Until August 2006, Pluto was defined as the outermost planet from the sun in the solar system. A decision by the International Astronomical Union in August 2006, however, effectively changed Pluto’s designation from planet to dwarf planet.
Pluto is one of the least well-understood objects in the solar system. It is a relatively tiny object when compared to the planets in the solar system, being only about two-thirds the size of Earth’s moon. Pluto also has a most unusual orbit that is very elliptical in shape. At certain times within its orbit, it is actually closer to the sun than the eighth planet Neptune.
Pluto has three other moons orbiting about it. Its long recognized moon, Charon, is so large that they essentially form a binary system. In 2005 two tiny moons of Pluto were discovered orbiting beyond Charon. How the four-body Pluto-Charon system formed and how the system acquired its special 2-to-3 orbital resonance with Neptune are, as of 2006, unanswered questions. Scientists continue to speculate about their origins, but we will probably not discover the secret of its origin until the NASA “New Frontiers” space mission called New Horizons, or other future missions, visits the Pluto-Charon system. On January 2006, when the mission was launched, Pluto was the only planet in the solar system that had not been visited by a space probe. Since then, Pluto has been demoted to a dwarf planet and the solar system has been reduced to only eight major planets.
In 2000 NASA canceled the previously planned Pluto Express mission. In all, since about 1985, at least four Pluto missions have been planned but, ultimately, canceled. After many setbacks in the early 2000s caused by the withdrawal of Congressional funds and the accumulation of technical problems, NASA scientists and engineers finally got the New Horizons mission approved and finalized. Administered by the Applied Physics Laboratory at Johns Hopkins University (Maryland), it launched successfully on January 19, 2006 from Cape Canaveral, Florida. It was sent directly into an Earth- and solar-escape trajectory on its way to Pluto, Charon, and the Kuiper Belt (a large region of icy, rocky bodies located past the orbit of Neptune and continuing past Pluto’s orbit).
The Pluto-Kuiper Belt mission, officially the name of the New Horizons mission, will be the first reconnaissance of Pluto and Charon. After leaving the two prime celestial bodies, the probe will go on to explore objects in the Kuiper Belt. As of September
2006, the Pluto-Kuiper Belt mission should encounter Pluto and Charon in the summer of 2015, with a target date (as of June 2006) of July 14. Observations of Kuiper Belt Objects past Pluto should occur approximately between 2016 and 2020.
Pluto has a very eccentric (non-circular) orbit about the sun. While its mean distance from the sun is 39.44 astronomical units (AU) (equivalently about 3,674,000,000 mi or 5,913,520,000 km), it can be as far as 49.19 AU from the sun and as close as 29.58 AU. The time required for Pluto to complete one orbit about the sun (its sidereal period) is 247.68 years (based on the Earth year of 365.256 days), and the time for the planet to repeat alignments with respect to Earth and the sun (its synodic period) is 366.73 days.
The large eccentricity of Pluto’s orbit can bring the planet closer to the sun than Neptune. Pluto, in fact, last edged closer to the sun than Neptune in January 1979. It remained the eighth most distant planet from the sun until February 11, 1999, at which time it regained its status as the ninth planet. It will remain more distant than Neptune until April 5, 2231.
On September 5, 1989, Pluto reached perihelion, its closest point to the sun, when it was at its brightest when viewed from Earth. Pluto is not a conspicuous night-sky object, and can only be viewed with telescopic aid. In fact, the Hubble Space Telescope can only record the largest features found on its surface. Under good viewing conditions, Pluto can be seen as a starlike point in any telescope having an objective diameter greater than 7.9 in (20 cm). Pluto moves only slowly through the constellations; due to the fact that it is both small and very distant from Earth.
At its closest approach to Earth, Pluto’s planetary disk is smaller than 0.25 arc seconds (that is, 0.00007°) across. Periodic variations in its brightness, however, have revealed that Pluto rotates once every 6.38675 days (based on a 24-hr Earth day). Pluto’s spin axis is inclined at 122.53° to the plane of its orbit about the sun and, consequently, its rotation is retrograde. The extreme tilt of Pluto’s spin-axis results in the Earth-based observer seeing different hemispheric projections as it moves around the sun. In the early 1950s, for example, Pluto presented its south pole towards Earth, while today we see its equatorial regions. In the year 2050 Pluto will present its north pole towards Earth.
Careful long-term monitoring of the variations in Pluto’s brightness indicate that the body is brightest when seen pole-on. This observation suggests that the poles are covered by reflective ices, and that Pluto has a dark patch (lower albedo) on, or near, its equator. It is highly likely that Pluto’s brightness variations undergo seasonal changes, but as yet, astronomers have only been able to monitor the planet during about one-sixth of one orbit about the sun.
