Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. However in 1992, after the dicovery of several similar sized objects in the Kuiper belt its status was up for debate.
It finally lost it status as a planet in 2006 when the International Astronomical Union (IAU) formally define the term "planet", this definition excluded Pluto and it was reclassified as a dwarf planet.
Lets take a look at it, we will enable the 80's mode and open the solar systems view. Notice that its orbit periodically takes it closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. It has an orbial period of 248 days.
Pluto is the largest but not most massive known dwarf planet. In 2005, Eris, a dwarf planet in the scattered disc was discovered and found to be 27% more massive than Pluto.
A TransNeptunian object (TNO) is defined to be any minor body in the Solar System that orbits the Sun at an average distance of more than Neptune. greater, lets visit the solar system view again, this time we will add in a few other TransNeptunian objects. We will add Eris, the most massive known object. 2007 OR10, the 4th largest TNO and the largest known body in the Solar System without a name and Haumea, a dwarf planet with a mass of about one-third that of Pluto.