PLUTO

Pluto, borrowing its name from the Roman God of the Underworld is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto now belongs to a distinct population known as the Kuiper belt and is also the largest among them.

In the 1840s, Urbain Le Verrier, using Newtonian Mechanics, predicted the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the end of the 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. After considerable delay in progress, on March 13, 1930, the Harvard College Observatory announced the discovery of Pluto.

Pluto is an unusual object in many respects. It is the smallest and the farthest from the Sun of all nine planets. Its orbit is unusually eccentric (as against the other eight planets) and inclined to the ecliptic. It is accompanied in this orbit by a moon, Charon, whose radius is half of Pluto’s. Its surface is among the coldest and the most contrasted of all planetary surfaces. Pluto is the only planet (or ex-planet) never visited by a space probe.

Like other members of the Kuiper belt, Pluto’s surface is composed of ice and rocks and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. Its highly eccentric and inclined orbit takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to occasionally come closer to the Sun than its neighbor, Neptune.

From its discovery in 1930 until 2006, Pluto was regarded as the Solar System's ninth planet. In the late 1970s, after the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many bodies 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 the 24^th of August in 2006, the International Astronomical Union (IAU) defined the meaning of a "planet" within the Solar System. This definition led to the omission of Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres. After the re-assortment, Pluto was added to the list of minor planets (many of which were discovered by the end of the 20^th century) and given the number 134340. A number of scientists continue to argue that Pluto should be classified as a planet.

Pluto and its largest moon, Charon, are sometimes considered together as a binary system because the barycentre of their orbits does not lie within either body. The IAU is yet to formalize a proper definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, both of which were discovered in 2005.

More Questions to Ponder (about the Planets of the Solar System)

• Why do planets rotate?
• How can we tell what the interiors of planets are like?
• How do planets capture satellites?
• How close do you have to come to a planet to be influenced by its gravity?