Pluto and Charon

Pluto was originally recognized as the ninth planet in the solar system. In 2006, the International Astronomical Union re-designated Pluto to the status of a dwarf planet. Pluto and its satellite Charon constitute a dual-object system located far from the sun. These bodies are different in size and composition from any of the planets of the solar system, more closely resembling the icy satellites of Neptune.

Overview

Pluto was discovered by American astronomer Clyde Tombaugh in 1930; its satellite Charon was detected in 1978 by James W. Christy of the US Naval Observatory. In the United States, “Charon” is pronounced SHAR-uhn, reminiscent of the discoverer’s wife, Charlene; in the rest of the world, the pronunciation KAR-uhn is usually preferred.

Less is known about Pluto than about any of the other planets. Earth-based telescopes cannot provide much information about Pluto and Charon, as they are too far away for surface details to appear even in the largest telescopes. Better images of Pluto have been obtained using the Hubble Space Telescope, which in 2005 detected two new small satellites of Pluto—Nix and Hydra. A fourth satellite, Kerberos, was discovered in 2011, and a fifth one, called Styx, was discovered in 2012 by scientists searching for potential hazards to spacecraft that would fly by Pluto. Hubble images have also provided the first indications of features on the surface of Pluto. Almost a dozen “provinces”—portions of Pluto with different albedos—have been discovered. Observations of Pluto have also revealed a northern polar cap, several dark spots, and a bright linear feature. The resolution of these Hubble images precluded a precise determination of the nature of these intriguing features until the New Horizons spacecraft flew through the Pluto-Charon system in 2015. New Horizons found what appeared to be mountains made of water ice, a large icecap in the shape of a heart near the equator, and glaciers of frozen gases such as carbon monoxide, methane, and nitrogen. There were also areas of the planet with few or no craters, suggesting geological activity in the recent past. This surprised many scientists, as the popular theory was that Pluto lacked the energy source required to sustain such activity.

Pluto is the smallest planet in the solar system, smaller even than Earth’s moon; it is usually considered the outermost planet. Pluto takes 247.7 Earth-years to orbit the sun and rotates on its axis once every 6.4 Earth-days. The orbit of Pluto is more eccentric than that of any other planet. Pluto’s orbital is so large that Pluto is sometimes closer to the sun than Neptune. This occurred between January 21, 1979, and March 14, 1999, when Pluto’s orbit took it farther from the sun than Neptune. Pluto will remain beyond the orbit of Neptune until 2226.

Distances between objects in the solar system are usually measured in astronomical units (AU). One astronomical unit is the average distance between Earth and the sun, or 150 million kilometers. Pluto can be as close to the sun as 29.64 AU and as far away as 49.24 AU. At Pluto’s distance, the sun appears as a star-like point, but a point more than one hundred times brighter than a full moon. The amount of solar energy received by Pluto varies greatly because of the large variation in distance between the sun and Pluto over the course of its lengthy “year” (orbit around the sun). This variation in solar energy is expected to cause the thickness of Pluto’s atmosphere to change markedly in different parts of its orbit.

Charon is about half the size of Pluto. The diameter of Pluto is approximately 2,306 kilometers, and Charon’s is approximately 1,207 kilometers. The average distance between Pluto and Charon is 19,700 kilometers. Because of this close proximity, along with their similar sizes, Pluto and Charon are sometimes referred to as a double dwarf planet system. Charon orbits Pluto in 6.4 days—the orbital period of Charon is the same as the rotation period of Pluto. As a result, Pluto always points the same face toward Charon. In fact, an observer on the surface of Pluto would always see Charon in the same position relative to the horizon.

The orbit of Charon is not in the plane of Pluto’s orbit about the sun. Instead, the plane swept out by Charon’s orbit is almost perpendicular to the plane swept out by Pluto’s orbit. When the plane of Charon’s orbit presents its edge to the Earth, a series of occultations and transits between Pluto and Charon occur. This results in a series of mutual eclipses being observed from the Earth. These mutual eclipses can be observed at two positions in Pluto’s orbit, and occur every 124 years. Mutual eclipses last for about six years. One series began in 1985 and made possible the measurement of the sizes of Pluto and Charon.

