Pluto

Pluto (IPA: BrE /ˈpluːtəʊ/, AmE /ˈplutoʊ/), also designated 134340 Pluto (see minor planet names), is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun. Originally considered a planet, it has since been recognised as the largest member of a distinct region called the Kuiper Belt. Approximately one-fifth the mass of the Earth's Moon, Pluto is smaller than seven of the Solar System's natural satellites. It is primarily composed of rock and ice. It has an eccentric orbit that takes it from 29 to 49 AU (4.3 to 7.3 billion km) from the Sun, and is highly inclined with respect to the planets. As a result, Pluto occasionally comes closer to the Sun than Neptune.

Pluto and its largest satellite, Charon, are often considered a binary system because they are closer in size than any of the other known celestial pair in the solar system, and because the barycenter of their orbits does not lie within either body.^[3] However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, and until it passes such a ruling, Charon remains a moon of Pluto.^[4] Pluto has two smaller moons, Nix and Hydra, discovered in 2005.^[5].

From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries however, many objects similar to Pluto were discovered in the outer solar system, most notably the trans-Neptunian object Eris which is slightly larger than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which was then reclassified under the new category of dwarf planet along with Eris and Ceres.^[6] Pluto is also classified as the prototype of a family of trans-Neptunian objects.^[7]^[8] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.^[9]^[10]

Discovery

Discovery photographs of Pluto

In 1930 Clyde Tombaugh was searching for a ninth planet as part of a project at Lowell Observatory. Tombaugh's work was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine if any objects had shifted position in that time. 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.^[11]

Incorrect prediction

The history of how Pluto was discovered is intertwined with the discoveries of Neptune and Uranus. In the 1840s, using Newtonian mechanics, both Urbain Le Verrier and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analyzing 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.^[12]

Observations of Neptune in the late 19th century had astronomers starting to speculate that Uranus's orbit was also being disturbed by another planet in addition to Neptune. In 1905, Lowell Observatory (founded by Percival Lowell in 1894) started an extensive project in search of a possible ninth planet, which Lowell termed "Planet X".^[13] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.^[14] The work continued after Lowell's death in 1916.

Pluto is too small to have the effect on Uranus's orbit that initiated the search. After the flyby of Neptune by Voyager 2 in 1989, it was conclusively demonstrated that the discrepancies in Uranus'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, which makes Tombaugh's discovery even more surprising.^[15]

Naming

Venetia Burney, the girl who named Pluto

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.^[13] 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.^[16]

The name Pluto was first suggested by Venetia Burney (later Venetia Phair), an eleven-year-old girl from Oxford, England.^[17] Venetia, who was interested in Classical mythology as well as astronomy, considered the name, the Roman equivalent of Hades, appropriate for such a presumably dark and cold world. She suggested it 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, who then cabled the suggestion to colleagues in America.^[18]

The object was officially named on March 24, 1930.^[19] Each member of the Lowell Observatory was allowed to vote on a shortlist of three: "Minerva" (which was already the name for an asteroid), "Cronus" (which had garnered a bad reputation after being suggested by an unpopular astronomer named Thomas Jefferson Jackson See) and Pluto. Pluto received every single vote.^[20] The name was announced on May 1, 1930.^[17] Upon the announcement, Madan gave Venetia five pounds as a reward.^[17]

The name that was 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, a desire echoed in the P-L monogram that is Pluto's astronomical symbol ().^[21] Pluto's astrological symbol resembles that of Neptune ( ), but has a circle in place of the middle prong of the trident ( ).

In the Chinese, Japanese, and Korean languages, the name was translated as underworld king star (冥王星), suggested by Houei Nojiri in 1930. In Vietnamese it is named after Yama (Sao Diêm Vương), the Guardian of Hell in Buddhist mythology. Yama (Devanāgarī यम) is also used in India, as it is the deity of Hell in Hindu mythologies.

Physical characteristics

The largest plutinos compared in size, albedo and colour.

Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.^[22]

Appearance

Pluto's mean apparent magnitude is 15.1 with a maximum of 13.56.^[23] To see it, a telescope of around 30 cm aperture is desirable.^[24] 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.^[25]

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.^[26] 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.^[27]

Mass and size

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

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1955, Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars.^[28] However, in 1976, David Cuikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, and found it matched that for methane ice, which meant Pluto had to be exceptionally bright, and therefore could not be more than 1 percent the mass of the Earth.^[28]^[29]

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

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. Once Charon's gravitational effect on Pluto was measured, estimates of Pluto's mass fell to 13.1 Yg; less than 0.24 percent that of the Earth.^[30] Observations were able to determine Pluto's diameter when it is at occultation with Charon.^[31] With the invention of adaptive optics astronomers were able to accurately determine its shape.^[32] Pluto's diameter is 2,390 km.

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 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 the dwarf planet Eris, a trans-Neptunian object discovered in 2005. See List of solar system objects by mass and List of solar system objects by radius.

Atmosphere

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 increases, 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,^[33] by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins colder than they expected.

