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The solar system is believed to have formed roughly 4.568 billion years ago.
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While the inner planets are rocky planets, the outer planets are referred to as gas giants.
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Inner planets have a mostly iron core, surrounded by a mantle.
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The surface of the inner planets is characterized by volcanoes, canyons, craters, and mountains, and earth is the only one known to have water on its surface. Earth and Mars both have polar ice caps that are permanent (unless global warming continues).
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Ceres is a dwarf planet that is believed to have surface features and a rocky core, making it very similar to the inner planets.
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Mercury is the closest inner planet to the Sun, followed by Venus, Earth, and Mars.
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The inner planets have no rings circling them and zero to a few moons. Earth has one moon, and Mars has two. Mercury and Venus do not have any moons.
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Mars, Earth and Venus have atmospheres that are able to generate weather.
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All of the inner planets have impact craters from the collision of space objects, and volcanoes and other tectonic surface features.
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Even though Mercury is the closest planet to the Sun, Venus is the hottest of all planets in the solar system because of its thick atmosphere. The atmosphere's thick, toxic environment enables it to trap the heat.
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The thick atmosphere also protects Venus from meteors as they tend to disintegrate before they can pass through to the planet's surface. This means that Venus has fewer crater impacts than on the other inner planets.
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Mercury has a thin atmosphere. It is the solar system's smallest planet. One day on Mercury is equal to 176 days on Earth.
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Earth is the densest planet, with Mercury coming in second place.
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Mercury is so close to the Sun that it can't be viewed by the Hubble Space Telescope as it would damage the telescope because of the brightness.
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Earth is the largest inner planet with a diameter of 3,963 miles, followed by Venus with a diameter of 3,760 miles. Third is Mars with a diameter of 2,110 miles, and Mercury is fourth with a diameter of 1,516 miles.
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The inner planets rotate slower than the outer planets, which makes them more round at their poles. The inner planets are also denser than the outer planets.
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