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For more uses of the term Pluto, see Pluto (disambiguation).
Pluto Astronomical symbol of Pluto
Pluto in True Color
Discovered by Clyde W. Tombaugh
Discovered on February 18, 1930
Orbital characteristics (Epoch J2000)
Semi-major axis 5,906,376,272 km
39.481 686 77 AU
Orbital circumference 36.530 Tm
244.186 AU
Eccentricity 0.248 807 66
Perihelion 4,436,824,613 km
29.658 340 67 AU
Aphelion 7,375,927,931 km
49.305 032 87 AU
Orbital period 90,613.3055 d (248.09 a)
Synodic period 366.73 d
Avg. Orbital Speed 4.666 km/s
Max. Orbital Speed 6.112 km/s
Min. Orbital Speed 3.676 km/s
Inclination 17.141 75°
(11.88° to Sun's equator)
Longitude of the
ascending node
110.303 47°
Argument of the
113.763 29°
Number of satellites 3
Physical characteristics
Diameter 2390 km
(0.188 Earths or 1,485 miles)
Surface area 1.795×107 km2
(0.033 Earths)
Volume 7.15×109 km3
(0.0066 Earths)
Mass 1.25×1022 kg
(0.0021 Earths)
Mean density 1.750 g/cm3
Equatorial gravity 0.58 m/s2
(0.059 gee)
Escape velocity 1.2 km/s
Rotation period 6.387 d (6 d 9 h 17.6 min)
Rotation velocity 47.18 km/h (at the equator)
Axial tilt 122.54° (to orbit)
115.60° (to the ecliptic)[1]
Right ascension
of North pole
133.02° (8 h 52 min 5 s)
Declination -9.09°
Albedo 0.30
Surface temp.
min mean max
33 K 44 K 55 K
Adjective Plutonian
Atmospheric characteristics
Atmospheric pressure 0.30 pascals (summer maximum)
Composition nitrogen, methane

Pluto is a small celestial body in the outer solar system. Discovered in 1930 and originally classified as a planet, its status is currently under dispute. Pluto has an eccentric orbit that is highly inclined in respect to the other planets and takes it inside the orbit of Neptune. Its largest moon is Charon, discovered in 1978; two smaller moons were discovered in 2005. Pluto's astronomical symbol is a P-L monogram, . This represents both the first two letters of the name Pluto and the initials of Percival Lowell, the man who lent his name to the observatory that was used to find Pluto. An alternate symbol resembles that of Neptune, but has a circle in place of the middle spoke in the top center.

Due to its size and unusual orbit, there has been debate regarding Pluto's classification as a major or a minor planet, and there is increasing momentum for recognizing "dual status." Pluto is classified as a trans-Neptunian object. As of July 31, 2005, one other trans-Neptunian object, 2003 UB313, had been found that is larger than Pluto.


Discovery and naming

Pluto was discovered by the astronomer Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930 when he compared photographic plates taken on January 23 and 29. After the observatory obtained confirming photographs, the news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The planet was later found on photographs dating back to March 19, 1915. Tombaugh was searching for a "Planet X" to explain discrepancies in the predicted orbit of Neptune. It is now known these discrepancies were an artifact of the slightly incorrect value then assumed for the mass of Neptune.

In the matter of Pluto the discretion of naming the new object belonged to Lowell Observatory and its director, Vesto Melvin Slipher, who, in the words of Tombaugh, was "urged to suggest a name for the new planet before someone else did". Soon suggestions began to pour in from all over the world. Constance Lowell, Percival's widow who had delayed the search through her lawsuit, proposed Zeus, then Lowell, and finally her own first name, none of which met with any enthusiasm. One young couple even wrote to ask that the planet be named after their newborn child. Mythological names were much to the fore: Cronus and Minerva (proposed by the New York Times, unaware that it had been proposed for Uranus some 150 years earlier) were high on the list. Also there were Artemis, Athene, Atlas, Cosmos, Hera, Hercules, Icarus, Idana, Odin, Pax, Persephone, Perseus, Prometheus, Tantalus, Vulcan, Zymal, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids. Virtually all the female names had been used up, and male names were usually reserved for objects with unusual orbits.

