Pluto

Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet due to the discovery that it is one of several large bodies within the Kuiper belt. Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a sixth (1/6) the mass of the Earth's Moon and a third (1/3) its volume. It has an highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune. As of 2011, it is 32.1 AU (5,770,000,000 km) from the Sun. From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres.

Physical Characteristics
Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. The distance from Sun is 5,770,000,000 km, 47 times of Earth and 1/4 of Neptune. However, due to its distance and small size Pluto is not visible on naked eye and only visible with a telescope, around 30 cm (12 in) aperture being desirable. It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11&amp;#176;.

Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth, while its diameter is 2,306 kilometres, or roughly 2/3 of the Moon. Pluto's atmosphere complicates determining its true solid size within a certain margin. Observations by the Hubble Space Telescope place Pluto's density at between 1,800 and 2,100 kg/m3 (1.8 and 2.1 g/cm3), suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass. Because radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice.

Moons
Pluto has four known natural satellites: Charon, first identified in 1978 by astronomer James Christy; Nix and Hydra, both discovered in 2005, and S/2011 P 1 (provisional name, also known as P4), identified by the Hubble Space Telescope in 2011.