Planets are objects revolving about a star or pulsar or black-hole. Planets can or cannot have moons. The only known planet known to host life is Earth. There are two main types of planets: rocky planets and gas giants. As the name explains, rocky planets are made up of rock. Equally explanatory is the name gas giants which are planets that are mainly made of gas but have molten cores. Some planets have moons which are smaller bodies of rocky material that orbit the planet.