The name retained for the planet is that of the Roman god Pluto, and it is also intended to evoke the initials of the astronomer Percival Lowell, who predicted that a planet would be found beyond Neptune. The name was first suggested by Venetia Burney, at the time an eleven-year-old girl from Oxford, England. Over the breakfast table, one morning her grandfather, who worked at Oxford University's Bodleian Library, was reading about the discovery of the new planet in the Times newspaper. He asked his granddaughter what she thought would be good name for it. Venetia thought that as it was so cold and so distant it should be named after the Roman God of the underworld. Professor Herbert Hall Turner cabled his colleagues in America with this suggestion, and after favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made by Slipher on May 1, 1930.

Because Pluto was nowhere near the asteroid belt (trans-Neptunian objects would not be discovered until much later), it never got an asteroidal provisional designation; had it obtained one, it would probably have been "1930 BD".


The orbits of the outer planets, showing Pluto's eccentricity, which causes it to cross Neptune's orbit
The orbits of the outer planets, showing Pluto's eccentricity, which causes it to cross Neptune's orbit

Pluto's orbit is unlike those of the other planets. It is highly inclined above the plane of the ecliptic, and highly eccentric (non-circular). The eccentricity of its orbit is such that it crosses the orbit of Neptune, and making Pluto only the eighth-most distant planet from the Sun for part of each orbit; the most recent occurrence of this phemonenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence only lasted fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was the eighth-most distant planet between April 30, 1483 and July 23, 1503, which is almost exactly the same length as the 1979 to 1999 period. Recent studies suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for approximately thirteen and twenty years with minor variations.

Pluto's orbit seen from the plane of the ecliptic, showing its high inclinaion compared to the other planets
Pluto's orbit seen from the plane of the ecliptic, showing its high inclinaion compared to the other planets

Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity start to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they're almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune is about to catch up to Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead of Neptune, and pulls ahead until they meet again on the other side of Pluto's orbit.

Physical characteristics

More than 75 years after its discovery, many facts about Pluto remain unknown, mainly due to the fact that it is the only planet that has not been visited by human spacecraft and that it is too far away for in-depth investigations with telescopes from earth. What is known are the few physical properties listed below.

Mass and size

Pluto is not only smaller and much less massive than every other planet, at less than 0.2 lunar masses it is also smaller and less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is more than twice the diameter, and a dozen times the mass, of Ceres, the largest minor planet in the asteroid belt, and it was larger than any other object known in the trans-Neptunian Kuiper belt until 2003 UB313 was announced in 2005. See List of solar system objects by mass and List of solar system objects by radius.

Pluto's mass and diameter could only be estimated for many decades after its discovery. The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Later Pluto's diameter was measured when it was occulted by Charon, and its disk can now be resolved by telescopes using adaptive optics.


Pluto's thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes. Pluto was found to have an atmosphere from an occultation observation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of earth.

In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy [2] and by Jim Elliot. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was further from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The current best hypothesis is that the south pole of Pluto came out of shadow for the first time in 120 years in 1987, and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.


Pluto's apparent magnitude is fainter than 14 m and therefore a telescope is required for observation. To be easily seen, a telescope of around 30cm aperture is desirable. It looks star-like even in very big telescopes, because its angular diameter is only 0.15″. The colour of Pluto is light brown with a very slight tint of yellow.

Stationary, retrograde Opposition Distance to Earth
Stationary, prograde Conjunction to Sun
March 24, 2004 June 11, 2004 29.80193 13.8 August 31, 2004 December 13, 2004
March 27, 2005 June 14, 2005 29.95761 13.8 September 3, 2005 December 16, 2005
March 29, 2006 June 16, 2006 30.12128 13.9 September 5, 2006 December 18, 2006
March 31, 2007 June 19, 2007 30.29202 13.9 September 7, 2007 December 21, 2007
April 2, 2008 June 20, 2008 30.46941 13.9 September 9, 2008 December 22, 2008
April 4, 2009 June 23, 2009 30.65286 13.9 September 11, 2009 December 24, 2009
April 7, 2010 June 25, 2010 30.84244 14.0 September 14, 2010 December 27, 2010
April 9, 2011 June 28, 2011 31.03813 14.0 September 16, 2011 December 29, 2011
April 10, 2012 June 29, 2012 31.24049 14.0 September 17, 2012 December 30, 2012
April 12, 2013 July 2, 2013 31.44959 14.0 September 20, 2013 January 1, 2014
April 15, 2014 July 4, 2014 31.66530 14.1 September 22, 2014 January 3, 2015
April 17, 2015 July 6, 2015 31.88724 14.1 September 24, 2015 January 6, 2016
April 18, 2016 July 7, 2016 32.11459 14.1 September 26, 2016 January 7, 2017
April 20, 2017 July 10, 2017 32.34681 14.2 September 28, 2017 January 9, 2018
April 23, 2018 July 12, 2018 32.58277 14.2 September 30, 2018 January 11, 2019
April 25, 2019 July 14, 2019 32.58277 14.2 October 2, 2019 January 13, 2020
April 26, 2020 July 15, 2020 33.06323 14.3 October 4, 2020 January 14, 2021

