Pluto

For other uses of this name, including in Greek mythology, see Pluto (disambiguation).

Pluto is a celestial body in the solar system, and is the prototype of a yet-to-be-named family of trans-Neptunian objects.

From its discovery in 1930 Pluto was considered the ninth planet of the solar system until it was determined on August 24, 2006 by the International Astronomical Union (IAU) that it is not a true planet.

After recent discoveries of other trans-Neptunian objects, most notably 2003 UB313, which is larger than Pluto, the IAU redefined the term 'planet'; this resulted in Pluto being reclassified as a dwarf planet.

Pluto 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. It is smaller than several natural satellites or moons in our solar system (see the list of solar system objects by radius). Pluto itself has a large moon named Charon (the ratio of the size of Charon to that of Pluto is the largest such ratio known in our Solar System, but see below); two smaller moons named Nix and Hydra were discovered in 2005.

The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first spacecraft from Earth to fly by Pluto; ETA is July 14 2015.

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 place from where, after initiating several earlier searches, Pluto was eventually discovered by Clyde Tombaugh. (Another symbol sometimes used for Pluto is an astrological symbol and not an astronomical one. It resembles that of Neptune, ♆, but has a circle in place of the middle prong of the trident.)

Pluto and its satellite Charon have often been considered a binary planet because they are more nearly equal in size than any other planet/moon combination in the Solar System, and because the two bodies orbit a point not within the surface of either. Under the rejected planet definition proposal, since they orbit each other around a center of mass that is outside either body, they would have been officially considered a binary planet system.

Discovery

Pluto's diameter is about 18% that of Earth.

The story and history of how Pluto was discovered begins with the discovery of Neptune. 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 the perturbations in the orbit of Uranus which could only have been caused by the gravitational pull of another massive planet. Thanks to their calculations, Neptune was discovered by Johann Gottfried Galle on September 23, 1846.

By the late 19th century, astronomers started speculating that Neptune's orbit too was being disturbed by another planet. 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 the calculations of the Indian astronomer V.B. Ketaker which predicted a location for the undiscovered planet. Although Lowell died in 1916, the search for the elusive planet continued.

Pluto was discovered after an extensive search for the hypothetical planet by the astronomer Clyde Tombaugh at Lowell Observatory in Arizona. Tombaugh systematically took pictures of the plane of the solar system: he took pictures in pairs, one or two weeks apart, and looked for objects that had moved. On February 18, 1930, Tombaugh was comparing photographic plates taken on January 23 and January 29 and discovered an object that had moved (he also referenced a lesser-quality photo taken on January 20 to confirm this movement). After the observatory obtained confirmatory photographs, the news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Pluto was later found on photographs dating back to March 19, 1915. Its mean distance from Earth and its mean daily motion turned out to be 39.48 AU and 14.283".

Ironically, Pluto is far too small to have the effect on Neptune's orbit that initiated the search. The discrepancies in Neptune's orbit observed by 19th century astronomers were due instead to an inaccurate estimate of Neptune's mass. Tombaugh's discovery is therefore even more surprising, given that Pluto's proximity to the region predicted by Pickering, Lowell and Ketakar was a coincidence.

File:Discovery Pluto.png

Discovery photographs of Pluto

Name

The right to name the new object belonged to the 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 object 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, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids.

File:Venetia phair.jpg

Venetia Burney, the girl who named Pluto

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, who predicted that a planet would be found beyond Neptune. The name was first suggested by Venetia Phair (née Burney), at the time an eleven-year-old girl from Oxford, England. Over the breakfast table one morning her grandfather Falconer Madan, a former librarian of Oxford University's Bodleian Library, was reading about the discovery of the new planet in the Times newspaper. Venetia, who was interested in Classical mythology as well as astronomy, suggested Pluto. 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 1930-05-01.

In the Chinese, Japanese, and Korean languages, the name was translated as Star of the King of the Dead (冥王星). In Vietnamese it is named after Yama (阎王星 Diêm Vương Tinh), the Guardian of Hell in Hindu traditions.

