The planets of the Solar System revolve around the Sun, each with unique characteristics and orbit. They are divided into rocky planets (or inner) and jovian planets (or outer). The first (in order of proximity to the Sun we have Mercury, Venus, Earth and Mars) are relatively small and consist mainly of rocky material. The seconds (Jupiter, Saturn, Uranus and Neptune) are composed of light elements such as hydrogen and methane that wrap probably a solid center core. Between Mars and Jupiter (then in the area that separates the inner planets from the external ones) there is the asteroid belt composed of a myriad of rocky bodies of irregular shape and size of less than 1000 km (but most of them do not come to 100 km). As our Earth, all the planets of the Solar System have an elliptical orbit. The orbits are almost all on the same plane called ecliptic and, for that reason, if you happen to see more planets in the sky at the same time, they all seem aligned.