Pluto's moons

Main article: Pluto's natural satellites
Diagram of the Plutonian system
Diagram of the Plutonian system

Pluto has three known natural satellites: Charon, first identified in 1978, and two smaller, as yet unnamed moons discovered in 2005.


The Pluto-Charon system is noteworthy for being the only planet/moon system in the solar system whose barycenter lies above the planet's surface, thus prompting some astronomers to label it a double planet (a term complicated by the discovery of two more Plutonian moons).

The Pluto-Charon system is also unusual among planetary systems in that they are tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.

The discovery of Charon allowed astronomers to determine the mass of the Pluto-Charon pair from their observed orbital period and separation by a straightforward application of Kepler's third law of planetary motion. The mass was found to be lower than even the lowest earlier estimates.

The discovery also led astronomers to alter their estimate of Pluto's size. Originally, it was believed that Pluto was larger than Mercury but smaller than Mars, but that calculation was based on the premise that a single object was being observed. Once it was realized that there were in fact two objects instead of one, the estimated size of Pluto was revised downward. Today, with modern adaptive optics, Pluto's disc can be resolved and thus its size can be directly determined.

Pluto and its primary satellite Charon
Pluto and its primary satellite Charon

Charon's discovery also resulted in the calculation of Pluto's albedo being revised upward; since the planet was now seen as being far smaller than originally estimated, by necessity its capacity to reflect light must be greater than what had been formerly believed. Current estimates place Pluto's albedo as marginally less than that of Venus, which is fairly high.

Previously, some researchers had theorized that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit. Today it is widely accepted that Pluto never orbited Neptune. Instead, Triton, the largest moon of Neptune, which shares many atmospherical and geological composition similarities with Pluto, may once have been a Kuiper belt object in a solar orbit.

The outer moons

Two additional moons were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005 and have received provisional designations of S/2005 P 1 and S/2005 P 2. They were confirmed with "precovery" Hubble images from June 14, 2002. Observations suggest they orbit Pluto at at least twice the distance Charon does. P2 stays about 49,000km from the planet, P1 lies even further away at 65,000km. The two candidate moons seem to orbit Pluto in an anti-clockwise direction. Preliminary observations are also consistent with the outer moons lying in the same orbital plane as Charon, and orbiting at distances two and three times farther away, with orbital resonances of 4:1 and 6:1 with Charon.

Both objects appear to be on the order of 50-150 km in diameter, compared to Charon's 1,200 km, and are thought to have masses less than 0.3% of Charon's (or 0.03% of Pluto's mass). The discovery team plans follow-up observations with Hubble in February 2006 to work out the precise orbits, but ground-based observatories will attempt to image the moons as well. Once the orbits are confirmed, the moons can be given permanent names. [3] [4]

Exploration of Pluto

Little is known about Pluto because of its great distance from Earth and because no exploratory spacecraft have visited Pluto yet. The Voyager 1 probe was originally intended to visit Pluto, but due to budget cuts and lack of interest — before the discovery of Pluto's moon, size, and atmosphere — the flyby was scrapped in order to faciltate a close flyby of Saturn's moon Titan.

The first spacecraft to visit Pluto will be NASA's New Horizons, a mission led by the Southwest Research Institute and the Johns Hopkins Applied Physics Laboratory.

