Pluto (minor-planet designation:134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodiesbeyond the orbit of Neptune. In 1930, it was the first objectdiscovered in the Kuiper belt, when it was declared the ninth planetfrom the Sun. Beginning in the 1990s, following the discovery ofseveral objects of similar size in the Kuiper belt including thedwarf planet Eris, and the scattered disc, Pluto's status as a planetwas questioned. In 2006 the International Astronomical Union (IAU)formally re-defined the term planet, reclassifying Pluto as a dwarfplanet.
Pluto is the ninth-largest andtenth-most-massive known object directly orbiting the Sun. It is thelargest known trans-Neptunian object by volume, but is less massivethan Eris. Like other Kuiper belt objects, Pluto is primarily made ofice and rock and is relatively small. Compared to Earth's Moon, Plutohas only one sixth the mass, and one third the volume.
Pluto has a moderately eccentric andinclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3billion kilometers; 2.8 to 4.6 billion miles) from the Sun. Lightfrom the Sun takes 5.5 hours to reach Pluto at its average distance(39.5 AU [5.91 billion km; 3.67 billion mi]). Pluto's eccentric orbitperiodically brings it closer to the Sun than Neptune, but a stableorbital resonance prevents them from colliding.
Pluto has five known moons. Charon thelargest, whose diameter is just over half that of Pluto, Styx, Nix,Kerberos, and Hydra. Pluto and Charon are sometimes considered abinary system because the barycenter of their orbits does not liewithin either body, and they are tidally locked.
The New Horizons became the firstspacecraft to perform a brief flyby of Pluto on July 14, 2015, makingdetailed measurements and observations of Pluto and its moons. InSeptember 2016, astronomers announced that the reddish-brown cap ofthe north pole of Charon is composed of tholins, organicmacro-molecules that may be ingredients for the emergence of life,produced from methane, nitrogen, and other gases released from theatmosphere of Pluto and transferred 19,000 km (12,000 mi) to theorbiting moon.
History
Discovery
In the 1840s, Urbain Le Verrier usedNewtonian mechanics to predict the position of the then-undiscoveredplanet Neptune after analyzing perturbations in the orbit of Uranus.Subsequent observations of Neptune in the late 19th century ledastronomers to speculate that Uranus's orbit was being disturbed byanother planet besides Neptune.
In 1906, Percival Lowell—a wealthyBostonian who had founded Lowell Observatory in Flagstaff, Arizona,in 1894—started an extensive project in search of a possible ninthplanet, which he termed "Planet X". By 1909, Lowelland William H. Pickering had suggested several possible celestialcoordinates for such a planet. Lowell and his observatory conductedhis search until his death in 1916, but to no avail. Unknown toLowell, his surveys had captured two faint images of Pluto on March19 and April 7, 1915, but they were not recognized for what theywere. There are fourteen other known precovery observations, with theearliest made by the Yerkes Observatory on August 20, 1909.
Percival's widow, Constance Lowell,entered into a ten-year legal battle with the Lowell Observatory overher husband's legacy, and the search for Planet X did not resumeuntil 1929. Vesto Melvin Slipher, the observatory director, gave thejob of locating Planet X to 23-year-old Clyde Tombaugh, who had justarrived at the observatory after Slipher had been impressed by asample of his astronomical drawings.
