2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy); The Planets

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 T m
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
(19% of Earth, or
1485 mi)
Surface area 1.795×107 km²
(0.033 Earths)
Volume 7.15×109 km³
(0.0066 Earths)
Mass (1.305±0.007)×1022 kg
(0.0021 Earths)
Mean density 2.03±0.06 g/cm³
Equatorial gravity 0.58 m/s²
(0.059 gee)
Escape velocity 1.2 km/s
Rotation period −6.387230 d
(6 d 9 h 17 m 36 s)
Rotation velocity 47.18 km/h (at the equator)
Axial tilt 119.59° (to orbit)
112.78° (to the ecliptic)
Right ascension
of North pole
(8 h 52 min 11 s)
Declination -6.145±0.02°
Albedo 0.49–0.66 (varies by 35%)
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
Note: This article contains special characters.

Pluto ( IPA: /ˈpluːtəʊ/), designated (134340) Pluto in the Minor Planet Centre catalogue, is the second-largest known dwarf planet in the solar system. It orbits between 29 and 49 AU from the Sun, and was the first Kuiper Belt Object to be discovered. Approximately one-fifth the mass of the Earth's Moon, Pluto is primarily composed of rock and ice. It has an eccentric orbit that is highly inclined with respect to the planets and takes it closer to the Sun than Neptune during a portion of its orbit. Pluto and its largest satellite, Charon, could be considered a binary system because they are closer in size than any of the other known planetoid/moon combinations in the solar system, and because the barycentre of their orbits does not lie within either body. However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, so Charon is regarded as a moon of Pluto. Two smaller moons, Nix and Hydra, were discovered in 2005. Pluto is smaller than several of the natural satellites or moons in our solar system (see the list of solar system objects by radius).

From its discovery by Clyde Tombaugh in 1930, Pluto was considered the ninth planet from the Sun. In the late 20th and early 21st century, many similar objects were discovered in the outer solar system, most notably the Trans-Neptunian object Eris which is slightly larger than Pluto. In August 2006 the IAU redefined the term "planet", and classified Pluto, Ceres, and Eris as dwarf planets. Pluto is also classified as the prototype of a family of trans-Neptunian objects. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.


In 1930 Clyde Tombaugh was working on a project searching for a ninth planet at Lowell Observatory. Tombaugh's work was to systematically take pictures of the celestial sky in pairs, one to two weeks apart, then look for objects that had moved between images. On February 18, 1930, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photo taken on January 20 helped confirm the movement. After the observatory worked to obtain further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Pluto would later be found on photographs dating back to March 19, 1915.

Relations to Neptune and Uranus

The history of how Pluto was discovered is intertwined with the discoveries of Neptune and Uranus. In the 1840s, using Newtonian mechanics, Urbain Le Verrier, and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Theorizing the perturbations were caused by the gravitational pull of another planet, Johann Gottfried Galle discovered Neptune on September 23, 1846.

Observations of Neptune in the late 19th century had astronomers starting to speculate that Neptune's orbit too was also being disturbed by another planet in a similar manner that Neptune was disturbing Uranus. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial coordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France published calculations by Indian astronomer V.B. Ketakar which predicted a location for an undiscovered planet.

Percival Lowell's influence

Internal structure of Pluto
Internal structure of Pluto

Percival Lowell would have significant influence on Pluto's discovery. In 1905, Lowell Observatory (founded by Lowell in 1894) started an extensive project in search of a possible ninth planet. The work continued after Lowell's death in 1916. Lowell was searching for a theoretical Planet X to match observations seen in Uranus and Neptune.

Pluto is too small to have the effect on Neptune's orbit that initiated the search. After the flyby of Neptune by Voyager 2 in 1989, it was conclusively demonstrated that the discrepancies in Neptune's orbit observed by 19th century astronomers were due instead to inaccurate estimates of Neptune's mass. Once found, Pluto's faintness and lack of a visible disk cast doubt on the idea that it could be Percival Lowell's Planet X. Lowell had made a prediction of Pluto's position in 1915 which was 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 still held today. Tombaugh's discovery is therefore even more surprising, given that Pluto's proximity to the region predicted by Pickering, Lowell, and Ketakar was likely a mere coincidence.


The right to name the new object belonged to the Lowell Observatory and its director, Vesto Melvin Slipher. Tombaugh urged Slipher to suggest a name quickly for the new object before someone else did. Name suggestions poured in from all over the world. Constance Lowell, Percival Lowell's widow, proposed Zeus, then Lowell, and finally her own first name, none of which met with any enthusiasm. Mythological names, such as Cronus and Minerva, were high on a list of considered names.

