Final answer:
Ceres is not an official dwarf planet.
Step-by-step explanation:
The correct answer is Ceres. Ceres is classified as a dwarf planet, so it is not an official dwarf planet. This information can be found in the text that mentions the five currently known dwarf planets: Eris, Haumea, Pluto, Ceres, and Makemake. The text also explains that Pluto is different from the other planets and is classified as a dwarf planet due to its small size and unique characteristics.