Pluto was found to have an atmosphere from an occultation observation in 1985; 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.^[34] From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of Earth.^[35]

In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy of the Paris Observatory^[36] and by James Elliot of MIT^[37] and Jay Pasachoff of Williams College.^[38] 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 most widely accepted hypothesis to explain this discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years; as a result extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.^[39] Another stellar occultation was observed by the MIT-Williams College team of James Elliot and Jay Pasachoff and a Southwest Research Institute team led by Leslie Young on 12 June 2006 from sites in Australia.^[40]

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.^[41]

Composition

Possible structure of Pluto.
1. Frozen nitrogen
2. Water ice
3. Silicate and water ice

Spectroscopic analysis of Pluto's surface reveals it is composed of over 98 percent nitrogen ice, with traces of methane and carbon monoxide.^[42]^[43] It 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 contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice. This makes Pluto the second most contrasted body in the Solar System after Iapetus.^[44] The Hubble Space Telescope places Pluto's density at between 1.8 and 2.1 g/cm³, suggesting its internal composition consists of roughly 50-70 percent rock and 50-30 percent ices.^[43] Because decay of radioactive minerals would eventually heat the ices enough for them to separate from rock, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. It is also possible that such heating may continue into the present time, creating a subsurface ocean of liquid water.^[45]

Orbit

Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in red) shows its large inclination to Neptune's orbit (in blue). The ecliptic is horizontal

Pluto's orbit is markedly different to those of the planets. The planets all orbit the Sun close to a flat reference plane called the ecliptic, and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it again lasted for 20 years.

Although this repeating pattern may suggest a regular structure in the short term (measured over several millennia), Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forwards and backwards in time), after intervals longer than 10-20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurable details of the present state of the solar system.^[46]^[47] For example, at some specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere inbetween), with no way for us to predict which. This does not mean that the orbit of Pluto itself is unstable, however; only that its position along that orbit is impossible to determine far into the future. In fact, several resonances and other dynamical effects conspire to keep Pluto's orbit stable, safe from planetary collision or scattering.

Neptune-avoiding orbit

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 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 bodies are on April 16, 2006; by April 2007 they will have changed by about 3 pixels (~1 AU).

Despite Pluto's orbit apparently crossing that of Neptune when viewed from directly above the ecliptic, the two objects cannot collide. This is because their orbits are aligned so that Pluto and Neptune can never approach closely. Several factors contribute to this.

At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed in a top-down projection (right), it is also the farthest above the ecliptic. This means Pluto's orbit actually passes above that of Neptune, preventing a collision.^[48] Indeed, the part of Pluto's orbit that lies as close or closer to the Sun than that of Neptune lies about 8 AU above the ecliptic,^[49] and so a similar distance above Neptune's orbit.^[50] Pluto's ascending node, the point at which the orbit crosses the ecliptic, is currently separated from Neptune's by over 21°;^[51] their descending nodes are separated by a similar angular distance (see diagram). Since Neptune's orbit is almost flat with respect to the ecliptic, Pluto is far above it by the time the two orbits cross.

However, this alone is not enough to protect Pluto; perturbations from the planets, particularly Neptune, would adjust Pluto's orbit (e.g. orbital precession), so that over millions of years a collision could be possible. Some other effect(s) must therefore be in place. The most significant of these is a mean motion resonance with Neptune.

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 between them to emphasize the closest approach in 1896.

Pluto lies in the 3:2 mean motion resonance of Neptune: for every 3 orbits of Neptune around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time that Pluto is near perihelion Neptune is around a quarter of an orbit (over 50°) "behind" Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance "ahead" of Pluto. In fact, the minimum separation of Pluto and Neptune is over 17 AU; Pluto actually comes closer (11 AU) to Uranus than it does to Neptune.^[50]

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years.^[52] This prevents their orbits from changing relative to one another - the cycle always repeats in the same way - and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined, the two bodies could never collide.^[50]

Other factors governing Pluto's orbit

Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto's and Neptune's orbits does not change.^[48]^[53] However, there are several other resonances and interactions which govern the details of their relative motion, and enhance Pluto's stability. These can principally be divided into arising from one of two further mechanisms (in addition to the 3:2 mean motion resonance).

Firstly, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°.^[53] This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the solar system, preventing encounters with Neptune. This is a consequence of the Kozai mechanism.^[48] Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (=90°-38°). The closest such angular separation occurs every 10,000 years.^[52]

Secondly, the longitudes of ascending node of the two bodies - the points where they cross the ecliptic - are in near-resonance with the above libration. When the two longitudes are the same - that is, when one could draw a straight line through both nodes and the Sun - Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance.^[48]

To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, in accordance with Kepler's third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune once again at the original node. The whole process takes about 20,000 years to complete.^[50]^[52]

Moons

Main article: Pluto's natural satellites

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 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.^[54]

The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius, the stable gravitational zone of Pluto's influence. 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."^[55]

Charon

The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, i.e. whose barycenter lies above the primary's surface (617 Patroclus is a smaller example).^[56] This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet.^[57] 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.^[58]

