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Pluto 1 Data Name – In Roman mythology, Pluto (Greek: Hades) is the god of the underworld of the dead. Discovery – The idea that a distant, ninth planet might exist emerged shortly after the discovery of Neptune. The person most identified with a search for “Planet X” was Percival Lowell. Lowell and his colleagues conducted a photographic search for such a planet for years without success. The search resumed in 1929, thirteen years after Lowell’s death. Clyde W. Tombaugh 2, a young astronomer, was hired to take an exhaustive series of photographs and to examine them for objects that were not stars. He exposed 14-by-17-inch plates by night, and by day, examined the images in an instrument called a blink comparator. By rapidly alternating a magnified view of two photographs of the same area of the sky taken at different times, any object that was not a distant star appeared to move against the background star field. Clyde Tombaugh’s diligence and patience was rewarded when a 15th-magnitude speck moved on photographic plates taken on January 23 and January 29 of 1930 3. The discovery of the ninth planet was announced six weeks later on March 13 (Percival Lowell’s birthday). Spacecraft Exploration – The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, became the first spacecraft to fly by Pluto on July 14, 2015. Location – Pluto’s average distance from the Sun is approximately 3,670,000,000 miles. Seen at this vast the distance, the Sun would be about 1,000 times fainter than it appears from Earth 4. Size – Pluto measures 1,473 miles in diameter. Orbital Period – 247.7 years. High Orbital Eccentricity and Inclination – Pluto is usually farther from the Sun than the planet Neptune. However, due to the high eccentricity of its orbit 5, it is closer than Neptune for about 20 years out of its 247.7-year orbit. Pluto crossed Neptune’s orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226. Moreover, as Pluto approaches perihelion, it reaches its maximum distance from the ecliptic due to its 17.14º orbital inclination 6. Rotational Period – 6.39 days (retrograde). 1 Inclination of Rotation Axis – 119.6º. Like Uranus, Pluto rotates with its poles almost in its orbital plane. Number of Satellites – 5 known moons. Atmosphere 7 Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. The surface pressure ranges from about one million to 100,000 times less than Earth’s atmospheric pressure. Pluto’s elliptical orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out. When Pluto is closer to the Sun, the temperature of Pluto’s solid surface increases, causing the ices to sublimate. Surface Composition – Pluto’s surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. Pluto’s surface is quite varied, with large differences in both brightness and color. The color varies between charcoal black, gray, and dark orange. This coloration is a result of hydrocarbons called tholins, which form when solar ultraviolet light and cosmic rays interact with methane in Pluto’s atmosphere (which precipitate out onto the surface) and its crust. Features – Notable geographical features of Pluto’s surface include a large bright area called Tombaugh Regio (the Heart) 8 and a dark, cratered terrain named Cthulhu Regio (the Whale) 9. Forming the the left lobe of Tombaugh Region is Sputnik Planum, a craterless, icy plain. Images taken by the New Horizons spacecraft revealed a number of features associated with the plain: Polygonal Structure 10 – Patterns of polygonal ice displaying wind-blown streaks indicating geysers and pitting resulting from the sublimation of ice. Nitrogen-Ice Glaciers 11 – Glaciers of what is probably nitrogen ice can be seen flowing into valleys and craters at the edge of the plain; the valleys appear to have been formed through erosion. Water-Ice Mountains 12 – Mountains nearly two miles high have been found along the southwestern and southern edges of Sputnik Planum. Water-ice is the only ice detected on Pluto that is strong enough at Plutonian temperatures (nearly -400º F) to support such heights. Cryovolcanism – Two possible cryovolcanoes, provisionally named Wright Mons 13 and Piccard Mons, have been identified near the south pole. Both are nearly 100 miles across and at least 2.5 miles high, the tallest peaks known on Pluto at present. They are lightly cratered and thus geologically young. 