Chapter 13: The Pluto System
Beyond the orbit of Neptune lies a genuinely unexplored planetary system centered on Pluto. Recently, Pluto has become the center of intense debate when the International Astronomical Union demoted Pluto to the status of a "dwarf planet," a class of small objects that includes the asteroid Ceres and a few recently discovered icy bodies in the outer solar system.
There are no detailed images of the surfaces of these small bodies because no spacecraft have yet visited their realm--this planetary incognita. The Voyager spacecraft did not fly by Pluto and its satellite Charon before leaving the solar system. Our best views of the dwarf planet have come from the Hubble Space Telescope. The surface features and many characteristics of Pluto will remain mysterious unless a flyby mission becomes reality in 2015 when NASA's New Horizons spacecraft reaches Pluto. In 1985, Pluto and Charon became an eclipsing binary system, with Charon passing before and behind Pluto as seen from Earth. This special geometry has allowed planetary scientists to calculate accurate orbital and physical parameters for the pair and to deduce something of the nature of these distant bodies. Detailed spectroscopic studies, conducted with telescopes on Earth, are also helping us probe the composition of the ices at its surface. In 2005, two other small satellites were discovered by the Hubble Space telescope.
Pluto is a place of extremes. It is far from the Sun, small, cold, and dark. Pluto is distinct from the outer planets in that it lacks a thick, hydrogen-rich atmosphere. It has a surface layer of frozen nitrogen and a tenuous atmosphere. Indeed, it is much more similar to the moons of Neptune than to any of the major planets. What follows is a brief summary of what we know about the nature of these distant objects, as well as some speculations. Pluto, Charon, and the other moons promise to be just as fascinating as the other bodies of the solar system. Their compositions and orbital evolution will prove to be key tracers of events in the ancient evolution of the outermost solar nebula.
1. Pluto is classed as one of the dwarf planets; it is icy, has several satellite companions, and a tenuous atmosphere. Our best images come from the Hubble Space Telescope.
2. Pluto and Charon form a double planet system with an elliptical orbit about the Sun. Pluto has a surface dominated by nitrogen ice and an atmosphere formed by vaporization of this same ice.
3. Pluto has a density significantly higher than those of the moons of Uranus and Saturn and about the same as Triton, suggesting that it contains a large proportion of rocky materials. In many ways, it must be similar to Triton. This may suggest that the Pluto and Triton formed as Sun-orbiting, icy planetesimals where the solids that condensed from the ancient nebula were enriched in rock compared to ice.
Since it was discovered in 1930 until 2006, Pluto was known as the most distant of the planets of our solar system. Pluto has a highly elliptical orbit that requires 250 years to complete (Figure 13.1) and it has not swept up all of the orbiting "debris" in its path. Hence, in 2006 it was grouped with the dwarf planets. The distance between Pluto and the Sun varies from a minimum of 4.5 billion kilometers to a maximum of 7.4 billion kilometers. So strongly does Pluto's orbit deviate from circular that periodically Pluto is closer to the Sun than Neptune. Besides being highly elliptical, Pluto's orbit is inclined nearly 20 degrees out of the plane of the solar system. Only asteroids and comets have similarly inclined orbits. Moreover, Pluto's orbit is in a 2:3 resonance with Neptune. Other orbital oddities ensure that the two never collide.
Pluto's orbit around the Sun is very elliptical; for about 20 years, Pluto is actually closer to the Sun than Neptune.
Controversy has surrounded Pluto since its discovery. Pluto is so small and distant that accurate measurements of its diameter are difficult to make from Earth. In the 1950s, Pluto was thought to be larger than Mercury (4800 km diameter) and about the same size as Saturn's largest satellite, Titan, with a diameter of about 5800 km. If this had been true, Pluto would have been more like Earth because the high calculated density suggested a rocky or even metallic composition. Decades of new observations show that Pluto is neither rocky nor Earthlike.