At its mean distance of about 40 AU from the sun, Pluto receives 1/1600 the amount of sunlight received at Earth. Consequently, Pluto is a very cold world, with a surface temperature that varies between←–391 and 346°F(–235 and←–210°C), and with an average temperature of←–369°F (–223°C). Spectroscopic observations indicate the presence of methane, nitrogen, and carbon monoxide ices on Pluto’s surface. Most surprisingly, however, and in spite of its small size and low escape velocity (0.75 mi/sec or 1.2 km/sec), Pluto is able to support a very tenuous atmosphere.
That Pluto might have a thin methane atmosphere was first suggested, on the basis of spectroscopic observations, in the early 1980s. Conclusive evidence for the existence of a Plutonian atmosphere was finally obtained, however, on June 9, 1988, when Pluto passed in front of a faint star producing what astronomers call a stellar occultation. As Pluto moved between the star and Earth, observers found that rather than simply vanishing from view, the star gradually dimmed. This observation indicates the presence of a Plutonian atmosphere. Indeed, Pluto’s atmosphere appears to have a tenuous outer layer and a more opaque layer near its surface, with a surface pressure of only about three microbars.
It has been suggested that Pluto only supports an atmosphere when it is near perihelion and that as the planet moves further away from the sun the atmosphere freezes. This freezing and thawing of Pluto’s atmosphere may explain why the planet has a relatively high surface albedo from about 40-60%. Essentially, the periodic freezing and thawing of Pluto’s atmosphere continually refreshes the methane ice at the planet’s surface.
As of 2006, scientists think that Pluto’s atmosphere consists primarily of nitrogen and methane, along with smaller amounts of ethane and carbon monoxide. When it is near perihelion its atmosphere may exist entirely as gases, but for the majority of its orbit the atmospheric gases are frozen into ice. When New Horizons arrives at Pluto, mission specialists will be able to examine Pluto in detail because its atmosphere will be gaseous (as opposed to frozen) in order to obtain the most information possible.
Speculations about the existence of a ninth planet arose soon after astronomers discovered that the planet Neptune (discovered in 1846 by German astronomer Johann Gottfried Galle) did not move in its orbit as predicted. The small differences between Neptune’s predicted and actual position were taken as evidence that an unseen object was introducing slight gravitational perturbations in the planet’s orbit. The first search for a trans-Neptunian planet appears to have been carried out by American astronomer David Peck Todd (1855–1939), of the U.S. Naval Observatory, in 1877. Todd conducted a visual search during 30 clear nights between November 1887 and March 1888, but he found nothing that looked like a planet.
The first systematic survey for a trans-Neptunian planet, using photographic plates, was carried out by American astronomer Percival Lowell (1855–1916), at the Flagstaff Observatory, in Arizona between 1905 and 1907. Lowell conducted a second survey at Flagstaff in 1914, but again, no new planet (which he called Planet X) was discovered. On the basis of predictions made by William Henry Pickering in 1909, American astronomer Milton Lasell Humason (1891–1972), at Mount Wilson Observatory, in California, carried out yet another photographic survey for a trans-Neptunian planet, with negative results, in 1919.
A third photographic survey to look for objects beyond the orbit of Neptune was initiated at Flagstaff Observatory in 1929. Clyde Tombaugh (1906–1997) was the young American astronomer placed in charge of the program. The survey technique that Tombaugh used entailed the exposure of several photographic plates, of the same region of the sky, on a number of different nights. In this way, an object moving about the sun will shift its position, with respect to the unmoving, background stars, when two plates of the same region of sky are compared. The object that we now know as Pluto was discovered through its “shift” on two plates taken during the nights of January 23rd and 29th, 1930. The announcement that a new planet had been discovered was delayed until March 13, 1930, to coincide with the 149th anniversary of the discovery of Uranus, and to mark the 78th anniversary of Lowell’s birth. It is reported that some of Tombaugh’s ashes were placed onboard the New Horizons spacecraft in honor of his discovery of Pluto.
Humason, it turns out in retrospect, was unlucky in his survey of 1919, in that a re-examination of his plates revealed that Pluto had, in fact, been recorded twice. Unfortunately for Humason, one image of Pluto fell on a flaw in the photographic plate, and the second image was obscured by a bright star.
After its discovery, it was immediately clear that the Pluto was much smaller and fainter than the theoreticians had suggested it should be. Indeed, a more refined analysis of Neptune’s orbit has revealed that no “extra” planetary perturbations are required to explain its orbital motion.
