Dwarf planet
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.
List of slow rotators (minor planets)
The periods given in this list are sourced from the "Light Curve Data Base" (LCDB), which contains lightcurve data for more than 15,000 bodies. Most minor planets have rotation periods between 2 and 20 hours. , a group of approximately 650 bodies, typically measuring 1–20 kilometers in diameter, have periods of more than 100 hours or 4 days. Among the , there are currently 14 bodies with a period longer than 1000 hours. According to the Minor Planet Center, the sharp lower limit of approximately 2.2 hours is due to the fact that most smaller bodies are thought to be rubble piles – conglomerations of smaller pieces, loosely coalesced under the influence of gravity – that fly apart if the period is shorter than this limit. The few minor planets rotating faster than 2.2 hours, therefor, can not be merely held together by self-gravity, but must be formed of a contiguous solid.