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THE INNER PLANETS The Inner Planets are Mercury, Venus, Earth and Mars. All four planets are very different from each other, but also have their similarities. Their main similarities are that they all have solid surfaces, are fairly similar in size (in comparison with the sizes of the Outer Planets) and are all quite close to the Sun. The picture below shows the orbits of the Inner Planets around the Sun, and their distances from the Sun and from each other, plus the orbit of Jupiter (the first of the Outer Planets), just about fitting onto the picture. This shows how much further away Jupiter is from the Inner Planets.
Each planet has its own characteristics, although none of them, apart from Earth, are known to be able to support life. Below is a short description of each planet. For a more detailed look at the planet, select on the planet's name (underlined in blue) or picture.
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Venus is the only planet in the Solar System to turn in a clockwise direction on its axis, and it does this very slowly, taking 243 Earth days to do so (so a day on Venus is 243 days long). This is even longer than a year on Venus, which is 225 Earth days long. Venus is covered in poisonous clouds containing sulphuric acid, and an atmosphere containing dangerous carbon monoxide. Venus' surface remained a mystery because of its thick cloud cover until 1990-1994 when special imaging equipment on the Magellan space craft managed to look through the clouds to reveal a volcanic surface. Temperatures on Venus reach up to 480°c because heat from the Sun cannot escape through the thick atmosphere, which acts like a greenhouse covering the entire planet.
Earth is the only planet in the Solar System known to contain life. It takes 23 hours and 56 minutes for a day to take place on Earth (the amount of time it takes the planet to turn completely on its axis) and 365 and a quarter days for it to complete one orbit around the Sun. Every four years, an extra day is added to the year to make up for the extra quarter of a day it takes for the planet to orbit the Sun. Earth is the only planet to contain water in its three forms, as a solid (ice), as a liquid (sea, rain, etc.) and as a gas (steam, clouds, atmosphere). It is still geologically living, meaning that volcanoes, earthquakes and weather still form and reform its surface. Its thin atmosphere is made up mainly of Nitrogen, giving the sky a blue colour, and Oxygen, placed into the atmosphere by the presence of life. Hundreds of years ago, people believed Earth to be the centre of the universe, although scientists later discovered the truth. Earth is the biggest of the Inner Planets, and has its own moon orbiting it.
Mars is a small red planet with a very thin atmosphere of Carbon Dioxide. It is believed that it once had flowing water, and would have appeared like Earth from the distance. Life has not been discovered on the planet, despite many missions aiming to find evidence of primitive forms of life. The planet has a rusty surface and a pink sky. It is covered in rocks and impact craters, but has seasons like Earth and water droplets can form frost, fog and clouds. The Solar System's biggest volcano, Olympus Mons, took place on Mars, millions of years ago when dinosaurs lived on Earth. Mars also has huge valleys, which people believe were created when volcanic eruptions tore up the ground. Mars takes 687 days to orbit the Sun (so a year is 687 days long on Mars), but a day on Mars is 24 hours and 37 minutes long, about forty minutes longer than a day on Earth. The planet has two small moons orbiting it.
After the Inner Planets is the Asteroid Belt, an area of rocks, the size of some of the smaller moons orbiting the planets. This belt is millions of kilometres wide, and separates the Inner Planets from the Outer Planets.
STATISTICS ABOUT THE INNER PLANETS:
AVERAGE DISTANCES FROM THE SUN (in kilometres):
DIAMETER AT EQUATOR (WIDTH) (in kilometres):
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