Pluto has a mean density of about 1750 kg/m3,or more than three times less than Earth’s mean density. It has an estimated surface gravity of about 0.58 m/ sec2 and an escape velocity of about 1.2 m/sec. Scientists estimate that Pluto may have a core of silicate rock about 1,050 mi (1,700 km) in diameter, which is surrounded by ices of water, methane, and carbon monoxide. The crust of Pluto may be a thin coating of nitrogen, methane, and carbon monoxide ice. Hubble Space Telescope photographs (taken in infrared) show light and dark patches on the surface of Pluto that may represent terrains of different composition and perhaps different ages as well. It is considered likely that Pluto has polar caps. While Pluto may have had some internal heating early in its history, that is likely long past and the planet is quite cold and geologically inactive. There is no reason to expect that Pluto has a magnetic field.
Charon, Pluto’s largest and closest moon, was discovered by American astronomer James W. Christy (1938–) in June, 1978. Working at the U.S. Naval Observatory in Flagstaff, Arizona, Christy noted that what appeared to be slight bulges, or “bumps,” on several highly magnified images of photographic plates taken of Pluto, which reappeared on a periodic basis. With this information, Christy realized that what had been dismissed previously, as far back as early 1965, as image distortions were actually composite images of Pluto and a companion moon. The body was temporarily designated as S/1978 P 1. Christy later suggested that the new moon be named Charon, after the mythical boatman that ferried the souls of the dead across the river Styx to Hades, where Pluto, God of the underworld, sat in judgment. In 1985, the name was accepted by the International Astronomical Union (an international astronomy body that officially names celestial bodies).
The orbit of Charon is circular (its orbital eccentricity is zero) and parallel to Pluto’s equator (its orbital inclination to Pluto is zero). Charon orbits Pluto once every 6.38725 days, which is also the rate at which Pluto spins on its axis. Charon is therefore in synchronous orbit about Pluto, so each sees only one side of the other. As seen from the satellite-facing hemisphere of Pluto, Charon hangs motionless in the sky, never setting nor rising. The average Pluto-Charon separation is 12,196 mi (19,640 km), which is about one-twentieth the distance between the Earth and the moon.
Soon after Charon was discovered astronomers realized that a series of mutual eclipses between Pluto and its satellite would be seen from Earth every 124 years. During these eclipse seasons, which last about five years each, observers on Earth would witness a complete series of passages of Charon across the surface of Pluto. The last eclipse season ended in 1990, and the next series of eclipses will take place in 2114.
By making precise measurements of the brightness variations that accompany Charon’s movement in front of and behind Pluto, astronomers have been able to construct detailed albedo (reflectivity) maps of the two bodies. They have also been able to derive accurate measurements of each component’s size. For example, Pluto has a diameter of 1,685 mi (2,390 km), making the planet about 1.5 times smaller than Earth’s moon, and two times smaller than the planet Mercury. Charon has a diameter of 737 mi (1,186 km).
Since Pluto has a satellite, Kepler’s third law of planetary motion can be used to determine its mass. A mass equivalent to about 1/500 that of Earth, or about 1/5 that of the moon, has been derived for Pluto. Charon’s mass is about one-eighth that of Pluto’s. Given the high mass ratio of 8:1 and the small relative separation between Pluto and Charon, the center of mass about which the two bodies rotate actually falls outside of the main body of Pluto. This indicates that rather than being a planet-satellite system, Pluto and Charon really constitute a binary system, or, in other words, a double body system.
Pluto has a bulk density of about 2 g/cm3, while Charon has a lower bulk density of about 1.2 g/cm3. This difference in densities indicates that while Pluto is probably composed of more rock than ice, while Charon is most likely made up of more ice than rock. In general terms, Pluto can be likened in internal structure to one of Jupiter’s Galilean moons, while Charon is more similar in structure to one of Saturn’s moons. In fact, astronomers believe that Pluto’s internal structure and surface appearance may be very similar to that of Triton, Neptune’s largest moon.
Charon’s surface is thought to be composed of water ice, nitrogen ice, and carbonmonoxide ice. Charon probably has a core composed of silicate rock, which is a minor component of the satellite’s mass. About the core, is a hypothetical mantle and cryosphere (ice layer) of water ice, nitrogen ice, and carbon-mon-oxide ice. It is likely that Charon has no internal heat source and that it has no appreciable magnetic field. Charon has no noticeable atmosphere. Its surface gravity is 0.31 m/sec2 and its escape velocity is 0.6 km/sec.