Late in 1978, a natural satellite of Pluto was discovered. This discovery substantially changed our notions of Pluto's size, density, and probable origin. The discovery photograph (Figure 13.2) shows Charon, the newly found moon, as extending or smearing the disk of Pluto. Charon is so close to Pluto and so far from Earth that its disk has not been resolved, or separated, from that of Pluto; it appears only as a bump on this speckled image constructed using an Arizona telescope. Much better images of Pluto and Charon have been obtained from the Hubble Space Telescope (Figure 13.3). This large telescope, placed in orbit by the Space Shuttle, is not hampered by the obscuring effects of Earth's atmosphere. Evidence from these photographs, and from careful observations of the pair as they pass in front of one another, shows conclusively that Pluto has a diameter of only about 2300 km. Thus, Pluto is smaller than seven planetary satellites, including Earth's Moon. Its closest analog is probably Triton, a satellite of Neptune.
This splotchy telescopic image of Pluto showed the existence of a moon orbiting this tiny, icy planet. The slight bulge at the top of the image is caused by Charon, Pluto's moon. The two bodies are so close that they cannot be resolved as separate objects by telescopes on Earth. (U.S. Naval Observatory)
Pluto and Charon are shown clearly as separate disks in this image constructed by the Hubble Space Telescope, although no surface features are visible. (NASA A: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility. B: STScI-PR96-09A)
Judging from telescopic observations, Charon has a diameter of almost 1200 km. These measurements show that Charon is more than half as large as Pluto, about which it orbits. Although by no means the largest planetary satellite in the solar system, Charon has the distinction of being the largest satellite as compared to its primary. It may be appropriate to think of Pluto and Charon as a double planet, unique in the solar system.
Charon orbits Pluto at a distance of only about 20,000 km (Earth's Moon orbits at a distance nearly 20 times as great). As seen from the surface of Pluto, Charon would be a dim, faintly glowing globe, but its apparent diameter would be nearly six times as large as the Moon appears from Earth. (In contrast, the Sun, forty times more distant than from Earth, is merely a bright star in Pluto's perennial twilight.) Like many other planetary satellites, Charon revolves once about Pluto for each rotation on its axis (6.4 days), and therefore keeps the same face pointed toward Pluto at all times. Unlike other systems, however, Pluto also keeps the same face pointed toward Charon, which therefore appears to remain locked in a fixed position in the sky, like a geosynchronous communication satellite. A viewer on the opposite side of Pluto would never see Charon. Both bodies exert substantial gravitational forces upon the other because of the similarity in their sizes and their relative proximity. The revolution of Charon about Pluto is from north to south, out of the plane of the solar system; Pluto, like Uranus, has a highly tilted spin axis (117 degrees), so that its equator is nearly perpendicular to its orbital plane. The tilt of the axis probably changes over time, but like Uranus, Pluto receives more solar energy at its poles than at its equator.
Two more moons (Nix and Hydra) were discovered in 2005 using images acquired by the Hubble Space Telescope. The moons are small, probably less than 100 km in diameter. Both lie beyond the orbit of Charon, but they orbit in the same plane as the larger satellite.
Early estimates of Pluto's density ranged from as high as 8 g/cm3 to as low as 1.2 g/cm3, greater than the total range for all other planets and satellites in the solar system. (Earth's density is only 5.5 g/cm3.) If near the high end of that range, Pluto should have been composed of rocky or even metallic components. A planet dominated by such components would be extremely difficult to reconcile with the nebular condensation model outlined in Chapter 2. Instead, this theory predicts that a small planet in the outer solar system would be composed of a mixture of rock and low density ices, like the satellites of the giant planets.
Careful measurements of the movement of Charon about Pluto allow the masses of Pluto and Charon to be calculated with greater accuracy. Pluto and Charon together have a mass that is only about 0.26% of Earth's mass and less than 20% the mass of the Moon. New measurements of the diameter of Pluto vary from 2300 to 2400 km. This may seem like a small difference but it translates into important differences in the calculated density and internal structure for Pluto. The mean density of the Pluto has been determined to be about 1.8 g/cm3. Data from the Hubble Space Telescope suggest that Charon's density is nearly identical.
