Pluto

The ninth planet from the Sun , Pluto is one of the least well understood objects in the solar system . It is the smallest of the major planets, and has a most unusual orbit . Pluto's companion moon , Charon, is so large that the pair essentially form a binary system. How the Pluto-Charon system formed and how the system acquired its special 2-to-3 orbital resonance with Neptune are unanswered questions at the present time. We will probably not know more until a planned NASA space mission visits the Pluto-Charon system. At this time, Pluto is the only planet in the solar system that has not been visited by a space probe .

In 2000 NASA canceled the previously planed Pluto Express mission. In order to make progress toward its goal of reaching Pluto with a probe by 2020, NASA scientists and engineers have created the New Horizons mission to be administered by Johns Hopkins University, Applied Physics Laboratory.

The Pluto-Kuiper Belt Mission will be the first reconnaissance of Pluto and Charon. The probe will go on to explore the Kuiper Belt. As of February 2003, the Pluto-Kuiper Belt mission was scheduled to launch in 2006, and to encounter Pluto and Charon as early as 2015. Observations of Kuiper Belt objects might occur approximately 11 years later.

Basic properties

Pluto has the most eccentric (non-circular) orbit of all the planets in our solar system. While the planet's mean distance from the Sun is 39.44 Astronomical Units (AU), 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 248.03 years, and the time for the planet to repeat alignments with respect to the earth and the Sun (its synodic period) is 366.7 days.

While commonly referred to as the ninth and outermost planet of our solar system, 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 of 1979, and remained the eighth most distant planet from the Sun until March of 1999. 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. Under good viewing conditions, Pluto can be seen as a star-like 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 the planet is both small and very distant.

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 the planet's brightness, however, have revealed that Pluto rotates once every 6.3827 days. Pluto's spin axis is inclined at 123° 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 the planet moves around the Sun. In the early 1950s, for example, Pluto presented its south pole towards the earth, today, we see its equatorial regions. In the year 2050 Pluto will present its north pole towards the Earth.

Careful long-term monitoring of the variations in Pluto's brightness indicate that the planet is brightest when seen pole-on. This observation suggests that the poles are covered by reflective ices, and that the planet 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 1/6 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 typical daytime surface temperature of about -351°F (-213°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 (0.68 mi/sec (1.1 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 the 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.

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 out. This freezing and thawing of Pluto's atmosphere may explain why the planet has a relatively high surface albedo of about 40%. Essentially the periodic freezing and thawing of Pluto's atmosphere continually refreshes the methane ice at the planet's surface.

The discovery of Pluto

Speculations about the existence of a ninth planet arose soon after astronomers discovered that the planet Neptune (discovered in 1846) 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 David Peck Todd, 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 the American astronomer Percival Lowell, at the Flagstaff Observatory, in Arizona between 1905 and 1907. No new planet was found, however. A second survey was conducted at Flagstaff in 1914, but again, no new planet was discovered. On the basis of predictions made by W. H. Pickering in 1909, Milton Humason, at Mount Wilson Observatory, 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 was the young 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 the planet 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 one-hundred-and-forty-ninth anniversary of the discovery of Uranus , and to mark the seventy-eighth anniversary of Lowell's birth. 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's characteristics

Pluto has a density of about two times that of water and it is estimated that Pluto may have a core of silicate rock about 1700 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

Charon, Pluto's companion moon, was discovered by James Christy in June, 1978. Working at the U.S. Naval Observatory in Flagstaff, Arizona, Christy noted that what appeared to be "bumps" on several photographic images taken of Pluto reappeared on a periodic basis. With this information, Christy realized that what had previously been dismissed as image distortions were really composite images of Pluto and a companion moon. Christy 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.

Charon orbits Pluto once every 6.39 days, which is also the rate at which Pluto spins on its axis. Charon is therefore in synchronous orbit about Pluto. 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 1/20 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, observes on Earth would witness a whole 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 components size; Pluto has a diameter of 1,413 mi (2,274 km), making the planet 1.5 times smaller than Earth's Moon, and two times smaller than 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 the Earth, or about 1/5 that of the Moon has been derived for Pluto. Charon's mass is about 1/8 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 planet.

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 a mixture of rock and ice, Charon is most probably an icy body. 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 characteristics

Charon's surface is thought to be composed of water ice, nitrogen ice, and carbon-monoxide 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-monoxide ice. It is likely that Charon has no internal heat source and that it has no appreciable magnetic field.

Pluto's strange orbit

The Pluto-Charon system has the strangest orbit of all the planets in the solar system. It has a large eccentricity and a high orbital inclination of 17.1° 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 led some astronomers to suggest that the two bodies may have formed in the same region of the solar nebula. Subsequently, it is argued, 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 either captured as satellites around Uranus and Neptune, or being ejected from the Solar System. One important, and as 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.

History of the Pluto-Charon system

Obviously, a history of the Pluto-Charon system is quite speculative. It is though perhaps that this double-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 (Charon being formed by coalesced debris in near Pluto space). This 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. Recently, some astronomers have stopped referring to Pluto as a planet and have called it a TNO. Until a space mission returns data and photographs from Pluto, Charon, and some TNOs, scientists may not be able to eliminate any of the completing hypotheses.

Resources

books

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge Univ. Press, 1999.

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Levy, David. Clyde Tombaugh: Discoverer of Planet Pluto. Tucson: The University of Arizona Press, 1991.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge: Cambridge University Press, 2002.

periodicals

Binzel, R.P. "Pluto." Scientific American (June 1990).

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003]. <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.

JPL. "New Horizons: The Mission." NASA Jet Propulsion Laboratory (JPL) [cited February 15, 2003]. <http://pluto.jhuapl.edu/mission.htm>.

Martin Beech

David T. King, Jr.

KEY TERMS

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Objective diameter

—The diameter of a tele scope'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.