Pluto (planet)

Pluto in True Color

Brightness map of Pluto

Discovered by Clyde W. Tombaugh
Discovered on February 18, 1930
Orbital characteristics (Epoch J2000)
Semi-major axis 5,906,376,272 km
39.481 686 77 AU
Orbital circumference 36.530 Tm
244.186 AU
Eccentricity 0.248 807 66
Perihelion 4,436,824,613 km
29.658 340 67 AU
Aphelion 7,375,927,931 km
49.305 032 87 AU
Orbital period 90,613.3058 d (248.09 a)
Synodic period 366.74 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 1
Physical characteristics
Diameter 2390 km
(0.180 Earths)
Surface area 1.795×107 km2
(0.033 Earths)
Volume 7.15×109 km3
(0.0066 Earths)
Mass 1.25×1022 kg
(0.0021 Earths)
Mean density 1.750 g/cm3
Equatorial gravity 0.58 m/s2
(0.059 gee)
Escape velocity 1.2 km/s
Rotation period 6.387 d (6 d 9 h 17.6 min)
Rotation velocity 47.18 km/h (at the equator)
Axial tilt 122.54° (to orbit)
115.60? (to the ecliptic)[1] (
Right ascension
of North pole
133.02° (8 h 52 min 5 s)
Declination -9.09°
Albedo 0.30
Surface temp.
min mean max
33 K 44 K 55 K
Atmospheric characteristics
Atmospheric pressure 0.15-0.30 pascals
Composition nitrogen, methane

Pluto is the ninth planet from the Sun in our solar system. Because Pluto is also the smallest planet in our solar system and has a highly eccentric orbit (which takes it inside the orbit of Neptune) there has been some debate regarding whether Pluto should be classified as a planet (see below for details). Its symbol is a combination of the name's first two letters, "P-L", which are also Percival Lowell's initials (Unicode: ♇ ), although some prefer a different symbol resembling that of Neptune but with a circle in the top center for the middle spoke.


Discovery and naming

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

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

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

Physical characteristics

Mass and size

Pluto is not only much smaller and less massive than every other planet, it is also smaller and less massive than seven moons of other planets: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is larger than any minor planet in the main asteroid belt, or any minor planet so far discovered in the trans-Neptunian Kuiper belt. See List of solar system objects by mass and List of solar system objects by radius.

Pluto's mass and diameter were unknown for many decades after its discovery and could only be estimated. The discovery of its satellite Charon permitted determining the mass for the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Meanwhile, its diameter is now known since telescopes using adaptive optics can resolve its disk.

Eccentric orbit

Pluto's highly eccentric orbit makes it the eighth-most distant planet from the Sun for part of each orbit; this most recently occurred from February 7, 1979 through February 11, 1999. Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity start to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they're almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune's 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.

Because of its small size and eccentric orbit, there has been some debate over whether it truly should be classified as a planet. There is mounting evidence that Pluto may in fact be a member of the Kuiper belt, only one of a large number of distant icy bodies. A subclass of such objects have been dubbed plutinos, after Pluto.


Pluto has an atmosphere when it is close to perihelion; the atmosphere may freeze out as Pluto moves farther from the Sun. It is thought by some that Pluto shares its atmosphere with its moon. Pluto was determined to have an atmosphere from an occultation observation in 1988. When a planet or asteroid occults a star, if it has no atmosphere, 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 pascal (Pa). This thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface.

In 2003, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy [2] ( and by Jim Elliot. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was farther away 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 in 1987 (for the first time in 120 years), and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.

Pluto's moon


Pluto has one natural satellite, Charon, first identified in 1978. Pluto and Charon are noteworthy for being the only planet/moon pair in the solar system whose barycenter lies above the planet's surface. Pluto and Charon are also unusual among planets in that they are tidally locked to each other. This means that Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon. Note that some binary asteroids may also possess both of these traits, and that the Jupiter/Sun barycenter is above the Sun's surface, so neither is completely unique.

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.

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

At one point some researchers suggested that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit, but it is now thought that Pluto was never Neptune's moon. Triton's retrograde orbit suggests that it was originally an independent body much like Pluto which was captured by Neptune. Triton also shares many atmospherical and geological composition similarities with Pluto.

