearly minus 200 degrees Celsius at night. Mercury rotates around its axis so slowly, that it takes it longer to rotate around its own axis than to go around the Sun. One day on Mercury lasts two Mercurian years! The surface of Mercury has many craters, which make it look similar to the surface of the Earth’s moon.

The second planet from the Sun — Venus — is about the same size as Earth. Venus has the densest atmosphere of all terrestrial planets in the solar system. This dense atmosphere, made mostly of carbon dioxide, traps so much heat from the Sun that Venus is the hottest planet in the whole solar system. Venus rotates very slowly, even more slowly than Mercury. One day on Venus lasts about 243 Earth days.

Also, Venus rotates in the opposite direction to other terrestrial planets in the solar system — from East to West. On the surface of Venus there are mountains, valleys, and tens of thousands of volcanoes. Venus and Mercury are the only planets in the solar system that don’t have any moons. The third planet from the Sun is our Earth. Earth has one natural satellite - the Moon.

Earth is the only planet we know of that harbours life. One reason life can survive here is the density and composition of Earth's atmosphere, which allows temperature to remain relatively stable. Another reason is the presence of liquid water, which covers about two thirds of the planet’s surface. The last terrestrial planet in our solar system is Mars. Its atmosphere is very thin, mostly made of carbon dioxide and nitrogen.

This makes Mars very cold, the average temperature is about minus 60 degrees Celsius. Mars has two small moons, Phobos and Deimos. The surface of Mars is covered with iron oxide dust, which gives Mars its unique red colour. Mars has the largest mountains in the solar system, as well as the biggest canyon. We know this because Mars is one of the best studied planets in the solar system.

Even now there are robots on Mars’s surface taking pictures and collecting data. Rocky planets of our solar system have been studied for centuries. We know quite a lot about them, but there is still so much to discover.