Pluto. From New Horizons’ Long Range Reconnaissance Imager (LORRI), 8 July 2015. True-color information added from the Ralph instrument, also aboard New Horizons.
|Date of discovery||1930|
|Name of discoverer||Clyde W. Tombaugh|
|Name origin||Greco-Roman god of wealth and the underworld|
|Order from primary||10|
|Semi-major axis||39.48168677 AU|
|Titius-Bode prediction||77.2 AU|
|Sidereal year||248.09 a|
|Synodic year||366.73 da (1.004 a)|
|Avg. orbital speed||4.666 km/s|
|Inclination||17.14175° to the ecliptic|
|Sidereal day||-6.387230 da|
|Rotational speed||47.18 km/h|
|Mass||1.305 * 1022 kg (0.218% earth)|
|Mean radius||1,185 km|
|Surface gravity||0.58 m/s² (0.0591 g)|
|Escape speed||1.2 km/s|
|Surface area||17,650,000 km² (3.460% earth)|
|Minimum temperature||33 K|
|Mean temperature||44 K|
|Maximum temperature||55 K|
|Number of moons||5 or more|
According to the International Astronomical Union, it no longer qualifies as a planet because it has not cleared its orbit of other objects. A 2015 flyby by U.S. spacecraft New Horizons found that Pluto has mountains that were formed relatively recently, suggesting active plate tectonics.
In 1930, Clyde W. Tombaugh at the Lowell Observatory surveyed the sky, looking for a planet beyond Neptune (called "Planet X") that other astronomers had predicted from calculations based on an erroneous value for Neptune's mass and its orbital patterns. Tombaugh knew nothing of the error, but found an object anyway; the object was soon named Pluto.
But even he realized that Pluto was not large enough to be the predicted Planet X. Astronomers continued to search in vain for it, until Voyager 2 made its flyby of Neptune, and the Jet Propulsion Laboratory scientists determined from this flyby that Neptune was significantly heavier than previously supposed. Current theory predicts no more planets other than the eight now known, but still allows for many other objects, both in the classic asteroid belt and in the Kuiper Belt, essentially a second belt of asteroids and comets.
The dwarf planet controversy
The discovery of Eris, a scatter-disk body 27% more massive than Pluto, caused a controversy concerning what does, and what does not, constitute a planet. If Pluto were to retain its historical designation of "planet," then Eris would also qualify. But Eris was held not to qualify because it was in a neighborhood with multiple other objects, a situation similar to those of similar bodies in the asteroid belt, namely Ceres and Eros.
In 2006, the International Astronomical Union passed the following resolution:
|“|| RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) A "planet"1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d)is not a satellite.
(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. 
Under those rather strict criteria, Pluto does not qualify. For that reason, Pluto is no longer considered a planet. It shares the new "dwarf planet" category with Eris, Ceres, Haumea and Makemake. 
Pluto was initially thought to have originally been a moon of Neptune that escaped into its own orbit around the Sun. But with the discovery of Charon in 1978, and the subsequent discoveries of its other moons Hydra and Nix, that theory became far less plausible. The current theory is that Pluto and Charon both formed from the solar nebula as the other planets in the solar system did. 
In popular fiction
Science fiction author Larry Niven once speculated on an "earlier generation" race of extraterrestrial beings that controlled the galaxy until it suffered mutual annihilation in a war with a revolting slave race. As part of that scenario, a member of that "slaver race" once had to make an emergency "landing" in a crippled spacecraft that could no longer brake to a safe approach speed. He then set a course for the earth, bailed out of his ship, and left the ship's autopilot with orders to crash-land on Neptune. Instead of that happening, the ship struck a moon of Neptune hard enough to knock it out of orbit; that moon became known as Pluto.
Pluto has also been a subject of speculation involving future efforts by humanity to colonize that body, efforts often complicated by the presence of extraterrestrial "campers" or even of pathogens, usually viruses, native to Pluto.
Pluto has at least three moons, the largest of which is Charon. (The size of Charon relative to Pluto is such that Pluto and Charon are often collectively considered to bea binary system. Due to their sizes, the center of rotation is not "inside" pluto, but rather above its surface. Rather than rotating around Pluto, Pluto and Charon rotate around a common center of gravity.) The others are named Hydra and Nix.
- ↑ http://www.space.com/scienceastronomy/070614_eris_mass.html
- ↑ "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.
- ↑ http://www.iau.org/public_press/news/detail/iau0807/
- ↑ http://science.jrank.org/pages/5352/Pluto.html
- ↑ http://www.britannica.com/eb/article-54312/Pluto
- Hamilton, Calvin J. "Entry for 'Pluto'." Views of the Solar System, 2007. Accessed January 21, 2008.