This value for Pluto's density partly supports the prediction that it is composed of ice and rock. Moreover, Pluto is more dense than the icy satellites of Saturn and Uranus, but very similar to Triton, which has a density of 2.06 g/cm3. A reasonable model of Pluto's internal structure that satisfies the density requirements calls for 70 to 80% rock with a density of about 3 g/cm3, with the remainder being low density ices of water, methane, nitrogen, and carbon monoxide. If this model is correct, Pluto contains a higher proportion of rocky material than the moons of Saturn or Uranus, which can be modeled as about 50/50 mixtures of ice and rock. Pluto apparently has more rock than even Ganymede and Callisto, the large moons of Jupiter. Condensation models for the solar nebula suggest that the outermost regions of the ancient nebula may have been water poor and rich in carbon monoxide. Carbon monoxide should not condense as readily as methane ice, thereby leaving the solids richer in denser silicates and poorer in low density ices. The rather high densities of both Pluto and Triton support this idea.
Pluto's interior may consist of up to 70 to 80% rock (1), with the remainder being water ice (2) and and outer layer of methane and nitrogen ice (3). This configuration, reminiscent of Triton and Europa, is suggested by its relatively high density. The model assumes that Pluto is differentiated into a rocky core and an icy mantle, but that is uncertain. The ices in Pluto are probably dominated by water ice, but spectroscopic studies show that bright nitrogen and methane ices are present. Sublimation of ice from the surface may give Pluto a thin atmosphere of nitrogen. (Wikipedia: http://en.wikipedia.org/wiki/Image:Pluto-cutaway.svg)
Figure 13.4 shows a possible internal structure for Pluto. This model assumes that Pluto is differentiated into a rocky core and an icy mantle, but even that is uncertain. Accretion of this small body may not have produced enough heat to melt the ice and rock mixture sufficiently to allow gravitational separation and planetary differentiation. However, the high rock content probably led to melting of water ice because of heat released by radioactive decay. The ices in Pluto probably consist largely of water ice, but spectroscopic studies, completed using a sensitive telescope on top of Hawaii's Mauna Kea volcano, show nitrogen (N2) ice is the most common ice at the surface. Water ice has been detected only on Charon, thus far. Carbon monoxide (CO) ice and methane (CH4) ice are also present at the surface of Pluto. The nitrogen and methane ice may be concentrated in a discrete icy shell on top of a mantle of mostly water ice. In fact, some areas on Pluto are nitrogen rich and others are methane rich (darker and redder) suggesting that distinct terranes exist, perhaps like the those on Triton. Temperatures at the surfaces of Pluto and Charon hover just below 40 K, reminding us that the image evoked by Pluto's hellish namesake (Hades of Greek mythology) has little to do with reality.
A tenuous atmosphere, probably of nitrogen, envelops Pluto. The pressure exerted by this atmosphere may only be a few microbars (1 x 10-6 bars). Sublimation of nitrogen ice at the surface must be the source of this wispy atmosphere. Because of Pluto's eccentric orbit, temperature increases are expected to occur when it comes closer to the Sun and temperature decreases as it moves farther from the Sun. Because of these small temperature changes, repetitive cycling of ice and gas between the surface and atmosphere probably occurs. During orbit-related climate changes, the atmosphere should periodically freeze and fall to the surface during an exotic snow storm. Pluto's atmosphere may also contain carbon monoxide. There may be very little methane in the atmosphere of Pluto, in spite of the presence of methane ice on the surface. Pluto may simply be too cold for significant amounts of methane to sublime. Our best estimates of its temperature lie between 35 and 37 degrees above absolute zero.
Because Pluto is such an oddity--an icy planet among the gas-rich outer planets--there has been much conjecture about its origin. The chaotic nature of Pluto's orbit even makes it difficult to know the original location of Pluto. It might have formed in its current orbit; or it might have accreted in a less eccentric orbit and then evolved into the present situation. Since its discovery, the highly inclined and elliptical orbit of Pluto has been cited as evidence that Pluto originated as a satellite of Neptune. Perhaps a chance, close encounter between Triton (either as a satellite of Neptune or as an outer solar system renegade) and proto Pluto ejected Pluto from Neptune's satellite system and pushed Triton into its retrograde orbit around Neptune. Other scientists speculate that a large impact on a primitive Neptune orbiting Pluto fragmented the satellite, ejecting the material from Neptune's gravitational grasp and creating the present Pluto-Charon double planet. These Neptune origin scenarios are extremely unlikely because they require two chance events to occur nearly simultaneously. First, some body is required to move Pluto out of Neptune's grasp, and then a quick means of shifting it into resonance before it collided with Neptune. Another problem is that if Pluto had been thrown from the Neptune system, Pluto and Charon should have fused together again in less than a million years.