The mission's launch window is between January 11 and February 14, 2006. Assuming it launches within the first 23 days of the window, it will benefit from a gravity assist from Jupiter, and arrive at Pluto in July 2015.

New Horizons weighs half a ton and will travel at speeds reaching 43,000 km/h (27,000 mph). It will use a remote sensing package that includes imaging instruments and a radio science investigation, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and characterize Pluto's neutral atmosphere and its escape rate. The mission plan also calls for a flyby of one or more Kuiper belt objects by 2022.

The New Horizons mission replaced the Pluto Kuiper Express mission, which was cancelled in 2000 because of increasing costs and launch vehicle delays.

The Pluto debate

Planet X?

Main article: Planet X

The planet Pluto was originally discovered in 1930 in the course of a search for a body sufficiently massive to account for supposed anomalies in the orbits of Uranus and Neptune. Once it was found, its faintness and failure to show a visible disc cast doubt on the idea that it could be Lowell's Planet X. Lowell had made a prediction of Pluto's position in 1915 which had turned out to be fairly close to its actual position at that time; however Ernest W. Brown concluded almost immediately that this was a coincidence, and this view is retained today. Lowell had also made earlier, different predictions of Planet X's position beginning in 1902. [5]

In the following decades estimates of the Plutonian mass and diameter were the subject of debate as telescopes and imaging systems improved. The consensus steadily favored smaller masses and diameters as time passed. Indeed, one observer waggishly pointed out that if the trend were extrapolated, the planet seemed to be in danger of vanishing altogether, a remark which proved possibly prophetic in light of later debates over Pluto's status as a "planet".

In an attempt to reconcile Pluto's small apparent size with its identification as Planet X, the theory of specular reflection was proposed. This held that observers were measuring only the diameter of a bright spot on the highly reflective surface of a much larger planet which could thereby be massive without having an exceptionally high density.

The uncertainty was conclusively resolved by the discovery of Pluto's satellite Charon in 1978. This made it possible to determine the combined mass of the Pluto-Charon system which turned out to be lower even than that anticipated by skeptics of the specular reflection theory, which was then rendered completely untenable. The accepted figure for Pluto's diameter today makes it considerably smaller than the Moon, with only a fraction of the Moon's mass on account of its being largely composed of ice. More recently, measurements of the path of Voyager 2 have shown that Neptune has a lower mass than previously believed and that when this lower mass is taken into account there is no anomalous movement of Uranus or Neptune.

Thus Pluto's discovery and Lowell's 1915 prediction were largely coincidental as Pluto actually has no role in what were believed to be anomalies in Neptune and Uranus' motion. Pluto's discovery was mostly due to the thoroughness and diligence of Tombaugh's search, which he continued for some time after the discovery and left him satisfied that no other planet of a comparable magnitude existed.

While Pluto's identification as Planet X began to be doubted soon after its discovery, and for some decades afterwards some considered that a hypothetical tenth planet might be the true Planet X which supposedly caused anomalies in Uranus and Neptune's position, Pluto's identity as the solar system's ninth planet was unquestioned until the 1990s.

Minor planet?

An image of Pluto and Charon taken with a 61" telescope; note the difficulty in resolution despite telescope size.  This small size is one of the reasons Pluto's planetary status is debated.
An image of Pluto and Charon taken with a 61" telescope; note the difficulty in resolution despite telescope size. This small size is one of the reasons Pluto's planetary status is debated.

Starting in September of 1992 scientists began discovering hundreds of other bodies in the area of the solar system beyond the orbit of Neptune. The second of these, after Pluto and Pluto's moon Charon, was (15760) 1992 QB1. The continued discovery of these objects, especially of plutinos, rekindled a debate that goes on to this day: is Pluto a major planet or simply one of the largest trans-Neptunian objects?

Trans-Neptunian objects are considered to be minor planets, so the question arose whether to consider Pluto to be one too. This planetary sciences debate landed in newspaper headlines, editorials, and on the Internet in the mid- to late-1990s. Thoughts that Pluto might be "demoted" to non-planet status created an emotional response in certain sectors of the public. Such news outlets as the BBC News Online, the Boston Globe, and USA Today all printed stories noting that the International Astronomical Union was considering dropping Pluto's planetary status. "Save Pluto" websites sprang up, and school children sent letters to astronomers and the IAU.