The name Pluto was first suggested by Venetia Burney (later Venetia Phair), at the time an eleven-year-old girl from Oxford, England. Venetia, who was interested in Classical mythology as well as astronomy, suggested the name, the Roman equivalent of Hades, in a conversation to her grandfather Falconer Madan, a former librarian of Oxford University's Bodleian Library. Madan passed the suggestion to Professor Herbert Hall Turner, Turner then cabled the suggestion to colleagues in America. After favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds as a reward.

The name retained for the object is that of the Roman god Pluto, and it is also intended to evoke the initials of the astronomer Percival Lowell. In the Chinese, Japanese, and Korean languages, the name was translated as death king star (冥王星), suggested by Houei Nojiri in 1930. China started official use of the name in 1933. Japan used the pronunciation プルートー (purūtō); later Tokyo Observatory decided to adopt it, and it became the official name in Japan in 1943. In Vietnamese it is named after Yama (Sao Diêm Vương), the Guardian of Hell in Buddhist mythology.


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, who had searched extensively for a ninth planet and who had founded Lowell Observatory, the observatory from which Tombaugh discovered Pluto. Besides its astronomical symbol Pluto also has an astrological symbol. Pluto's astrological symbol resembles that of Neptune ( ), but has a circle in place of the middle prong of the trident ( ).

Physical characteristics

Diagram of Pluto (top left) and its moons (top right) compared in size, albedo and color index with the largest plutinos: Orcus (bottom left) and Ixion (bottom right).
Diagram of Pluto (top left) and its moons (top right) compared in size, albedo and colour index with the largest plutinos: Orcus (bottom left) and Ixion (bottom right).

Many details about Pluto remain unknown, mainly due to the fact that it has not been visited up close by spacecraft. Pluto's distance from Earth makes in-depth investigation difficult.


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

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

Distance and limits on telescope technology make it currently impossible to directly photograph surface details on Pluto. Images from the Hubble Space Telescope barely show any distinguishable surface definitions or markings. The best images of Pluto derive from brightness maps created from close observations of eclipses by its largest moon, Charon. Using computer processing, observations are made in brightness factors as Pluto is eclipsed by Charon. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a gray spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.

Mass and size

Pluto's volume is about 0.66% that of Earth's
Pluto's volume is about 0.66% that of Earth's

Pluto's diameter and mass were incorrectly overestimated for many decades after its discovery. Initially it was thought to be relatively large, with a mass comparable to Earth, but over time the estimates were revised sharply downward as observations were refined.

The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by application of Newton's formulation of Kepler's third law. 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 two objects instead of one, the estimated size of Pluto was revised downward. Observations were able to determine Pluto's diameter when it is at occultation with Charon, and its shape can be resolved by telescopes using adaptive optics.

Pluto (bottom right) compared in size to the largest moons in the solar system (from left to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton.
Pluto (bottom right) compared in size to the largest moons in the solar system (from left to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton.

Among the objects of the Solar System, Pluto is not only smaller and much less massive than any planet, but at less than 0.2 lunar masses it is also smaller and less massive than seven of the moons: Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of Ceres, a dwarf planet in the asteroid belt. However, it is smaller than trans-Neptunian Kuiper belt object Eris, discovered in 2005. See List of solar system objects by mass and List of solar system objects by radius.


Pluto does not have a significant atmosphere. It has a thin envelope of gas that is most likely made up of nitrogen, methane, and carbon monoxide, that develops in equilibrium with solid nitrogen and carbon monoxide ices on the surface as it approaches the Sun. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes and falls to the ground. When it returns to a closer proximity to the Sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to sublimate into gas—creating an anti-greenhouse effect. Much as sweat evaporating from the surface of human skin, this sublimation has a cooling effect and scientists have recently discovered, by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins less than they expected.

Pluto was found to have an atmosphere from an occultation observation in 1985 (IAU Circ. 4097; MNRAS 276, 571); the finding was confirmed and significantly strengthened by extensive observations of another occultation 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 of the Paris Observatory and by James Elliot of MIT and Jay Pasachoff of Williams College. 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.

In October, 2006, the spectroscopic discovery of ethane (C2H6) on Pluto's surface, presented by Dale Cruikshank of NASA/Ames Research Centre (a New Horizons co-investigator) and colleagues was announced. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane (CH4) on Pluto's surface and suspended in its atmosphere.

The MIT-Williams College team of James Elliot and Jay Pasachoff and a Southwest Research Institute team led by Leslie Young observed a further occultation of a star by Pluto on 12 June 2006 from sites in Australia. (Elliot, J. L., Person, M. J., Gulbis, A. A. S., Adams, E. R., Kramer, E. A., Zuluaga, C. A., Pike, R. E., Pasachoff, J. M., Souza, S. P., Babcock, B. A., Gangestad, J. W., Jaskot, A. E., Francis, P. J., Lucas, R., Bosh, A. S. 2006, "The Size of Pluto's Atmosphere As Revealed by the 2006 June 12 Occultation," Pasadena Division of Planetary Sciences meeting, October 2006.)


