The ninth planet in our solar system is PLUTO, which was discovered in 1930 by Clyde Tombaugh. It is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun. It is the only planet in the Solar System that has not been visited by a spacecraft. PLUTO is the farthest known planet from the Sun and is composed primarily of rock and ice.