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.
Smallest organisms
Frogs include the smallest vertebrates known. The smallest known frog species is "Paedophryne amauensis", with a snout-vent length reported as 7.7 mm, which occurs among leaf-litter in the tropical montane forests of New Guinea. Other very small frogs include "Brachycephalus didactylus" from Brazil (reported as 9.6-9.8 mm), several species of "Eleutherodactylus" such as "Eleutherodactylus iberia" (around 10mm) and "Eleutherodactylus limbatus" (8.5–12 mm) and "Eleutherodactylus orientalis" (12.5 mm) from Cuba, Gardiner's Frog "Sechellophryne gardineri" from the Seychelles (up to 11 mm), several species of "Stumpffia" such as "Stumpffia tridactyla"(8.6–12  mm) and "Stumpffia pygmaea"(10–12.5 mm；females:11 mm) and "Wakea madinika"(males:11–13 mm；females:mm 15–16 mm) from Madagascar. "Paedophryne swiftorum"(Body length 8.5 mm) is not included in the smallest vertebrates known with other nine species of frogs. The two species "Microhyla borneensis"(males:10.6–13 mm；females:16–19 mm) and "Arthroleptella rugosa"(males:11.9–14.1 mm；females:15.5mm) were once the smallest known frogs from the Old World. In general these extremely small frogs occur in tropical forest and montane environments. There is relatively little data on size variation among individuals, growth from metamorphosis to adulthood or size variation among populations in these species. Additional studies and the discovery of further minute frog species are likely to change the rank order of this list.