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comment by asdfoster

It's one of the biggest comets! Big enough that it's reached hydrostatic equilibrium and has become round, which is coincidentally the criteria required for it to be called a "dwarf planet" as well. Compositionally, it is indistinguishable from a comet, and formed in the same region as most comets (the Kuiper Belt - the same place that 67P was formed!). If it were moved closer to the sun, the ices on its surface would begin outgassing, just like a comet, however because of its larger mass and escape velocity, the gasses would be mostly bound to the surface instead of making a tail (at first, of course over time that atmosphere would escape and Pluto would lose enough mass that its atmosphere would begin to form a tail).

https://en.wikipedia.org/wiki/Pluto