- There are eight planets in our solar system. In order from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
- They range in size from Mercury at about one-third the size of the Earth, to Jupiter which is about eleven Earths across.
- The word “planet” comes from ancient Greek astronomers, who called them asteres planetai, which means “wandering stars”. This is because over time, planets appear to move across the sky relative to the background stars due to their orbital motion around the Sun.
- The planets all orbit the Sun in the same direction (“prograde” – which is clockwise when viewed from the north celestial pole).
- This is no coincidence, as they formed from the same spinning cloud of gas and dust as the Sun, around 4.6 billion years ago.
- While Mercury, Venus, Mars, Jupiter and Saturn are all bright enough to see with the unaided eye, Uranus and Neptune will require binoculars or a telescope – though these will also allow you to see features like the clouds on Jupiter, and the rings of Saturn.
The four innermost planets (Mercury, Venus, Earth and Mars) are known as “Terrestrial” planets, which means “Earth-like”. They have metal cores that are made from iron and nickel, while their mantle and crust are composed of silicates (rocky material). This layering is known as differentiation and occurred during the formation of the solar system, when the heavier materials sank towards the middle of the molten proto-planets, while lighter materials floated towards the surface.
For more information, click on the terrestrial planet below:
The outer four planets are very different to those huddled closer to the Sun. Jupiter, Saturn, Uranus and Neptune are all much larger and are dominated by their thick atmospheres. These planets formed past what is known as the frost line, which is the distance from the Sun where many volatile compounds freeze, becoming a solid. This meant that there was much more solid material available for the growing outer planets to accumulate, increasing their mass considerably and allowing their stronger gravity to sweep up and hold onto large amounts of hydrogen and helium gas that were present in the early solar system.
While all four outer planets are often referred to as gas giants, Uranus and Neptune‘s distance from the Sun and cold temperature allows them to maintain a larger proportion of volatile compounds in their atmosphere, leading to their other name – ice giants.
For more information on the giant planets, click below:
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