Pluto was founded by ancient Greek astronomers thousands of years ago. So I'll go with Pluto the planet, despite it now being a dwarf planet.
Pluto, when it was discovered was a planet because it orbited the Sun. However, since then, more and more objects were found that also orbited the Sun. When a larger body than Pluto was found (Eris) it became clear that not all objects could be called a planet. So a definition was decided [See related question - what is a planet] and Pluto did not fulfil this criteria. Pluto is still Pluto - it's just it has changed from being a street to an avenue.
The planet that was an outer planet, but was more like an inner planet was Pluto because of its size. The planet Pluto is no longer considered to be a planet.
August 24, 2006 is the date when Pluto was consider no more a planet.
In 2006 Pluto was degraded to the status of dwarf planet(there is another dwarf planet, Eris, that is more massive than Pluto).
The closest planet to Pluto is Neptune, the closest object would be Chiron which is Pluto's "moon", although it is little more than a captured asteroid.
a dwarf planet
Pluto is not considered a planet any more.
Mercury, which is over twenty times the mass of (minor planet) Pluto.
A dwarf planet is a planet that is too small to be considered a real planet, and is more like a very large asteroid that orbits the Sun between Mars and Jupiter. Pluto was first considered a planet, but, then when scientists found a lot of similar size objects orbiting the Sun beyond Pluto, Pluto was then reclassified a dwarf planet. Pluto is a lot more like a large moon, than a small planet. It's orbit, for example, is not round like the other planets, but, gets nearer to the Sun, (at times) than Neptune (the eighth planet).
Earth is the only planet with one moon. Pluto use to be classified as a planet. It had only one discovered moon for a while, but astronomers have since found more moons belonging to Pluto.
This could apply to Pluto. However, Pluto is no longer considered a planet and is more icy than rocky.
If by this, you mean the planet next closest to the sun, the answer is Neptune! Of course, Pluto isn't a planet any more. Hope this helps!