A planet is in the Solar System.
Mercury is the smallest and innermost planet in the Solar System . Solar System . The four smaller inner planets , Mercury , Venus , Earth and Mars , are terrestrial planets , being primarily composed of rock and metal . Venus is the second planet from the Sun , orbiting it every 224.7 Earth days . It has the longest rotation period ( 243 days ) of any planet in the Solar System and rotates in the opposite direction to most other planets .