2 Interior Pluto’s size and density (1.87 g/cm3) suggest that it is composed of about 70% rock and 30% ice (mainly water-ice). Though Pluto is a small body, a sufficient amount of heat to melt the ice for differentiation was most likely generated by the decay of radioactive material associated with its high rock content. Modeling of Pluto’s internal structure consists of a rocky core overlain by a mantle and crust of ice 14. It is possible that sufficient heating continues today, creating a layer of liquid water at the core-mantle boundary. Origin Escaped Moon – In an early hypothesis, Pluto was an escaped moon of Neptune, knocked out of orbit by its current moon, Triton. However, the 3:2 orbital resonance between Neptune and Pluto makes this hypothesis implausible. Specifically, Neptune completes three 165-year orbits as Pluto completes two 247.7-year orbits; and computer simulations of the motions of these worlds back to the origin of the solar system show that they have never been close, nor can they ever be. Trans-Neptunian Object – Researchers have proposed a theory that links Pluto to trans-Neptunian objects (TNOs). A TNO is any object in the solar system that orbits the Sun at a greater distance on average than Neptune. They are remnants from the Solar System’s formation. The majority of TNOs are thought to be low-density mixtures of rock and frozen volatiles (ices), such as methane, ammonia, and water, with some organic (carbon-containing) surface material such as tholin. Hundreds of TNOs have been discovered since the early 1990s 15. Some of these TNOs are quite large, and a few of them have moons. Like Pluto, approximately one-quarter of TNOs have 3:2 orbital resonances with Neptune. They are called “plutinos”. Computer simulations of the formation of the solar system suggest that gravitational interactions between the planets and a disk of planetesimals caused the planets to migrate to more distant orbits. In the process, Neptune trapped a number of these planetesimals into a 3:2 orbital resonance. Kuiper Belt – The first astronomers to suggest the existence of a “belt” of trans-Plutonian objects (note: not Neptune) were Frederick Leonard in 1930 and Kenneth Edgeworth in 1943. However, Gerard Kuiper in 1951 published a paper mentioning objects beyond Pluto, and because of this paper, the belt became generally known as the Kuiper belt. Moreover, the bodies that comprise the region are referred to as Kuiper belt objects (KBOs). The largest KBOs include Pluto, its moon Charon, Makemake, and Quaoar (about 800 miles in diameter). Neptune’s moon Triton and Saturn’s moon Phoebe are commonly thought to be captured KBOs. Studies have revealed the Kuiper belt region is shaped like a thick disk, extending outward from the orbit of Neptune at 30 AU to 50 AU. 3 TNO Divisions – The Kuiper belt, scattered disk, and Oort cloud (discussed later) are names for divisions of TNOs. The scattered disk is a dynamically unstable region overlapping the Kuiper belt, and thought to have formed when KBOs were “scattered” by gravitational interactions with Neptune into highly eccentric and inclined orbits. The TNO, Eris is classified as a scattered disk object (SDO). Its orbit is quite eccentric (bringing it to within 37.91 AU of the Sun, a typical perihelion for SDOs) and extremely inclined (tilted at an angle of about 44º to the ecliptic). Moons Five Moons 16 – Pluto has five known natural satellites: Charon was first identified in 1978 by astronomer James Christy. Nix and Hydra were discovered in 2005 by the Hubble Space Telescope Pluto Companion Search Team. The team also discovered Kerberos in July 2011 and Styx in July 2012. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons. However, Kerberos has a much lower albedo than the other moons of Pluto, which is difficult to explain with a giant collision. Charon 17 – Charon has a diameter of about 750 miles (slightly more than half as large as Pluto) 18. By no means the largest planetary satellite in the solar system, it has the distinction of being the largest satellite as compared to its primary. Charon appears not to have an atmosphere. Its surface is covered in large part by water-ice, making it quite different from that of Pluto. New Horizons revealed very few craters on Charon, indicating that it is geologically active and young. Its surface also has several canyons up to 5 miles deep and cliffs extending nearly 600 miles. However, the most intriguing feature is a depression with a peak in the middle of unknown origin 19. Charon is mainly an icy body and contains less