The surface of Pluto is remarkably heterogeneous, as evidenced by its lightcurve, maps of its surface constructed from Hubble Space Telescope observations, and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon has more methane ice, while the opposite face has more ices of nitrogen and carbon monoxide. This makes Pluto the second most contrasted body in the Solar System after Iapetus.


Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in red) shows how steeply inclined the orbit is in comparison to Neptune's more normal orbit (in blue)
Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in red) shows how steeply inclined the orbit is in comparison to Neptune's more normal orbit (in blue)

Pluto's orbit is very unusual in comparison to the planets of the solar system. The planets orbit the Sun close to an imaginary flat plane called the plane of the ecliptic, and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined above the ecliptic (up to 17° above it) and very eccentric (non-circular). Owing to the orbit’s inclination, Pluto's perihelion is well above (~8.0 AU) the ecliptic. The high eccentricity means that part of Pluto's orbit is closer to the Sun than Neptune's.

Heliocentric distance

Orbit of Pluto – polar view. This 'view from above' shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and also shows how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic. The positions of both are marked as of April 16, 2006; in April 2007 they will have changed by about 1 pixel.
Orbit of Pluto – polar view. This 'view from above' shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and also shows how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic. The positions of both are marked as of April 16, 2006; in April 2007 they will have changed by about 1 pixel.

Near perihelion, Pluto gets closer to the Sun than Neptune; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence lasted only fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was closer to the Sun than Neptune 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 orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity starts 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 are 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 with 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.

Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named " plutinos", after Pluto.

Trans-Neptunian object

This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance to facilitate comparison. The closest approach is in 1896.
This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance to facilitate comparison. The closest approach is in 1896.

Pluto's orbit is often described as 'crossing' that of Neptune. In fact, Pluto's nodes (the points at which the orbit crosses the ecliptic) are both situated outside Neptune’s orbit and are separated by a distance of 6.4 AU (that is, over six times the distance of the Earth from the Sun). Furthermore, due to the orbital resonance between them, Pluto executes 2 full cycles while Neptune makes 3; this means that when Neptune reaches the 'closest' point on the orbit, Pluto remains far behind and when Pluto in turn reaches that point, Neptune is far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this point in its orbit.

The actual closest approach between Neptune and Pluto occurs at the opposite part of the orbit, some 30 years after Pluto's aphelion (its last aphelion was in 1866) when Neptune catches up with Pluto (i.e. Neptune and Pluto have similar longitudes). The minimum distance was 18.9 AU in June 1896. In other words, Pluto never approaches Neptune much closer than it approaches Saturn.

Comet comparison

The Kuiper belt is believed to be the source for all short-period comets, and Pluto, like other Kuiper Belt objects, shares features in common with comets. The solar wind is gradually blowing Pluto's surface into space, in the manner of a comet. If Pluto were placed near the Sun, it would develop a tail, like comets do.


Pluto and its three known moons. Pluto and Charon are the bright objects in the center, the two smaller moons are at the right and bottom, farther out.
Pluto and its three known moons. Pluto and Charon are the bright objects in the centre, the two smaller moons are at the right and bottom, farther out.

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.


The Pluto-Charon system is noteworthy for being the largest of the few binary systems in the solar system, i.e. the barycenter lies above the primary's surface ( 617 Patroclus is a smaller example). This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet. The system is also unusual among planetary systems in that they are both tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.

Some researchers have theorized that Pluto and Charon were moons of Neptune that were knocked out of Neptunian orbit when Triton was captured. 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. Today it is widely accepted that Pluto never orbited Neptune.

Pluto and Charon, compared to Earth's Moon

( Pronunciation key)

Orbital radius (km) Orbital period (days)
Pluto ploo'-toe
(65% Moon)
(18% Moon)
(0.6% Moon)
(25% Moon)
Charon shair'-ən
(35% Moon)
(2% Moon)
(5% Moon)

Nix and Hydra

Diagram of the Plutonian system. P 1 is Hydra, and P 2 is Nix.
Diagram of the Plutonian system. P 1 is Hydra, and P 2 is Nix.

Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, and received provisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially christened Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometers from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.

