The planets in the Solar System form two very different groups — inner and outer. The inner planets, often called terrestrial planets, are composed mainly of rock and metal, with solid surfaces, no rings and few satellites. The outer planets, called Jovian or Gas Giants, are much larger than their inner neighbours. They are composed primarily of hydrogen and helium, have very deep atmospheres, rings and lots of satellites.
Our solar system consists of many planets, one of which is Earth. The total number of planets is eight although there have been disagreements to this statement with some saying there are more than eight (the opponents of the theory that Pluto is not a planet). Whatever the case, when we talk about planets we divide them into two groups; inner planets and outer planets. This classification is relative to the planets’ position with respect to the Sun. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We shall now make clear which of these are inner planets and which are outer planets and what actually differentiates them.
Inner planets are those planets which are closest to the sun and include the first four planets (Mercury, Venus, Earth and Mars) in order of increasing distance from the Sun. Mercury is the closest, followed by Venus, Earth and then Mars. Outer planets are those which are further away from the Sun and include the next four planets in order of increasing distance from the Sun (Jupiter, Saturn, Uranus and Neptune), with Neptune being the furthest.
The inner planets are made up of rock and metal and are therefore solid. These planets move slowly as they are considered to be heavy. They have an average diameter of about 13000 km as they are small planets. On the other hand, the outer planets are said to be made of gases and they are not really solid. The gases which make them up are Hydrogen and Helium; huge balloons floating in the space are considered as giant gas planets by people and they have an average diameter of about 48000 km.
Furthermore, the inner planets are warmer than outer planets simply due to the fact that they are closer to the Sun. Outer planets are composed of lighter elements such as gases and inner planets are composed of heavy elements such as iron. Inner planets have fewer moons, small, silicate surface, nickel-iron core, higher density and rotate more slowly compared to outer planets. Outer planets have a greater number of moons, no solid part; rotate faster, have a lower density as well as rings in some cases (Jupiter and Saturn). Outer planets are significantly bigger than the inner planets as Jupiter is measured to be 88846 miles in diameter and Mercury is measured to be 3031 miles in diameter.
There is significant difference between the rotation and the orbit of the two types of planets. For example, for Jupiter it would take 9 hours and 55 minutes for a day to complete (or to complete one rotation) and on Venus it would take 234 hours for a day to complete. (The time period of a day is that compared to the standard 24 hour day on Earth.) The inner planets take lesser time to orbit the Sun whereas the planets which are far away need more time as they have to cover more ground. For example, Jupiter takes 164 Earth years to complete one orbit!