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}}: ''This article is about the [[dwarf planet]]. For the Greco-Roman god of that name, see [[Pluto (god)]].''

}}

:''This article is about the [[dwarf planet]]. For the Roman god, see [[Pluto (god)]].''

''' 134340 Pluto''' is a [[dwarf planet]] in the Kuiper belt. It was once classified as one of the nine major planets, but lost this status in 2006. It is smaller than all eight major planets. NASA once thought it smaller than [[Eris]], a recently discovered dwarf planet.<ref>http://www.space.com/scienceastronomy/070614_eris_mass.html</ref> But that turns out not to be true. In fact, Pluto is the largest object in all the [[Kuiper Belt]].

'''134340 Pluto''' is a [[dwarf planet]] in the Kuiper belt of the outer solar system. It was discovered in 1930 by [[American]] astronomer [[Clyde W. Tombaugh|Clyde Tombaugh]]. It is orbited by a large moon called Charon as well as by two smaller moons, Nix and Hydra. Charon was discovered in 1978.

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.) (To read more about the legend of Hades/Pluto the Olympian, read [[Pluto (god)|here]].)

Pluto was recognized as one of the nine major planets from its discovery. A group of astronomers from the [[International Astronomical Union]] reclassified it as a [[dwarf planet]] in 2006 after the discovery of [[Eris]]. Not all planetary scientists agree with this reclassification as there is still controversy over the [[International Astronomical Union | IAU's]] own definition of what qualifies as a planet. Pluto is the Roman god of the underworld. In 2015 the U.S. spacecraft ''New Horizons'' passed through the Plutonian system.  

According to the [[International Astronomical Union]], it no longer qualifies as a [[planet]] because it has not cleared its orbit of other objects. But the IAU decided that under circumstances at least one observer calls suspicious. (See below.)

Pluto was discovered by [[American]] astronomer [[Clyde W. Tombaugh]] on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona. Tombaugh's daughter Annette proposed the name of that object: Pluto. Pluto remains one of the only planets ever to have been discovered or named by [[American|Americans]] although the [International Astronomical Union|IAU]] now claims it is a [[dwarf planet]]

In 2015 the U.S. spacecraft ''New Horizons'' passed through the Plutonian system. The [[United States]] thus became the first nation-state to complete a reconnaissance of the original nine named planets. ''New Horizons'' found many signs that are ''not'' consistent with the conventional favored theory of Pluto's origin. Pluto has mountains that have not eroded. That suggests the body is active. Pluto also has a frozen lake of carbon monoxide that no one has yet been able to explain.

Tombaugh was searching for "Planet X," a planet beyond [[Neptune]] that was believed to be influencing Neptune's orbit. Percival Lowell, founder of the observatory where Tombaugh worked, had calculated possible sky positions for Planet X based on observations of Neptune's orbit. However, Tombaugh did not make use of Lowell's calculations. Instead, he used the observatory's telescope to photograph the opposition point to the sun every night for a year, thus conducting a full sky survey of the Zodiac.

When astronomers realized that Pluto is not large enough to influence Neptune's orbit, the search for Planet X was resumed. The idea of Planet X was debunked in 1989, when [[Voyager 2]] flew by Neptune. NASA scientists determined that Neptune was significantly heavier than previously supposed, invalidating the earlier orbital calculations by Lowell and others.

On February 18, 1930, [[Clyde W. Tombaugh]] at the Lowell Observatory surveyed the sky, looking for a planet beyond [[Neptune]] (called "Planet X"). Other astronomers had inferred such an object 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. Tombaugh's daughter Annette proposed the name of that object: Pluto.

Even Tombaugh 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 [[comet]]s.

Pluto is smaller any of the eight of the major planets. NASA once thought it smaller than [[Eris]], a recently discovered dwarf planet.<ref>http://www.space.com/scienceastronomy/070614_eris_mass.html</ref> But that turns out not to be true. In fact, Pluto is the largest object in the [[Kuiper Belt]].

== The dwarf planet controversy ==

 

 

 

= The [[dwarf planet]] controversy =

[[Image:Pluto3.gif|left|thumb|Pluto(center) with moon [[Charon]]. NASA image]]

[[Image:Pluto3.gif|left|thumb|Pluto(center) with moon [[Charon]]. NASA image]]

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]].

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 [[Eris]].

In 2006, the [[International Astronomical Union]] passed the following resolution:{{cquote|'''RESOLUTION 5A'''

In 2006, the [[International Astronomical Union]] passed the following resolution:{{Quotebox|'''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:

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.

* (1) A "planet"¹ is a celestial body that

(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.

*  (a) is in orbit around the Sun,

(3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

 

 

* (3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

 

²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

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.<ref>"[http://www.iau.org/iau0602.423.0.html IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting]," ''International Astronomical Union'', 2005. Accessed January 14, 2008.</ref> }}

 

³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.  

