A planet is a celestial body generally orbiting a star or in rare cases a black or white hole. Planets can be as small as 5000 km to more than 100,000 km. They can be composed of rock, gas, metal, molten rock or something else. They have an atmosphere which may be breathable. Life exists on billions of planets in the universe. Some planets however do not orbit anything but float freely in space, these planets are called rogue planets or nomad planets. Some planets orbit two stars, this is called a binary star system, triple star systems are also found. Quadruple and more are considered rare.