Answer: Ceres

Although Pluto might be the most famous of the dwarf planets in our solar system—especially after the 2006 classification controversy that demoted it from “planet” to “dwarf planet”—it wasn’t the first one to be discovered.

The earliest observation of a dwarf planet dates to 1801 when it was discovered on New Year’s Day by Italian mathematician and astronomer Giuseppe Piazzi. Piazzi observed Ceres a total of 24 times, with the final time being February 11, 1801 when illness interrupted his observations. Based on those observations, he was unable to clearly determine if it was a planet or comet and opted for a more conservative estimate that it was a comet passing through our solar system. Later that same year, Carl Friedrich Gauss developed an efficient method of orbit calculation, which helped determine that it was in fact a dwarf planet.

In addition to being the first dwarf planet discovered in our solar system, Ceres has the distinction of being the smallest, which makes its place as first-discovered even more unique. The next dwarf planet, Pluto, wasn’t discovered until 1930. Nearly eighty years passed before any new dwarf planets were discovered, but between 2004 and 2005, three were added to the roster: Eris, Haumea, and Makemake. Why the long period between discoveries? Although scientists estimate there are dozens of dwarf planets in our solar system, their tiny size (Ceres, for example, is approximately 1.28 percent the size of the Moon) and enormous orbits (Eris takes nearly 560 years to orbit the Sun) make them particularly difficult to pin down and observe.