 

 

 

 

&mdash; ''International Astronomical Union''}}<ref>"[http://www.iau.org/iau0602.423.0.html IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting]," ''International Astronomical Union'', 2005. Accessed January 14, 2008.</ref><ref>https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf ''International Astronomical Union'', 2005. Accessed February 9, 2022</ref>

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.<ref>http://www.iau.org/public_press/news/detail/iau0807/</ref>

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.<ref>http://www.iau.org/public_press/news/detail/iau0807/</ref>

Brown suspects the IAU wanted to demote Pluto, because Pluto violates, by its very existence, every conventional theory of planetary origin.

Brown suspects the IAU wanted to demote Pluto, because Pluto violates, by its very existence, every conventional theory of planetary origin.

== Orbital characteristics ==

== Orbital characteristics ==

Pluto lies in a highly eccentric (nearly 25 percent) orbit around the sun. In fact, its [[apsis|perihelion]] lies inside the [[semi-major axis]] of the orbit of [[Neptune]].<ref>http://science.jrank.org/pages/5352/Pluto.html</ref>

Pluto lies in a highly eccentric (nearly 25 percent) orbit around the sun. In fact, its [[apsis|perihelion]] lies inside the [[semi-major axis]] of the orbit of [[Neptune]].<ref>http://science.jrank.org/pages/5352/Pluto.html</ref>

Conventional astronomers attribute the rust color of Pluto to a class of substances called "tholins" (literally, "mud-colored stuff") forming from the bombardment of hypothesized atmospheric constituents by ultraviolet radiation. But Walter T. Brown has another theory: the substance giving Pluto its rust color really ''is'' iron rust. Which, he says, formed far closer to the [[Sun]] than Pluto now orbits. (More on that below.)

Conventional astronomers attribute the rust color of Pluto to a class of substances called "tholins" (literally, "mud-colored stuff") forming from the bombardment of hypothesized atmospheric constituents by ultraviolet radiation. But Walter T. Brown has another theory: the substance giving Pluto its rust color really ''is'' iron rust. Which, he says, formed far closer to the [[Sun]] than Pluto now orbits. (More on that below.)

== Observation and Exploration ==

== Observation and Exploration ==

On January 19, 2006, the [[New Horizons mission]] was launched. Lead investigator Alan Stern and his team collected one U.S. Customary [[ounce]] of the ashes of Clyde Tombaugh, sealed them in a canister, and installed this on the upper deck of the New Horizons probe. Alan Stern inscribed it thus:{{cquote|Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's "third zone." Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997).<ref name=CNN>Mullen J, "[http://www.cnn.com/2015/07/13/us/nasa-pluto-new-horizons-clyde-tombaugh-ashes/ Pluto discoverer's ashes are aboard New Horizons probe]," [[Cable News Network]]. Published 14 July 2015. Retrieved 5 August 2015.</ref>}}

On January 19, 2006, the [[New Horizons mission]] was launched. Lead investigator Alan Stern and his team collected one U.S. Customary [[ounce]] of the ashes of Clyde Tombaugh, sealed them in a canister, and installed this on the upper deck of the New Horizons probe. Alan Stern inscribed it thus:{{cquote|Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's "third zone." Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997).<ref name=CNN>Mullen J, "[https://www.cnn.com/2015/07/13/us/nasa-pluto-new-horizons-clyde-tombaugh-ashes/ Pluto discoverer's ashes are aboard New Horizons probe]," [[Cable News Network]]. Published 14 July 2015. Retrieved 5 August 2015.</ref>}}

Pluto is its prime target, but mission planners are considering plans to study at least one other [[Kuiper belt]] object.<ref name=Jenner>Jenner, Lynn. "[http://www.nasa.gov/mission_pages/newhorizons/main/index.html NASA - New Horizons]." [[National Aeronautics and Space Administration|NASA]], October 9, 2007. Accessed July 3, 2008.</ref><ref name=Talbert>[http://www.nasa.gov/mission_pages/newhorizons/main/index.html New Horizons official site], ed. Tricia Talbert/NASA. First accessed 10 July 2015.</ref>

Pluto is its prime target, but mission planners are considering plans to study at least one other [[Kuiper belt]] object.<ref name=Jenner>Jenner, Lynn. "[https://www.nasa.gov/mission_pages/newhorizons/main/index.html NASA - New Horizons]." [[National Aeronautics and Space Administration|NASA]], October 9, 2007. Accessed July 3, 2008.</ref><ref name=Talbert>[https://www.nasa.gov/mission_pages/newhorizons/main/index.html New Horizons official site], ed. Tricia Talbert/NASA. First accessed 10 July 2015.</ref>

On 15 January 2015, New Horizons began its final approach to Pluto. On 14 July 2015, New Horizons passed directly through the Plutonian system and took the best photographs of it to date. Mission controllers expect New Horizons to transmit its thousands of high-resolution photographs and other data by the end of 2016.

On 15 January 2015, New Horizons began its final approach to Pluto. On 14 July 2015, New Horizons passed directly through the Plutonian system and took the best photographs of it to date. Mission controllers expect New Horizons to transmit its thousands of high-resolution photographs and other data by the end of 2016.

New Horizons now awaits further orders to alter its course to reconnoiter any other Kuiper Belt object it might be able to reach.

New Horizons now awaits further orders to alter its course to reconnoiter any other Kuiper Belt object it might be able to reach.

== In popular fiction ==

== In popular fiction ==