The surface temperature of Pluto is somewhat uncertain because the fraction of sunlight that it reflects (known as the albedo) is uncertain. Pluto’s surface temperature may range from forty-five to sixty kelvins (approximately-228 degrees Celsius to -213 degrees Celsius, or -379 degrees Fahrenheit to -352 degrees Fahrenheit). The uncertainty arises because the surface composition of Pluto and the extent of its atmosphere are uncertain. Pluto's temperature could dip as low as thirty-three kelvins (-240 degrees Celsius or -400 degrees Fahrenheit); measurements conducted by Smithsonian astronomers using the Submillimeter Array in 2006 placed its surface temperature at an average of forty-three kelvins (-230 degrees Celsius or -382 degrees Fahrenheit). The surface temperature of Charon is easier to measure because Charon lacks an atmosphere; the Submillimeter Array measured it to be about ten degrees warmer than Pluto, at fifty-three kelvins (-220 degrees Celsius or -364 degrees Fahrenheit).

The density of the Pluto-Charon system has been calculated to be 2.05 grams per cubic centimeter. This density, about twice that of water, indicates that Pluto and Charon are composed of a variety of ice and that more than half of their mass could be made up of rocky material. The surface of Pluto has, in fact, been determined to contain methane ice. It is thought that, rather than the methane being uniformly distributed over the surface, there are two large polar ice caps made of methane and a thin, warmer equatorial region, where the methane has become depleted, leaving water ice.

It was not known at first whether Pluto had an atmosphere. In the late 1980s, scientists observed a star passing directly behind the dwarf planet and saw that it dimmed before being eclipsed by Pluto. This indicated that Pluto did have an atmosphere. Periodically, this method has also been used to measure the variable thickness of Pluto's atmosphere using a calculation based on how much the star's light is dimmed. Pluto's atmosphere is thought to be made up of tiny droplets of hydrocarbons. These are made of the remnants of methane molecules split by the sun. This atmosphere is believed to have several distinct layers, although scientists do not yet know why this is. When New Horizons passed Pluto, NASA used radio waves to measure the thickness of the atmosphere, finding that its mass had halved since the previous measurement (using the star method) two years prior. This may be due to Pluto's atmospheric molecules freezing and falling to the surface of the planet as Pluto gets farther from the sun.

The surface of Charon has been determined to be covered with water ice; no frozen methane has been detected. It is expected, however, that the interior of Charon contains methane. The composition of Charon is similar to that of some of the satellites of the Jovian planets. In fact, the surface of Charon appears to be almost identical in composition to that of Miranda, one of the satellites of Uranus. Charon is not expected to trap an atmosphere, even temporarily. It is difficult to make an exact determination, but an upper limit of no more than one-twentieth of the thickness of Pluto’s atmosphere has been determined. Pluto’s equatorial region is depleted in methane and thought to have the same composition as its similarly methane-depleted satellite, Charon. New Horizons found large canyons on Charon.

Methods of Study

Most of the information currently available about the dual dwarf planet has been derived from the electronic recording of telescopic images of Pluto and then computer processing of these images. The rotation period of Pluto was measured by noting that the brightness of Pluto varies periodically with the rotation period of the planet. The brightness varies because the surface distribution of methane ice and water ice is not uniform, and different ices reflect different amounts of light.

Charon was discovered while James Christy examined some electronic images of Pluto. He noticed a bump on the edge of Pluto that appeared to move; this “bump” was Charon. Ground-based telescopes were unable to separate Pluto and Charon into well-resolved images, however, because of the Earth’s atmosphere.

The atmospheres of Pluto and Charon have been studied by two different methods: occultations and spectroscopy. An occultation occurs when the light from an astronomical object is extinguished by another celestial object, such as when Pluto passes in front of a star. The observation of occultations is the standard technique used to determine whether a planet has an atmosphere or rings. If the planet has no atmosphere, it is possible to observe the star with undiminished brightness until the disk of the planet crosses it. It then disappears completely and reappears with its usual brightness. If a planet has an atmosphere, light from the star dims gradually as starlight passes through the atmosphere of the planet. When it reappears, it is faint and brightens as the planet moves farther away from the star. The atmosphere of Pluto was first detected in this manner.