Exploration of Pluto

Little is known about Pluto because of its great distance from Earth and because no exploratory spacecraft have visited Pluto yet. In 2001, NASA approved preliminary studies for a mission called "New Horizons" to Pluto, to be conducted by the Southwest Research Institute. The current schedule has it launching in 2006 and arriving at Pluto in 2015. It will weigh half a ton and will travel at speeds reaching 43,000 km/h (27,000 mph). The spacecraft would use a remote sensing package that includes imaging instruments and a radio science investigation, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and characterize Pluto's neutral atmosphere and its escape rate. The mission plan also calls for a flyby of Kuiper Belt Objects by 2022.

Originally the Voyager 1 probe was planned to visit Pluto, but due to budget cuts and lack of interest the flyby was cancelled. It was redirected for a close flyby of Saturn's moon Titan.

The Pluto debate

Planet X?

The planet Pluto was originally discovered in 1930 in the course of a search for a body sufficiently massive to account for supposed anomalies in the orbits of Uranus and Neptune. Once it was found, its faintness and failure to show a visible disc cast doubt on the idea that it could be Lowell's Planet X. Lowell had made a prediction of Pluto's position in 1915 which had turned out to be fairly close to its actual position at that time; however Ernest W. Brown concluded almost immediately that this was a coincidence, and this view is retained today. Lowell had also made earlier, different predictions of Planet X's position beginning in 1902. [3] (

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

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

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

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

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

Minor planet?

In September of 1992 scientists began discovering hundreds of other, smaller, icy bodies in the area of the solar system beyond the orbit of Neptune. These objects are now deemed members of the Kuiper belt and are accordingly known as Kuiper Belt Objects (KBOs). The continued discovery of these objects, especially of Plutinos, began a debate that goes on to this day: is Pluto a planet or simply the largest (known) example of a Kuiper belt object?

Kuiper belt objects are minor planets, so the question arose as to whether to consider Pluto to be one too. This planetary sciences debate landed in newspaper headlines, editorials, and on the Internet in early 1999. Thoughts that Pluto might be "demoted" to non-planet status created an emotional response in certain sectors of the public. Such news outlets as the BBC News Online, the Boston Globe, and USA Today all printed stories noting that the International Astronomical Union was considering dropping Pluto's planetary status. "Save Pluto" websites sprang up, and school children sent letters to astronomers and the IAU.

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

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

Some scientists argue that "planet", from the Greek for "wanderer", is a designation that does not depend upon an object's particular size, formation or orbit. Yet others argue that not only is Pluto a planet but also some moons like Titan, Europa or Triton, or even the larger asteroids. Some agree that an astronomical object more than about 360 km in diameter, at which point the object has a tendency to become round under its own gravity, should be known as a planet. This would include several moons and a handful of asteroids. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and Ceres, which would also include Pluto but not moons.

New discoveries

Continuing discoveries in the Kuiper belt and beyond keep rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1280 km diameter, making it a bit more than half the size of Pluto. Another recent discovery, 90482 Orcus, is probably even larger. On March 15, 2004, Michael E. Brown, Chadwick A. Trujillo and David L. Rabinowitz announced their discovery of 90377 Sedna, much farther out than the Kuiper belt. An upper limit of 1800 km has been placed on Sedna's diameter, close to Pluto's 2320 km. Sedna could be the largest object yet discovered in the solar system since Pluto in 1930.

Some astronomers think it is only a matter of time before a Kuiper Belt object larger than Pluto is discovered. The last remaining distinguishing feature of Pluto would likely then be its moon, Charon, and its atmosphere. However, these characteristics may not be unique to Pluto: several other Kuiper belt objects (not including Sedna) are known to have satellites.

It is interesting to note that, historically, the first four asteroids (1 Ceres, 2 Pallas, 3 Juno and 4 Vesta) were considered planets for several decades (their size was not accurately known at the time). Some astronomy texts in the early 19th century referred to the existence of eleven planets (including Uranus and the first four asteroids). In 1845, the first new asteroid in 38 years was discovered (5 Astraea), just one year before Neptune, and soon every year brought a few more asteroid discoveries. Although they are still called "minor planets", they are no longer considered "planets". Thus there is precedent for the sort of "demotion" that some propose for Pluto (although Pluto has more than twice the diameter of Ceres and more than 10 times its mass).

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

See also Definition of Planet.

Pluto in Fiction and Film

Pluto in astrology

Main article: Planets in astrology#Pluto

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