After Charon was discovered, astronomers were able to accurately measure the mass of the Pluto-Charon system. Their individual masses were not able to be determined until Nix and Hydra were discovered in late 2005. Pluto’s mass is about 1.25 × 1026 kg, while the mass of Charon is approximately 1.52 × 1021 kg. From these statistics, Charon’s composition is estimated to be about 55% rock and 45% ice, while Pluto is about 70% rock and 30% ice. While Pluto shows pinkish colors and areas of dark and light, Charon looks basically a uniform gray color.
The Pluto-Charon system has the strangest orbit of all bodies rotating about the sun in the solar system. It has a large eccentricity (0.2488, which is much greater than Earth’s value of 0.0167) and a high orbital inclination of 17.16° to the ecliptic. These extreme orbital characteristics suggest that since its formation the Pluto-Charon system may have undergone some considerable orbital evolution.
Shortly after Pluto was first discovered, astronomers realized that unless some special conditions prevailed, Pluto would occasionally undergo close encounters with Neptune, and consequently suffer rapid orbital evolution. In the mid-1960s, however, it was discovered that Pluto is in a special 2-to-3 resonance with Neptune. That is, for every three orbits that Neptune completes about the sun, Pluto completes two. This resonance ensures that Neptune always overtakes Pluto in its orbit when Pluto is at aphelion, and that the two planets are never closer than about 17 AU. How this orbital arrangement evolved is presently unclear.
The close structural compatibility of Pluto and Triton (i.e., they have the same size, mass, and composition) has lead some astronomers to suggest that the two bodies may have formed in the same region of the solar nebula. Subsequently, it is sometimes argued that Triton was captured to become a moon of Neptune, while Pluto managed to settle into its present orbit about the sun. Numerical calculations have shown that small, moon-sized objects that formed with low inclination, circular orbits beyond Neptune do evolve, within a few hundred million years, to orbits similar to that of Pluto’s. This result suggests that Pluto is the lone survivor of a (small) population of moon-sized objects that formed beyond Neptune, its other companions being captured as satellites around Uranus and Neptune, or being ejected from the solar system. One important, and yet unsolved, snag with the orbital evolution scenario just outlined is that Pluto and Charon have different internal structures, implying that they formed in different regions of the solar nebula. It is presently not at all clear how the Pluto-Charon system formed.
Using a specially designed computer, Gerald Sussman and Jack Wisdom of the Massachusetts Institute of Technology, have modeled the long-term orbital motion of Pluto. Sussman and Wisdom set the computer to follow Pluto’s orbital motion over a time span equivalent to 845 million years; interestingly they found that Pluto’s orbit is chaotic on a time scale of several tens of millions of years.
Obviously, a history of the Pluto-Charon system is quite speculative. One theory states that the double-dwarf planet system may have originated in a more nearly circular orbit, and that a subsequent catastrophic impact changed the orbit to highly elliptical and perhaps separated the two masses. It is further speculated that a large object rammed into Pluto around 4.5 billion years ago and, in the process, much of Pluto’s mass was discarded and Charon was formed from coalesced debris in near Pluto space. This idea may also account for the strongly inclined spin axis of Pluto.
Another hypothesis holds that Pluto accreted in orbit around Neptune and may have been ejected in the Triton capture event that is thought to have reorganized the Neptunian system. The lack of a large “original” satellite of Neptune (Triton is thought to have been captured) is a point in favor of this hypothesis.
It is also possible that Pluto-Charon are simply part of a class of icy Trans-Neptunian objects (TNOs) that are rather close to the Sun as compared with others probably out there in the Ort cloud beyond the edge of the solar system. Until the New Horizons space mission, or other future missions, returns data and photographs from Pluto, Charon, and some TNOs, scientists may not be able to eliminate any of the completing hypotheses.
In late 2005, an astronomy team used the Hubble Space Telescope to discover two tiny moons orbiting Pluto. They were temporarily designated S/2005 P1 and S/2005 P2 but have since then been approved by the International Astronomical Union to be called Nix and Hydra, respectively. In the tradition of naming bodies in the far reaches of the solar system after underworld characters, the team originally chose the figures Nyx, the Greek goddess of the night (and mother of Charon), and Hydra, a nine-headed monster that guarded the entrance to the underworld. (The team altered one name after finding that Nyx was already a name for a small asteroid.)
Nix and Hydra are estimated to be between 30 and 125 mi (48–200 km) in diameter and about 27,000 mi (44,000 km) away from Pluto—about two to three times further away than Charon. Hydra is the further out of the two satellites. The two new moons appear to move counterclockwise around Pluto, as does Charon. When viewed with cameras onboard Hubble, the moons are about 5,000 times fainter than Pluto. However, scientists conclude that these numbers are currently only rough estimates.