Spectroscopy involves the analysis of light reflected by Pluto. Different wavelengths are reflected by different degrees, and some are completely absent from the reflected light. The spectrum of reflected light can be used to identify chemical elements and compounds present on the surface of a planet and in its atmosphere. This procedure works because each element or compound produces a unique spectrum that can be measured in the laboratory. The infrared spectrum of Pluto has also been probed to add to the information. The main problem encountered in the Pluto-Charon system is that normally the spectra of Pluto and Charon are obtained simultaneously. Mutual eclipse events described above have enabled the spectrum of Pluto alone to be obtained when Charon is behind Pluto. This Pluto spectrum can then be subtracted from the usual combined spectra to obtain the spectrum of Charon. Using this method, scientists have been able to determine the different surface compositions of Pluto and Charon.

Occultations could also be used to measure the sizes of Pluto and Charon, but instead scientists have used the series of mutual eclipses. The rotation period of Charon about Pluto is known, so if the durations of the eclipses of Charon by Pluto are timed (and vice-versa), these times can then be used to estimate the diameters of Pluto and Charon. Masses of the outer planets are usually measured by their effects on the orbits of planets closer to the sun. This method, however, has not worked in the case of Pluto and Charon, because their combined mass is too small to have an observable effect on the next closest planet, Neptune. Fortunately, however, the discovery of Charon made it possible to determine the mass of Pluto from the orbital period of Charon. Kepler’s third law of planetary motion states that the square of a planet’s orbital period divided by the cube of its orbital radius is equal to a constant. The constant depends on the mass of the object orbited; hence, scientists have found that the mass of Pluto is about one five-hundredth Earth’s mass. The mass of Charon has been determined from its size by assuming it has the same density as Pluto.

Prior to Pluto’s re-designation in 2006 to dwarf planet status, it was the only planet yet to be explored by spacecraft. Indeed, due to the tremendous distance between Pluto and Earth, the best way to gain information about Pluto and Charon would be an instrumented spacecraft flying close to the illusive system or even perhaps entering long-term orbit about Pluto. A number of proposals were entertained to conduct the flyby, including one that would use a from Jupiter in order to get to Pluto before its orbit took it so far from the sun that its atmosphere would freeze and cover the surface. A Pluto flyby was approved and funded, then canceled, followed by a new proposal that in turn failed to come to fruition. The New Horizons spacecraft was later proposed, approved, designed, and then launched on January 19, 2006. The spacecraft left Earth behind with the fastest speed ever attained by a human-made object. It passed Earth's moon in mere hours and reached Jupiter in only one year. New Horizons arrived in the Pluto-Charon system in 2015. Scientific instruments incorporated into the spacecraft include a long-range reconnaissance imager, a near-infrared imaging spectrometer, an ultraviolet imaging spectrometer, an electrostatic analyzer, a time-of-flight ion and electron sensor, a radio science experiment, and a dust counter. Incredibly, the total mass of these instruments is a mere thirty-one kilograms, and they operate on only twenty-one watts of electrical power. In the summer of 2014, on its trajectory toward Pluto, the craft's long-range reconnaissance imager captured images of Hydra.

On July 14, 2015, New Horizons became the first spacecraft to perform a detailed observation of Pluto. It would fly approximately 7,800 kilometers above the planet's surface. For the next 15 months, the mission took the first high-resolution photographs of the planet as well as of its moons Nix, Hydra, Kerberos, and Styx. The imagery revealed an immensely more complex environment than previously contemplated. This included the possiblility of an internal ocean of ice water. Later that year, New Horizons was redirected to position the spacecraft to encounter the Keiper Belt Object, Arrokoth. It successfully accomplished this feat on January 1, 2019. In doing so, Arrokoth became the most distant object from Earth ever imaged by an explorer spacecraft. In June 2020, further evidence was reported that Pluto may have had a subsurface ocean. In April 2022, the New Horizons mission was extended and it continues to travel beyond the solar system.

