A planet is an astronomical object that orbits around a star. Although all the planets of a star have the same origin, as they are born from the same interstellar cloud, their final composition may be very different.
In the solar system we basically find two types of planets. To discover what they are, we invite you to put your hand into the two holes on the table.
We start with the hole on the right. This solid, rocky surface is characteristic of the planets that are closest to the Sun: Mercury, Venus, the Earth and Mars, called inner or rocky planets. Their composition is due to the fact that during the formation of the solar system the heaviest chemical elements remained close to the Sun, which gave rise to denser, smaller, more arid, rocky planets. The ‘rara avis’ of the group is our home: the Earth, which meets a series of exceptional conditions that enable there to be liquid water on the surface, essential for the appearance of vegetable and animal life. Its geological and biological activity and wind and water erosion, mean that the Earth’s surface is constantly changing.
We now go on to the hole on the left. Can you feel anything? In fact, this is the gas that forms the planets that are furthest from the Sun, like Jupiter, Saturn, Uranus and Neptune, which are known as Jovian planets or gas giants. These planets have no solid surface and are made up, to a large extent, of Hydrogen and Helium, the most abundant elements in the universe. It is thought that the large amount of available matter and their remoteness from the Sun enabled them to grow much faster than the rocky planets, and create gigantic balls of gas.
Next, on the route you will be able to learn in detail about the surface of another heavenly body: the Moon and its relation with the Earth.