rock by proportion than Pluto. This supports the idea that Charon was created by the impact of another body into Pluto’s icy mantle. There are two conflicting theories about Charon’s internal structure: some scientists think it to be a differentiated body like Pluto, with a rocky core and an icy mantle, whereas others think it to be uniform throughout. Name – In Greek mythology, Charon is the boatman who ferries souls across the river Styx to the underworld. The ch is pronounced with an initial k sound, like in chaos. However, James Christy himself pronounced the ch in the moon’s name as sh, after his wife Charlene. The sh pronunciation is common among astronomers who speak English, and is the prescribed pronunciation at NASA and of the New Horizons mission team. Synchronous Rotation and Revolution 20 – Charon orbits Pluto in the same 6.39 days that Pluto takes to rotate, indicating that Pluto and its satellite are locked in what is called synchronous rotation and revolution. Like two ballroom dancers, Pluto and Charon face each other as they waltz around the Sun. It appears that Charon was able to pull Pluto into a tidal lock because of its large relative mass. 4 If hypothetical Plutonians happened to live on the hemisphere directed away from Charon, they wouldn’t even know that Pluto had a moon. Conversely, for inhabitants of the Charon-facing side, the moon would always be in the sky at the same place. Some artificial satellites are given “geosynchronous” orbits so they can function as television relays. Classification Dwarf Planet 21 – On August 24, 2006, the International Astronomical Union (IAU) passed a resolution reclassifying Pluto as the ninth planet from the Sun to a new category of celestial bodies in the solar system referred to as dwarf planets. The IAU has also classified four other bodies of the solar system as dwarf planets: the asteroid Ceres (discussed later) and the TNOs Eris, Makemake, and Haumea. The resolution describes a dwarf planet as an object that: (1) is in orbit around the Sun, (2) has sufficient mass for its gravity to overcome rigid body forces so that it assumes a spherical shape, and (3) has not “cleared the neighborhood” around its orbit. The phrase “clearing the neighborhood” refers to an orbiting body “sweeping out” its orbital region over time by gravitationally interacting with smaller bodies nearby. In other words, over many orbital cycles, a large body (planet) will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit. As a consequence, it does not then share its orbital region with other bodies of significant size, except for its own satellites. Eris 22 – Because Eris appeared to be larger than Pluto (in fact, Eris is slightly smaller), NASA initially described it as the Solar System’s tenth planet. This, along with the prospect of discovering other objects of similar size in the future, motivated the IAU to define the term “planet” more precisely and introduce the new category of dwarf planet. Eris has an orbital period of 558.04 years. Eris’ average distance from the Sun is 67.78 AU, with a maximum possible distance (aphelion) of 97.65 AU. Haumea 23 – Although evidence suggests that it is not spherical, Haumea is thought to have sufficient gravity to overcome rigid body forces. Its suspected “ellipsoidal” shape is believed to be a result of an unusually rapid rotation (about 4 hours); similar to a water balloon stretching out when tossed with a spin. Other Candidates – It is suspected that about 150 known TNOs are dwarf planets. Furthermore, scientists speculate that thousands of TNOs may exist that can be classified as dwarf planets. 5 Charon’s Status – On account of its large mass relative to Pluto, Charon revolves with Pluto around a center of mass (barycenter) located in space between Pluto and Charon rather than around a point located within Pluto 24. Since neither object truly orbits the other, it has been argued that Charon should be considered to be more than just a satellite of Pluto. Under the IAU draft proposal in 2006, Charon would have been classified as a planet, since the draft explicitly defined a planetary satellite as one in which the barycenter lies within the major body. In the final definition, Pluto was reclassified as a dwarf planet, but the formal definition of a planetary satellite was not decided upon. Therefore, the status of Charon remains uncertain, as there is presently no clear definition of what distinguishes a satellite system from a “binary system”. 6