"Dwarf planet" is a new category of astronomical object defined by the International Astronomical Union in 2006. A dwarf planet has sufficient mass to be rounded by its own gravity, but (unlike a planet) has not cleared its orbit of other matter.
The currently known dwarf planets are Ceres, Pluto, Eris, Makemake, and Haumea. The latter four are icy bodies beyond Neptune; Ceres is the largest object in the asteroid belt. Many more dwarf planets are expected to exist in the outer Solar System, with estimates of how many we will eventually discover ranging between 50 and 2,000. The large number is probably a factor in the decision to reclassify Pluto as a dwarf planet instead of a planet - you can't exactly expect school children to memorize a list of all 1,372 planets, after all.
A sixth dwarf planet, currently designated 2015 RR245 until a cool mythological name can be chosen, was discovered in 2015
Game and Story Use
- The first human exploration of one of the dwarf planets (perhaps Ceres or Pluto) might be a good SF campaign.
- A new dwarf planet far beyond Pluto might be discovered, carrying ancient ruins of an alien civilization.
- Star Trek, Star Wars, and most other sci-fi settings tend to kind of assume that each star system has half a dozen to a dozen planets - but if the number is actually hundreds or thousands, that's bound to have some implications. Sure most won't be habitable, but they're at least potential resource sites and worthy of some level of exploration.