Orbit

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 – 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 16th, 2006; in April 2007 they will have barely changed by about 1 pixel.

Pluto's orbit is very unusual in comparison to the planets in 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

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 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.

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.

Distance from Neptune

A view from around 10° above the ecliptic, showing the closest points of Pluto's and Neptune's orbits (in red and blue respectively). They are separated by about 6.4 AU.

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 the 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.

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.

Physical characteristics

More than seventy-six years after its discovery, many facts about Pluto remain unknown, mainly due to the fact that it has not been visited by 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.

Appearance

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 30 cm aperture is desirable. It looks star-like even in very large telescopes because its angular diameter is only 0.15″. The color of Pluto is light brown with a very slight tint of yellow.

Because of its distance, it is nearly impossible to directly photograph surface details on Pluto. Even Hubble Space Telescope images barely show any surface markings. The best images of Pluto derive from brightness maps created from observations of eclipses by its largest moon, Charon: eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a gray spot. Using this technique, one only has to measure the total average brightness of the Pluto-Charon system and track changes in brightness over time. Computer processing is then used to match brightness changes against the known position of the eclipsing moon.

Mass and size

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

File:Pluto to moon.png

Pluto to the Moon.

Pluto's mass and diameter could only be estimated for many decades after its discovery. Initially it was thought to be quite large, with a mass comparable to Earth, but over time the estimates were revised sharply downward as observations were refined. Indeed, in 1980 A.J. Dessler published a tongue-in-cheek analysis of Pluto's historical mass estimates and predicted that it would soon disappear completely.

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 disc can now be resolved by telescopes using adaptive optics.

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 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, a dwarf 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.

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

Atmosphere

Pluto's thin atmosphere is most likely made up of nitrogen, methane, 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 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 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 Jim 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.

Composition

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.

Pluto's moons

Main article: Pluto's natural satellites

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

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

Charon

The Pluto-Charon system is noteworthy for being the only system in the solar system whose barycenter lies above the primary's surface. This and the large size of Charon relative to Pluto prompted some astronomers to label it a dwarf double planet.

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.

File:Plutoncharon.jpg

Pluto and its largest satellite Charon

Charon's discovery also resulted in the calculation of Pluto's albedo being revised upward; since Pluto 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.

Pluto and Charon, compared to Earth's Moon
Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (days)
Pluto	ploo'-toe /ˈpluːtɔʊ/	2306 (65% Moon)	1.3×1022 (18% Moon)	2390 (0.6% Moon)	6.3872 (25% Moon)
Charon	shair'-ən /ˈʃɛərən/	1205 (35% Moon)	1.5×1021 (2% Moon)	19,570 (5% Moon)

Previously, some researchers had theorized that Pluto and its moon 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.

An occultation of a star by Charon in 2005, observed in South America by teams from MIT-Williams College, the Paris Observatory, and the Southwest Research Institute has led to improved knowledge of Charon's parameters.

Nix and Hydra

File:Plutonianmoons.jpg

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.

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

Two additional moons 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. They were confirmed with a series of "precovery" Hubble images from June 2002 through May 2003, which led to their orbits being determined. Additional follow-up observations were made in February and March 2006, confirming the orbit solutions. 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.

Hydra is sometimes brighter than Nix and thus may be larger in some dimensions; alternately different parts of its surface may vary in brightness. Their sizes are not known but can be estimated from likely 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 the brighter orientation of Hydra. Likely 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 time variable ring system. Small body impacts can create debris that can form into a ring system. After deep optical survey from the Advanced Camera for Surveys, ACS, of the Hubble Space Telescope, no ring system was detected. 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. This may become a problem for the New Horizons space probe. There is potential for micrometeorite damage that can damage or disable the probe.

Limits on additional moons

In imaging the Plutonian system, the Hubble observations placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km¹ exist beyond the glare of Pluto 5 arcseconds from the dwarf planet, assuming Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometers.