The most probable explanation is that Pluto and Triton accreted from materials that condensed in the same frigid part of the outer solar nebula as sun-orbiting planets. In this part of the nebula, the ratio of ice/rock that condensed was low, explaining why Pluto has so much rocky material inside it. In sub-nebulas that form around major planets, the ice/ratio is much higher, like that found in the moon systems around Uranus and Saturn. After accretion, Triton, with its retrograde orbit, was captured by Neptune's gravitational field. Pluto was subsequently captured in a 3:2 orbital resonance with Neptune. In this view, Pluto is little more than a big comet. Triton and Pluto may be outer solar system "asteroids," the last survivors of the multitude of icy planetesimals that accreted to form the outermost planets.
But where did Charon come from? Did Charon form by fission from a rapidly spinning molten Pluto? This seems unlikely because it is difficult to understand how such rapid spin rates could be achieved. Did Charon accrete as a separate planetesimal in orbit around Pluto? Probably not. This hypothesis would be unable to explain Charon's odd orbital properties. The simplest solution seems to be a catastrophic collisional origin of Charon from a once larger Pluto. Fragments blasted away from Pluto may have reaccreted in orbit to form Charon. A massive collision might also explain why Pluto's spin axis is tipped at an odd angle. We have invoked this same sort of chance event to explain the origin of our own Moon.
Many questions about the nature and origin of Pluto and Charon remain unanswered or even unformulated. One day a prolonged visit to Pluto could be extremely important to our perceptions of the outer solar system. Unlike Triton, Pluto probably was never heated by tidal massaging and therefore, it may come closer than any other planet to preserving primordial abundances of the ices that condensed from the nebula billions of years ago. The enigma of Pluto is likely to persist until a spacecraft, with cameras, spectrometers, and other instruments, can penetrate this remote part of the solar system and scrutinizes Pluto. The goals for such a mission have already been formulated and a late 1990s launch date has been discussed by NASA. A small spacecraft could reach Pluto in 6 or 7 years after launch. If this mission to the last unexplored planet is successfully funded, the quality of the new images and other data will exceed that obtained by Voyager at Triton.
Important questions that require answers, include
- What is the composition of Pluto?
- What are the sizes and densities of Pluto and Charon?
- What is the internal structure of Pluto? Is it differentiated?
- Was Pluto ever geologically active like Triton?
- What was the impact history of Pluto?
- Are the features of Charon and Pluto different as a result of different surface compositions?
Though answers to these and other questions may come from a flyby mission, the only certain thing, according to one of Pluto's investigators, is that we will be surprised by what we find. Until then, the results of Voyager investigations of Triton provide insights useful for interpreting the natures of Pluto and Charon. Continued Earth based investigations will yield most data for improving our understanding of these planets on the edge of the solar system.
1. Why is it appropriate to consider Pluto-Charon as a double planet?
2. How did the discovery of Charon change our view of the nature of Pluto?
3. In terms of its origin (not composition), is the atmosphere of Pluto more like that of Mars or Jupiter?
4. What other bodies in the Solar System is Pluto most like? What does this imply about its origin?
5. Early estimates of the density of Pluto were greater than the density of the Earth. If these estimates had been supported by later measurements, would they conflict with the theory for planet formation from a solar nebula?
6. Summarize why Pluto should, or should not, be considered a planet.
Binzel, R. P. 1990. Pluto. Scientific American. Vol. 262, p. 50
Mulholland, D. 1982. The Ice Planet. Science. Vol. 82, p. 64 68. (Discusses Pluto and the discovery of Charon.)
Sobel, D., 1993, The last world. Discover. No. 5, p. 68-76.
Tombaugh, C. W., and P. Moore. 1980. Out of the Darkness: The Planet Pluto. Harrisburg, PA: Stackpole Books. (An account of Pluto co authored by its discoverer.)