The main differences between inner and outer planets are primarily based on their respective locations in the solar system, as well as their physical characteristics and compositions.
Inner planets, also known as terrestrial planets, are those that are located closer to the Sun. In our solar system, the inner planets include Mercury, Venus, Earth, and Mars. These planets are characterized by their relatively smaller sizes, solid surfaces, and higher densities. They are primarily composed of rock and metal, and they have thinner or no atmospheres compared to the outer planets. The inner planets also have shorter orbital periods and faster rotation rates.
On the other hand, the outer planets, also known as gas giants or giant planets, are located farther from the Sun. In our solar system, the outer planets include Jupiter, Saturn, Uranus, and Neptune. These planets are much larger in size compared to the inner planets and have thick atmospheres composed mostly of hydrogen and helium, with traces of other compounds. The outer planets have lower densities and lack solid surfaces, as their atmospheres transition into dense fluid or slushy regions surrounding their cores. They have longer orbital periods and slower rotation rates compared to the inner planets.
Regarding the absence of a planet between Mars and Jupiter, this region is occupied by a large asteroid belt. The asteroid belt is a collection of numerous rocky and metallic objects called asteroids, which range in size from tiny particles to objects several hundred kilometers in diameter. It is widely believed that the gravitational influence of Jupiter, the largest planet in our solar system, prevented the formation of a planet in this region by perturbing the material and preventing it from accreting into a single planet. Instead, the gravitational interactions between Jupiter and the material in the asteroid belt kept it fragmented, leading to the formation of numerous smaller bodies such as asteroids.