¹Or a maximum of 37 kilometers with an albedo of 0.04

This is a striking distribution. Moons could potentially orbit Pluto up to the 53% (or 69%, if retrograde) of the Hill sphere radius (stable gravitational zone of influence) of 6.0 million kilometers. 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."

Exploration of Pluto

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

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 Charon or Pluto's size and atmosphere — the flyby was scrapped in order to facilitate a close flyby of Saturn's moon Titan.

In 1989, NASA also considered sending Voyager 2 to Pluto using a gravitational assist from Neptune, but this would not permit the best flyby of Triton for scientific observations. It was eventually decided to calculate the Neptune and Triton visit for the best results around that planet, regardless of the consequences to the craft's final trajectory.

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 launched on January 19, 2006 (while Pluto was still classified as a Planet). It will benefit from a gravity assist from Jupiter, and the closest approach to Pluto will be on July 14, 2015, 11:59 UTC. Observations of Pluto will start 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 would also photograph the surfaces of Pluto and Charon. 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. If launched, Pluto Kuiper Express was to reach Pluto around 2012, and the Kuiper Belt around 2017.

Planetary status

Planet X?

Main article: Planet X

Pluto was originally discovered in 1930 in the course of a search for a body sufficiently massive to account for perceived anomalies in the orbits of Uranus and Neptune. Once it was found, its 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 retained today. Lowell had also made earlier, different predictions of Planet X's position beginning in 1902.

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.

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 greater mass than previously believed and that when the updated 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 diligence of Tombaugh's search.

Reclassification to a dwarf 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 has been revoked.

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. Due to its small size and unusual orbit, however, there is some debate regarding Pluto's classification as a major or as a minor planet, which included some momentum for recognizing dual status. Pluto is one of the largest known members of the main Kuiper Belt, the area of the solar system beyond the orbit of Neptune; one scattered Kuiper Belt object, 2003 UB313, has been found that is larger than Pluto, and they have several companions (as of August 2006 there are 783 KBOs) in the Kuiper Belt, including 2003 EL61 and 2005 FY9.

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 sublimating 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.

Hundreds of other Kuiper Belt objects have been discovered since September 1992, the first being (15760) 1992 QB1. The continued discovery of these objects, especially that of plutinos, rekindled the debate on Pluto's status as 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 also. 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 Johannes Andersen 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 centered on whether a "planet", from the Greek for "wanderer", is an appellation that depends upon an object's particular size, formation, or orbit. Some argued that not only was Pluto a major planet but also some moons like Titan, Europa or Triton, or even the larger asteroids were as well. Others argued that the term "minor planet" should refer to an astronomical object more than about 360 kilometers in diameter, at which point the object has a tendency to become round under its own gravity; 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 4,879.4 kilometres and Ceres, the largest known asteroid with a mean diameter of 950 kilometers. This definition would include Pluto but not most moons.

Pluto had qualified as a planet under a draft definition that had been proposed by a committee of the IAU, but which was not accepted by the IAU General Assembly. The re-classification adopted by the assembly, however, has been disputed by some members who had left the conference on the grounds that the timetable had been "hijacked" with only 4% of eligible voters being left in the conference room at the time the resolution was adopted .

New discoveries

Pluto compared to 2003 UB313, 2005 FY9, 2003 EL61, Sedna, Quaoar, and Earth

Continuing discoveries in the Trans-Neptunian region kept rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1,280 kilometers diameter, making it a bit more than half the size of Pluto. Another discovery, 90482 Orcus, was probably even larger. In 2004 the discoverers of 90377 Sedna, an extremely distant object well beyond the other known Trans-Neptunian objects, placed an upper limit of 1,800 kilometers on its diameter, close to Pluto's 2,320 kilometers.