With the confirmation of these newly discovered bodies, astronomers are more confident in being able to learn more about the nature of Pluto and its origin. Based on this discovery, and knowing that Pluto resides within the heart of the Kuiper Belt, astronomers are already speculating that many larger bodies within the Kuiper Belt may have several moons orbiting them like Pluto.
Pluto remained a planet for over 75 years because of such planet like features as a spherical shape, atmosphere, internal rocky core structure with extended ice layers, and one or more moons. It also remained a planet due simply to the fact that it was always called a planet. However, the scientific community began questioning its status near the end of the twentieth century.
Early in the 1990s, many tiny bodies beyond the orbit of Neptune (what are called trans-Neptunian objects) were discovered. These icy bodies were similar in composition and size to Pluto. In addition, hundreds of exoplanets (planets orbiting stars other than the sun) were found to exist. These discoveries added a wide variety of sizes and characteristics when describing planets. Finally, in 2005, a body called 2003UB313— which was larger than Pluto (the solar system’s smallest planet at the time)—was found outside Neptune’s orbit.
On August 24, 2006, members of the International Astronomical Union (IAU) passed a resolution to officially define a planet as any “celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.” Consequently, Pluto was disqualified from being a planet due to its highly elliptical orbit that overlapped Neptune’s orbit. Instead, Pluto was recognized by the IAU as a dwarf planet: “a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.” Controversy about this decision continues to run rampant in the astronomy community— some in favor and others vehemently against it.
Because of this new status, Charon and Pluto are now sometimes considered a double dwarf planet system, although Charon has yet to be recognized as a dwarf planet. Nix and Hrdra are small enough and at a large enough distance from Pluto to be still considered satellites of Pluto.
NASA’s New Horizon’s spacecraft will cross the orbits of all of the planets past Earth on its way to fly past Pluto and Charon in 2015. It will be the first spacecraft to visit the Pluto-Charon system. The spacecraft is expected to fly within 6,200 mi (10,000 km) of Pluto
Objective diameter— The diameter of a telescope’s main light-collecting lens, or mirror.
Occultation— The passing of one astronomical object (e.g., a planet or asteroid) in front of another.
Retrograde rotation— Axial spin that is directed in the opposite sense to that of the orbital motion.
Solar nebula— The primordial cloud of gas and dust out of which our solar system formed.
with a relative velocity of 8.6 mi/sec (13.78 km/sec) at closest approach. When it flies by Charon it will come as close as 16,800 mi (27,000 km). The New Horizon’s unmanned spacecraft traveled to the moon’s orbit in about nine hours, as compared to the three days it took the Apollo manned spacecraft. It is the fastest spacecraft ever launched.
The spacecraft was built by Southwest Research Institute (Texas) and the Johns Hopkins Applied Physics Laboratory (Maryland). The seven science instruments onboard the grand piano-sized spacecraft will help scientists understand the global geology and morphology of Pluto and Charon, along with the chemical compositions of their surfaces and atmospheres. The Venetia Burney Student Dust Counter (VBSDC) will measure space dust encountered by the spacecraft during its entire mission, while the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) will measure the composition and density of plasma (ions) escaping the atmosphere of Pluto. The Solar Wind Around Pluto (SWAP) instrument will measure Pluto’s atmospheric escape rate and observe its interaction with the solar wind.
The Long Range Reconnaissance Imager (LORRI) will visibly map portions of Pluto’s surface and provide high resolution geologic data, while the Radio Science Experiment (REX) will measure the composition of Pluto’s atmosphere, along with its temperature, with the use of radio waves. An ultraviolet imaging spectrometer (nicknamed ALICE) will analyze the composition and structure of Pluto’s atmosphere and will examine the atmospheres around Charon and various Kuiper Belt Objects (KBOs). A visible and infrared imager/spectrometer (nicknamed RALPH) will provide various maps based on temperature, composition, and visual color of Pluto. Many other measurements are also possible for the spacecraft.
After passing by Pluto and Charon, New Horizons will travel further into the Kuiper Belt. Over the next four to five years, the spacecraft is expected to examine several KBOs. In this far region of the solar system, it will take uncompressed images over nine months to transmit to Earth, while highly compressed images can be transmitted in a few days. Pluto is the largest body in the Kuiper Belt, the outer zone of the solar system. The National Academy of Sciences considers the exploration of this region of space, especially of Pluto, to be the highest priority for future planetary missions.
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