Mercury (planet)
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm, only slightly less than Earth's density of 5.515 g/cm. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm versus Earth's 4.4 g/cm.
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.