On July 29, 2005, a Trans-Neptunian object called 2003 UB313 (nicknamed "Xena") was announced, which on the basis of its magnitude and simple albedo considerations was assumed to be larger than Pluto. This caused its discoverers to call it the "10th planet" of the solar system, although there was no consensus at the time on whether to call it a planet, and others considered the discovery to be the strongest argument for demoting Pluto to the status of a minor planet. 2003 UB₃₁₃ was the largest object yet discovered in the solar system since Neptune in 1846, although it is only slightly larger than Pluto. 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 2003 UB₃₁₃'s spectrum suggests that it has a similar surface composition to Pluto, as well as a moon (nicknamed "Gabrielle"), 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 2003 UB313, Pluto, and 2005 FY9 (nicknamed "Easterbunny").

Precedents for reclassifying planets

There is historical precedent for reclassifying what was originally called a planet, in the light of subsequent discoveries. Many bodies in the asteroid belt (including 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta) were considered to be planets for several decades (in part because their sizes were not accurately known at the time). In 1845, however, an asteroid (5 Astraea) was discovered for the first time in thirty-eight years. Neptune was discovered a year later, 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" (though Ceres, along with Pluto, was recently reclassified as a "dwarf planet"). Some see in this a precedent for noting that Pluto is just the most prominent member of the Kuiper belt.

As its discoverer reportedly said before his death in 1997: "It's there. Whatever it is. It is there."

International Astronomical Union meeting

Main article: 2006 redefinition of planet

In order to put these matters to rest, in August 2006, 3,000 astronomers and scientists of the International Astronomical Union (IAU) gathered in Prague, Czech Republic to deliberate. The organization planned to publish an official definition of "planet", ruling on whether to call Pluto a planet, dwarf planet or a Kuiper Belt Object (KBO). The draft proposal classified Pluto along with 2003 UB313 and any other spherical TNOs that may be discovered, as planets, although they would have been informally labelled 'plutons'. The asteroid 1 Ceres and Pluto's moon, Charon, would also have been considered planets.

On August 24, 2006, however, the previous draft was reversed, according to the newly passed rule, Pluto was demoted from planetary status to a dwarf planet and is accompanied by Ceres and 2003 UB313 in this category. There are three main conditions for an object to be called a 'planet', according to the IAU resolution.

1. The object must be in orbit around a star, but not be a star itself.
2. The object must be massive enough to be a sphere by its own gravitational force.
3. It must have cleared the neighborhood around its orbit.

Pluto fails to meet these conditions (primarily, condition 3) but will act as a prototype for a yet to be named category of Trans-Neptunian objects.

See also

• Pluto in popular culture
• Pluto in astrology
• Solar eclipses on Pluto
• Aspects of Pluto

References

• Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". New Straits Times, p. F17.
• Kaufmann III, William J., "Universe", 2nd Edition, pp.302–303
• Chhabra et al., "Prediction of Pluto by Ketakar", Indian Journal of History of Science, 19(1), pp.18–26, 1984

External links

• NASA's Pluto fact sheet
• Lowell Observatory History, Mission, Important Discoveries
• David H. Freedman, "When is a Planet Not a Planet?", Atlantic Monthly, February 1998.
• NASA's Pluto-Kuiper Belt Mission – launching in early 2006.
• Pluto in Science Fiction – Bibliography of science fiction which is set on Pluto.
• Windows to the universe – Pluto
• Space.com article on second and third moons
• NASA's Hubble Reveals Possible New Moons Around Pluto – Hubble Press Release
• Possible New Moons
• Elliot et al., The recent expansion of Pluto's atmosphere
• Pasachoff et al., "The Structure of Pluto's Atmosphere from the 2002 August 21 Stellar Occultation," Astron. J., 129, 1718-1723.
• The Girl who named a Planet – BBC News
• Planets – Pluto A kid's guide to Pluto.
• A simulation of Pluto's orbit, showing its resonance with Neptune.
• NPR article on IAU's anticipated recommendation of Pluto's status as a planet.
• Experts meet to decide Pluto fate - BBC News 14th August 2006
• Geologists fight for their plutons