Pluto is smaller than another object.
Pluto . In 2005 , Eris , a dwarf planet in the scattered disc which is 27 % more massive than Pluto , was discovered . Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . It is the largest known trans-Neptunian object by volume but is less massive than Eris . Like other Kuiper belt objects , Pluto is primarily made of ice and rock and is relatively small -- about one-sixth the mass of the Moon and one-third its volume .