Pluto. Computer artwork of the dwarf planet Pluto. Pluto, which used to be a planet, was demoted to the status of dwarf planet in August 2006. A dwarf planet is defined as an object large enough to be spherical due to its gravity, and with an independent orbit around the sun (ie. not a satellite of a planet). A true planet has to dominate its orbit around the Sun. Pluto fails because it is one of many Kuiper Belt objects in the outer solar system. Pluto is composed of rock and ice and has many craters on its surface.