On February 3, 1999, Brian Marsden of the Minor Planet Center inadvertently fueled the debate when he issued an editorial in the Minor Planet Electronic Circular 1999-C03 noting that the 10,000th minor planet was about to be numbered and this called for a large celebration (the IAU celebrates every thousandth numbered minor planet in some way). He suggested that Pluto be honored with the number 10,000, giving it "dual citizenship" of sorts as both a major and a minor planet.

Between the media reports and the Minor Planet Electronic Circulars, IAU General Secretary Joannes Anderson issued a press release that same day, stating there were no plans to change Pluto's planetary status. Eventually, the number 10,000 was assigned to an "ordinary" asteroid, 10000 Myriostos.

The debate centers on how a "planet", from the Greek for "wanderer", is an appellation that depends upon an object's particular size, formation, or orbit. Some argue that not only is Pluto a major planet but also some moons like Titan, Europa or Triton, or even the larger asteroids. Some argue that an astronomical object more than about 360 km in diameter, at which point the object has a tendency to become round under its own gravity, should be known as a major planet; this would include several moons and a handful of asteroids. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury, the smallest terrestrial planet with a diameter of 4879.4 km and Ceres, the largest known asteroid with a mean diameter of 950 km, which would include Pluto but not most moons.

New discoveries

Continuing discoveries in the transneptunian region keep rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1280 km diameter, making it a bit more than half the size of Pluto. Another recent discovery, 90482 Orcus, is probably even larger. In 2004 the discoverers of 90377 Sedna, an extremely distant object well beyond the other known transneptunian objects, placed an upper limit of 1800 km on its diameter, close to Pluto's 2320 km.

On July 29, 2005, a Trans-Neptunian object called 2003 UB313 was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be larger than Pluto. This caused its discoverers to call it the "10th planet" of the solar system, although there is no consensus yet on whether to call it a planet, and others consider the new discovery to be the strongest argument yet for demoting Pluto to the status of a minor planet. 2003 UB313 could be the largest object yet discovered in the solar system since Neptune in 1846. The last remaining distinguishing feature of Pluto is now its large moon, Charon, and its atmosphere; these characteristics may not, however, be unique to Pluto: several other transneptunian objects are known to have satellites; and 2003 UB313's spectrum suggests that it has a similar surface composition to Pluto, as well as a moon discovered in September of 2005.

There is some historical precedent for "demoting" a "planet" in the light of subsequent discoveries. The first four asteroids (1 Ceres, 2 Pallas, 3 Juno and 4 Vesta) were considered to be planets for several decades (in part because their sizes were not accurately known at the time). However, in 1845, the first new asteroid in 38 years was discovered (5 Astraea), just one year before Neptune, and soon every year brought more asteroid discoveries. It was soon recognized that Ceres and the others were just the most prominent members of a populous asteroid belt, and although asteroids are also known as "minor planets", they are no longer considered "planets". Some see in this a precedent for noting that Pluto is just the most prominent member of the Kuiper belt.

On the other hand, it may very well be that regardless of future astronomical discoveries, Pluto will remain grandfathered as a planet in much the same way that Europe is considered a separate continent for historical reasons although geographically it makes more sense, from first principles, to consider both Europe and Asia to comprise the single continent of Eurasia.

Pluto in Fiction and Film

See also

Wikimedia Commons has media related to:


  • Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". New Straits Times, p. F17.

External links

Pluto's natural satellites [edit ]
Charon | (S/2005 P 2) | (S/2005 P 1)

Large trans-Neptunian objectsedit
Kuiper belt: Pluto (Charon) | Orcus | Ixion | 2002 UX25 | Varuna
2002 TX300 | 2003 EL61 | Quaoar | 2005 FY9 | 2002 AW197
Scattered disc: 2003 UB313 | Sedna
 See also Triton, astronomical objects and the solar system's list of objects, sorted by radius or mass
For pronunciation, see: Centaur and TNO pronunciation.
† Current MPC classification. Some consider Sedna an Oort cloud object.

Our Solar Systemedit
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belt
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Scattered disc | Oort cloud
 See also astronomical objects and the solar system's list of objects, sorted by radius or mass

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