Pluto and Charon, compared to Earth's Moon
Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km) (barycentric)	Orbital period (d)
Pluto	ploo'-toe /ˈpluːtəʊ/	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)	6.3872 (25% Moon)

Nix and Hydra

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 named 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.^[59]

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.^[60]

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

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.^[61]

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.^[62]

Possibility of 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.^[63]

Kuiper belt

Main article: Kuiper belt

Artist's conception of the Kuiper belt, with Pluto's orbit (yellow) contrasted with Neptune's (white)

Pluto's origin and identity have long puzzled astronomers. In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by Triton, Neptune's largest moon. This notion has been heavily criticised, since, as explained above, Pluto never actually comes near the planet in its orbit.^[64]

Beginning in 1992, astronomers began to discover a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt after one of the astronomers who first hypothesised its existence, is believed to be the source for all short-period comets. Astronomers now believe Pluto to be the largest of the Kuiper belt objects. 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.^[65] If Pluto were placed near the Sun, it would develop a tail, like comets do.^[66]

Though Pluto is the largest of the Kuiper belt objects discovered so far, Triton, which is slightly larger than Pluto, shares many atmospherical and geological composition similarities with Pluto and is believed to be a captured Kuiper belt object.^[67] Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.^[68]

Exploration of Pluto

Main article: New Horizons

New Horizons, launched on January 19, 2006

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.^[69] In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.^[70]

First Pluto sighting from New Horizons

The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. In early 2007 the craft made use of a gravity assist from Jupiter, and its closest approach to Pluto will be on July 14, 2015. Scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).^[71] The images, taken from a distance of approximately 4.2 billion kilometres (2.6 billion miles), confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

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. Some of 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.^[62]

Planetary status controversy

See also: Definition of planet

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.

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.^[72] 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.^[73] Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after Uranus, Neptune, and Pluto. The Disney character Pluto, introduced in 1930, was also named in honour of the planet.^[74]

New discoveries ignite debate

Pluto compared to Eris, (136472) 2005 FY9, (136108) 2003 EL61, Sedna, Orcus, Quaoar, and Varuna compared to Earth (artist's impressions; no detailed photographs exist).

Pluto compared to Eris, (136472) 2005 FY₉, (136108) 2003 EL₆₁, Sedna, Orcus, Quaoar, and Varuna compared to Earth (artist's impressions; no detailed photographs exist).

The discovery of the Kuiper belt and Pluto's relation to it led many to question whether Pluto could be considered separately from others in its population. In 2002, the KBO 50000 Quaoar was discovered, with a diameter of roughly 1,280 kilometres, about half that of Pluto.^[75] In 2004, the discoverers of 90377 Sedna placed an upper limit of 1,800 kilometres on its diameter, near Pluto's diameter of 2,320 kilometres.^[76]

On July 29, 2005, a Trans-Neptunian object later named Eris was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be slightly larger than Pluto.^[77] 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.^[78] Others in the astronomical community considered the discovery to be the strongest argument for reclassifying Pluto as a minor planet.^[79]

The last remaining distinguishing features of Pluto were 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.^[80] It also possesses 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").

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.^[81]

IAU Decision

Main article: 2006 redefinition of planet

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

The object must be in orbit around the Sun.
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.
It must have cleared the neighbourhood around its orbit.^[82]

Pluto fails to meet the third condition.^[83] 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.

There has been resistance amongst the astronomical community towards the reclassification,^[84] dubbed the "Great Pluto War" by some astronomers.^[85]^[86]

New Horizons spacecraft

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."^[87] 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.^[88] However, his own published writing has supported the new list of planets, as "our solar system clearly contains" eight planets that have cleared their neighbourhoods, however, he does not deny planetary classification to objects like Pluto & Ceres that have not "cleared their neighbourhood".^[89] Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitoners against the definition.^[90] 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."^[91]

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.^[92] The U.S. state of New Mexico's House of Representatives passed a resolution declaring that Pluto will always be considered a planet while overhead of the state, with March 13th being known as "Pluto Planet Day".^[93] 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.^[94]

The verb "pluto" (preterite and past participle: "plutoed") was coined in the aftermath of the decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", an example being "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."^[95]

References

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External links

Find more information on Pluto by searching Wikipedia's sister projects
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Astronomy Education Review, Vol. 5 No. 2: Teaching What a Planet Is: A Roundtable on the Educational Implications of the New Definition of a Planet
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.
"Why Pluto Had To Go" – An opinion piece by astronomer Bryan Gaensler in Cosmos Magazine.* 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
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.
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.
NPR segment discussing the linguistic and cultural aspects of Pluto's planethood.
Experts meet to decide Pluto fate - BBC News 14th August 2006
Geologists fight for their plutons
Pluto died much as it lived -- famous, but an oddball
IAU General Secretary, Dr Luboš Perek's view of the decision

v • d • e Moons of Pluto and Eris
Pluto's moons Charon · Nix · Hydra Eris Dysnomia

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Small bodies: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Astronomy Portal