There are 8 major planets in the solar system and they have a few things in common. In the order of increasing distance from the sun, these planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn and Neptune. They all directly orbit the Sun and they have mass that don’t go below 38% of Earth’s mass.

All these 8 planets are believed to have formed 4.6 billion years ago, in a big explosion, which is called “the big bang”. There are several other theories that assume the creation of the solar system or of the whole universe. It is believed that the solar system started as a giant swirling cloud of dust and gas. This cloud, also called the “Solar Nebula” moved rapidly and the denser particles accumulated in its center. The core became the sun. The rest of the cloud continued to revolve around the sun and solid rocky particles in the cloud bumped and joined together. They formed planetisimals, which later came together and formed these spherical bodies that we now call planets.

Thus, the planets also started as clouds of gas, dust and rocky particles spinning rapidly just like the solar nebula. The denser materials clumped together, forming the core. The lighter materials steered into the surface. The gravitational draw that comes from the core causes the cloud of dust to compress towards the center. This is the reason why all planets are perfect globes. Every planet has a core, which could consist of molten rock or solid and cold materials. Outside the core, is an outer layer or mantle. The Earth’s mantle continuously moves around the core and it acts like a big dynamo that produces magnetic force towards the center of the Earth. Then, there is the outer covering which is called the crust. For the Earth, the crust is rocky and solid.

The planets are divided into two groups. The 4 innermost planets, namely Mercury, Venus, Earth and Mars are the terrestrial or Earth-like planets. From the word terra which means Earth, these planets have solid, rocky crusts that envelope their fluid mantle. These planets have magnetic chemicals such as nickel and iron.

The second group is called the Jovian planets. It means Jupiter-like. These planets are the ice giants, Neptune and Uranus; and the gas giants, Jupiter and Saturn. It is believed that their position from the sun, being the outermost planets in the solar system, has affected their geological composition. Unlike the terrestrial planets, the Jovian planets have no solid crusts. Instead, gases such as hydrogen and helium are abundant in their crusts and these light elements diffuse into their atmospheres. They all have rocky cores. The gas giants have a mantle made of metallic hydrogen while the ice giants have mantles that consist of ammonia, water, methane and different kinds of ice.

Pluto, which was originally thought to be a planet, is now considered a dwarf or minor planet. It is after scientists learned that other objects that have almost the same mass as Pluto also revolve around the sun near Pluto’s orbit.

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