Observations of Nix and Hydra are ongoing to determine individual characteristics. Hydra is sometimes brighter than Nix, speculating that it either is larger in dimension or different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity with Charon suggests a 35% albedo similar to Charon's; this results in diameter estimates of 46 kilometers for Nix and 61 kilometers for brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km respectively. At the larger end of this range, the inferred masses are less than 0.3% of Charon's mass, or 0.03% of Pluto's.

With the discovery of the two small moons, Pluto may possess a variable ring system. Small body impacts can create debris that can form into a ring system. Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggests that no ring system is present. If such a system exists, it is either tenuous like the Rings of Jupiter, or it is tightly confined to less than 1000km in width.


Artist's concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on left).
Artist's concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on left).

The distribution of Plutonian moons is highly unusual compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius (stable gravitational zone of influence) of 6.0 million kilometers. In simple terms, an imaginary sphere is drawn around an object to represent the potential of an object to have other objects orbit it stably. For example, Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty."

Additional moons?

In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometers.

Exploration of Pluto

Photo of New Horizons, the first probe to Pluto, launched on January 19, 2006 (it is planned to reach Pluto in July 2015)
Photo of New Horizons, the first probe to Pluto, launched on January 19, 2006 (it is planned to reach Pluto in July 2015)

Pluto presents significant challenges for space craft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, which resulted in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.

The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. The craft will benefit from a gravity assist from Jupiter, and the closest approach to Pluto will be on July 14, 2015. Observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter.

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, 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. New Horizons will also photograph the surfaces of Pluto and Charon. The ashes of Pluto's discoverer, Clyde W. Tombaugh, are aboard the spacecraft.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. With the relatively low escape velocity of Nix and Hydra, collisions with Kuiper belt debris may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for micrometeorite damage that could damage or disable the probe.

Planetary status controversy

Pluto's official status as a planet has been a constant subject of controversy, fueled by the past lack of a clear definition of planet, since at least as early as 1992, when the first Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further discoveries intensified the debate in the 21st century.

Omission from museum models

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of 8 planets without Pluto. The controversy made headlines in the media at the time.

Commemoration as a planet

Pluto is shown as a planet on the Pioneer plaque, an inscription on the space probes Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give information about the origin of the probes to any alien civilization that might in the future encounter the vehicles, includes a diagram of our solar system, showing nine planets. Similarly, an analog image contained within the Voyager Golden Record included on the probes Voyager 1 and Voyager 2 (also launched in the 1970s) includes data regarding Pluto and again shows it as the ninth planet.

Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after Uranus, Neptune, and Pluto.

New discoveries ignite debate

Pluto compared to Eris, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, and Varuna compared to Earth (artist's impressions; no detailed photographs exist).
Pluto compared to Eris, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, and Varuna compared to Earth (artist's impressions; no detailed photographs exist).

Continuing advances in telescope technology allowed for further discoveries of Trans-Neptunian objects in the 21st century, some of comparable size to that of Pluto. In 2002, 50000 Quaoar was discovered, with a 1,280 kilometers diameter, making it a bit more than half the size of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit of 1,800 kilometers on its diameter, near Pluto's diameter of 2,320 kilometers.

On July 29, 2005, a Trans-Neptunian object called Eris was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be slightly larger than Pluto. This was the largest object discovered in the solar system since Neptune in 1846. Discoverers and media initially called it the "tenth planet", although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery to be the strongest argument for reclassifying Pluto as a minor planet.

The last remaining distinguishing feature of Pluto was now its large moon, Charon, and its atmosphere; these characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites; and Eris' spectrum suggests that it has a similar surface composition to Pluto, as well as a moon, Dysnomia, discovered in September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind Eris, Pluto, and 2005 FY9 (nicknamed "Easterbunny").

IAU Decision

There are three main conditions for an object to be called a 'planet', according to the IAU resolution passed August 24, 2006.

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighbourhood around its orbit.

Pluto fails to meet the third condition.

The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as prototype for a yet-to-be-named category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.

Prior to this decision several other definitions had been proposed, some of which might have ruled out planetary status for Earth or Mercury or may have classified several of the asteroids as planets. This version was democratically chosen in a successful attempt at avoiding these non-traditional results.

Impact of the IAU decision

There has been resistance amongst the astronomical community towards the reclassification. Alan Stern, principal investigator with NASA's " New Horizons" mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks" albeit "for technical reasons." Stern's current contention is that by the terms of the new definition Earth, Mars, Jupiter and Neptune, all of which share their orbits with asteroids would be excluded. However, his own published writing has supported the new list of planets, as "our solar system clearly contains" eight planets that have cleared their neighborhoods. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."

Among the general public, reception is mixed amidst widespread media coverage. Some have accepted the reclassification, while some are seeking to overturn the decision, with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes. Others reject the change for sentimental reasons, citing that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.

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