Inner and Outer Planets

by Abby Cessna on July 12, 2009

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Eight Planets

Eight Planets


The eight planets in our Solar System are divided into two groups – the inner planets and the outer planets. The inner planets are the first four planets closest to the Sun, which includes Mercury, Venus, Earth, and Mars. The outer planets are Jupiter, Saturn, Uranus, and Neptune. The inner and outer planets are separated by the asteroid belt, which is a region of space where thousands of asteroids can be found.

The inner planets and outer planets are characterized by different features. The 4 inner planets are called terrestrial planets because they have a solid surface and are similar to Earth. These planets are composed of heavy metal, such as iron and nickel and have few or no moons. Mercury, the smallest planet, has no moons and is comprised mostly of iron and nickel. It is one of the densest planets in the Solar System. Venus, known for its brightness, has a rocky surface similar to the Moon, which is hidden by its thick yellow atmosphere. Like Mercury, Venus has no moon. Earth is a rocky planet with a molten core, an atmosphere that allows life to flourish, and only one moon. The last inner planet, Mars, has two moons called Phobos and Deimos. It is a rocky planet with a red color caused by a high concentration of iron in the rocks that comprise the surface.

The outer planets, also called Jovian planets or gas giants, are gaseous with no solid surfaces and only liquid cores. The outer planets are so much larger than the inner planets that they comprise 99% of the mass of the celestial objects orbiting our Sun. All of the outer planets have rings, although Saturn’s are the most noticeable. Jupiter is distinctive for a number of reasons. In addition to being the largest and most massive planet, it also has the most moons – 63 discovered┬áso far. Jupiter’s rings are very faint and difficult to see. Saturn is best known for its distinctive ring system, which can seen from Earth by the naked eye. Uranus, which is the only planet to rotate on its size, has 27 known moons. Many satellites have names from mythology, but Uranus’ moons are named after characters from the works of Alexander Pope and Shakespeare. Some of the satellites are Titania, Oberon, and Umbriel. Neptune is the last outer planet and the final planet in the Solar System. With only 13 moons, it has the least number of satellites of any of the outer planets.

Universe Today has more articles on the planets including inner planets and outer planets.

If you are looking for more articles, check out the planets and inner and outer planets.

Astronomy Cast has episodes on all the planets, including Jupiter.

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