Context

Once considered the outermost planet, Pluto remains the most difficult to investigate. What has been learned about it indicates that it is different from all the other planets. The other four planets of the outer solar system—the “gas giants” Jupiter, Saturn, Uranus, Neptune—have low densities and are composed primarily of gases. The density of Pluto is greater, indicating the presence of rocky material. Nevertheless, the density is lower than the densities of the terrestrial planets of the inner solar system (Mercury, Venus, Earth, and Mars). Pluto has features in common with the Galilean satellites of Jupiter and some of the satellites of Saturn, Uranus, and Neptune, but none has exactly the same makeup. Charon is in many ways similar to an asteroid. In fact, models developed for asteroids covered with water ice are applicable to Charon, and they have been used to gain a deeper understanding of this moon.

This pair of small worlds may seem insignificant in comparison with the other, larger planets of the outer solar system. However, if scientists are ever to develop a complete understanding of the origins and evolution of the solar system, they will need detailed knowledge of all its members. While Pluto has been considered the farthest planet from the sun, in many ways it has more in common with comets, meteoroids, and asteroids than it does with the other major planets. Pluto could well be one of a large number of similar objects in the outer solar system—including those in the Kuiper belt, where Pluto lies, and in the Oort cloud. The Oort cloud is a spherical swarm of comets far from the sun, possibly a light-year away. In developing an understanding of Pluto and Charon, scientists may be laying the groundwork for a better understanding of the Oort cloud.

The study of the Pluto-Charon system may also answer some questions about Neptune and its satellites. One of the theories for the origin of Pluto is that it was once a satellite of Neptune. That appears less likely since scientists have learned that Pluto has its own satellite. However, an understanding of its eccentric orbit would provide a definitive answer. The mysteries of the solar system include the eccentric orbit of Neptune’s satellite Nereid and the clockwise direction (as viewed from the north pole) of the orbit of another satellite, Triton, when all the other large satellites of the solar system orbit their planets in a counterclockwise direction. Both of these oddities could be explained by a collision in which Pluto broke free. However, that theory lost favor with the discovery of Quaoar, Sedna, and Eris, and the realization that the Kuiper belt is likely populated with almost countless bodies of lesser size than these three icy objects. The common belief presently held is that Pluto and Charon were created in the early Kuiper belt by a giant impact, not unlike the impact theory for the formation of the Earth and its moon. More research is needed to pinpoint the origin of this unusual dual-object system in the outer solar system.

It was in part for these reasons that the International Astronomical Union (IAU) moved to reclassify Pluto as a dwarf planet even though it still met many of the restated criteria for classification as a planet. That definition now requires that a body orbit about the sun, that it is in hydrostatic equilibrium, and that it has cleared out its environment. Pluto meets only the first two of those criteria. In addition, since 2000 five objects roughly the same size as Pluto have been discovered well beyond Pluto’s orbit. These bodies (Quaoar, Sedna, Eris, Makemake, and Haumea) are believed to be Kuiper belt objects. Pluto is likely the first of the Kuiper belt objects to have been discovered. As such, it is the model for the rest of what may be a huge class of similar objects at ever-increasing distance. Some have coined the term “plutinos” to describe icy bodies coming from the Kuiper belt. Because of the new classification scheme adopted by the IAU in 2006, the solar system officially consists of eight planets, at least five dwarf planets (Ceres, Pluto, Eris, Haumea, and Makemake), and numerous satellites, asteroids, comets, and minor bodies.

In an attempt to clarify its controversial planetary classification and minimize the worldwide discontent expressed over Pluto’s elimination from full-fledged planetary status, an executive meeting of the International Astronomical Union (IAU) held in June 2008 in Oslo, Norway. This conference proposed defining a plutoid as a solar system body beyond the orbit of Neptune having enough mass to assume a nearly spherical shape but not able to clean its orbit of other material, as do the eight planets. Pluto would therefore be the first plutoid object. Eris would be the second. Under this definition, Ceres would remain a dwarf planet but could not be considered to be a plutoid, since it exists in the between Mars and Jupiter. For many, this proposal did little to mitigate earlier objections. This classification scheme left the inner solar system with only the one dwarf planet, which until 2006 most astronomers had been quite content to consider to be the largest asteroid. More plutoids have been expected to be found, located even farther out in the Kuiper belt, as technology permits their detection. The period following the IAU’s adoption of the reclassification scheme has seen much controversy, not only among professional astronomers but also among amateur astronomers, teachers, and schoolchildren. In 2024, the US state of Arizona named Pluto as its official state planet.

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