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Pluto sri.jpg
Mosaic, true-color image of Pluto based on eclipses of Pluto by its satellite, Charon.
Date of discovery 19301930
5690 He
5933 AM
Name of discoverer Clyde W. Tombaugh
Name origin Greco-Roman god of wealth and the underworld
Orbital characteristics
Celestial class Dwarf planet
Primary Sun
Order from primary 10
Perihelion 4,436,824,613 km29.658 AU
2,756,914,999.528 mi
Aphelion 7,375,927,931 km49.305 AU
4,583,189,132.342 mi
Semi-major axis 5,906,376,272 km39.482 AU
3,670,052,065.935 mi
Titius-Bode prediction 77.2 AU
Circumference 188.925 AU28,262,777,589.75 km
17,561,675,806.882 mi
Orbital eccentricity 0.24880766
Sidereal year 248.09 a90,614.873 da
Synodic year 366.73 da1.004 a
Avg. orbital speed 4.666 km/s16,797.6 km/h
2.899 mi/s
10,437.545 mph
Inclination 17.14175°0.299 rad
19.046 grad
to the ecliptic
Rotational characteristics
Sidereal day -6.387230 da-153.294 h
Rotation speed 47.18 km/h0.0131 km/s
0.00814 mi/s
29.316 mph
Axial tilt 119.591°2.087 rad
132.879 grad
Physical characteristics
Mass 1.305 * 1022 kg0.00218 M⊕
0.178 M☾
Mean density 2,030 kg/m³2.03 g/ml
126.729 lb/ft³
Mean radius 1,195 km742.539 mi
Surface gravity 0.58 m/s²1.903 ft/s²
0.0591 g
Escape speed 1.2 km/s4,320 km/h
0.746 mi/s
2,684.324 mph
Surface area 17,950,000 km²6,930,533.746 mi²
0.0352 A⊕
0.473 A☾
Minimum temperature 33 K-240.15 °C
-400.27 °F
59.4 °R
Mean temperature 44 K-229.15 °C
-380.47 °F
79.2 °R
Maximum temperature 55 K-218.15 °C
-360.67 °F
99 °R
Number of moons 3
Color #EEBB88

Pluto is the second-largest dwarf planet in the solar system. (The recently-discovered Eris is larger.) Its name literally means "god of wealth" and is one of the two names for one of the brothers of Zeus; the other is Hades, or "god of the underworld." (The Romans used only the name Pluto for this particular Greek god.)

According to the International Astronomical Union, it no longer qualifies as a planet because it has not cleared its orbit of other objects.



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. Tombaugh knew nothing of the error, but found an object anyway. This object was later given the name 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 that of a similar body in the asteroid belt, namely Ceres.

In 2006, the International Astronomical Union passed the following resolution:


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. [1]

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 and Ceres.

Problems for uniformitarianism

Pluto was initially supposed to have been a moon of Neptune. 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.

Another hypothesis has held that Pluto entered the solar system from outside, and the Sun captured it. But if such a mass dived into the solar system from outside, then it would have followed a hyperbolic path. That it did not pass back out of the solar system to complete the hyperbola begs explanation.

Observation and Exploration

On January 19, 2006, the New Horizons mission was launched. Pluto is its prime target, but mission planners are considering plans to study at least one other Kuiper belt object.[2]

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.


Table of satellites, in order from the innermost to the outermost:
Name Perihadion Aphadion Eccentricity Sidereal month Inclination Mass Sidereal day
Charon 1964000019,640 km1.312853e-4 AU
12,203.73 mi
1964000019,640 km1.312853e-4 AU
12,203.73 mi
00 6.387256.387 da0.0175 a 00 °0 rad
0 grad
1.9E+210.0259 M☾1.9e+21 kg
3.17939e-4 M⊕
551858.4153.294 h6.387 da
Nix 4857800048,578 km3.247239e-4 AU
30,184.97 mi
4877200048,772 km3.260207e-4 AU
30,305.516 mi
0.0020.002 24.856224.856 da0.0681 a 0.0006981317007980.04 °6.981317e-4 rad
0.0444 grad
5.0E+166.804295e-7 M☾5.0e+16 kg
8.366815e-9 M⊕
Hydra (moon) 6445800064,458 km4.308751e-4 AU
40,052.344 mi
6510200065,102 km4.3518e-4 AU
40,452.507 mi
0.0050.005 38.20638.206 da0.105 a 0.003839724354390.22 °0.00384 rad
0.244 grad
5.0E+166.804295e-7 M☾5.0e+16 kg
8.366815e-9 M⊕
Use a JavaScript-enabled browser to view this element. Browse the result list directly.DECADECENTURYCharon1978-07-02T00:00:000Date of discovery 2 July 19782 July 1978
27 Sivan 5738 He
26 Tammuz 5981 AM

Discoverer James W. Christy
Name origin mythic ferryman of the River Styx
Celestial class Solar system, Moon
Hydra (moon)2005-05-15T00:00:000Date of discovery 15 May 200515 May 2005
6 Iyar 5765 He
6 Sivan 6008 AM

Discoverer Hubble Space Telescope Pluto Companion Search Team
Name origin Mythical 9-headed monster that guarded the underworld; second initial of "New Horizons" mission
Celestial class Solar system, Moon
Nix2005-05-15T00:00:000Date of discovery 15 May 200515 May 2005
6 Iyar 5765 He
6 Sivan 6008 AM

Discoverer Hubble Space Telescope Pluto Companion Search Team
Name origin Greek Nyx goddess of night and mother of Charon (Egyptian spelling); first initial of "New Horizons" mission
Celestial class Solar system, Moon


  1. "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.
  2. Jenner, Lynn. "NASA - New Horizons." NASA, October 9, 2007. Accessed July 3, 2008.

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