Salem Press

Solar System
Dwarf Planets

Categories: Planets and Planetology; Small Bodies

The discovery of many new bodies orbiting the Sun beyond the orbit of formerly outermost Neptune—including at least one larger than Pluto—created a crisis in astronomy. It became evident that a new definition was required to distinguish these objects from traditional planets. The term "dwarf planet" was introduced to include planetary objects smaller than planets but larger than asteroids, resulting in the demotion of Pluto from its status as a planet.

Overview
The concept of a planet has a long history, leading to a total of nine planets in the solar system until discoveries in the early twenty-first century led to new definitions that excluded Pluto. The word "planet" originates from a Greek word meaning "wanderer" and for centuries was applied to celestial objects that shifted positions relative to the "fixed" stars. In classical antiquity, seven such objects were identified and were associated with mythical gods: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Latin names for the seven days of the week were based on these seven celestial deities. In Greek thought, the planets were believed to orbit the Earth along complex paths determined by a combination of circles.

During the scientific revolution of the sixteenth and seventeenth centuries, it was shown that five of the classical planets revolve around the Sun in elliptical orbits, along with the Earth-Moon system. Late in the eighteenth century, British astronomer William Herschel, aided by his sister Caroline, discovered Uranus, the first planet to be discovered with the aid of a telescope. Early in the nineteenth century, Sicilian astronomer Giuseppe Piazzi discovered what he thought was a new planet, smaller than Mercury and orbiting the Sun between Mars and Jupiter. He called it Ceres. However, when many smaller bodies with similar orbits were discovered in the next few decades, they were called asteroids, and Ceres was demoted from its status as a planet. The asteroids are believed to be remnants from the formation of the solar system.

By the middle of the nineteenth century, investigations into slight deviations in the elliptical orbit of Uranus led to the discovery of Neptune by the German astronomer Johann Galle. Using Sir Isaac Newton's law of universal gravitation, astronomers were able to determine the masses of all but two of the eight known planets from the motions of their satellites, with Jupiter as the most massive, at 318 times the Earth's mass. Perceived deviations in the orbit of Neptune led to the discovery of Pluto in 1930 by American astronomer Clyde Tombaugh. Pluto's orbit differed from those of the other planets, with its large inclination from the ecliptic plane and its highly elliptical shape that brings it closer to the Sun than Neptune during some 20 years of its 248-year period. It was also found to be much smaller than the outer gas giant planets and to consist mostly of icy materials.

In 1977 Charles Kowal discovered a small, icy planetoid orbiting the Sun between Jupiter and Uranus, later named Chiron. In the 1990's several similar, cometlike objects were found between Jupiter and Neptune and are now called centaurs. Pluto's status as the ninth planet began to be suspect in 1978 when its satellite Charon was discovered, and Pluto's mass was found to be only 0.2 percent of Earth's mass. That is much less than even Mercury, at 5.5 percent of the mass of the Earth. Pluto's mass was too small to have produced deviations in Neptune's orbit, which were then found to be negligible.

Then, in 1992 after a five-year search using digital cameras and computerized analysis, David Jewitt and Jane Luu of the Massachusetts Institute of Technology (MIT) discovered the first of many similar icy objects beyond Neptune in a region called the Kuiper belt. Existence of such a region had been predicted by Dutch American astronomer Gerard Kuiper. It is similar to the asteroid belt between Mars and Jupiter but about twenty times wider and populated by icy objects rather than the rocky and metallic bodies found in the asteroid belt. The Kuiper belt extends from the orbit of Neptune between 30 and about 55 astronomical units (AU) and is believed to contain thousands of objects larger than 100 kilometers in diameter.

More than 130 Kuiper belt objects (KBOs) have been found with nearly the same 248-year period as Pluto at about 40 AU from the Sun. These "plutinos" complete their orbits twice during three orbits of Neptune, referred to as a 2:3 gravitational resonance. KBOs with other resonances, such as 3:5 and 4:7, are called cubewanos, and a few objects are found beyond a 1:2 resonance at 55 AU and with 330-year periods. Some objects have been found beyond 55 AU but are believed to have been scattered from the Kuiper belt into a region called the scattered disk containing scattered disk objects (SDOs). Planetesimal objects in these latter two regions (KBOs and SDOs) are called trans-Neptunian objects (TNOs).

Astronomers began to view Pluto as the largest member of the new class of plutinos, and some started to question its status as a planet. In 2003 a team from the California Institute of Technology (CalTech), working at Mount Palomar Observatory north of San Diego and led by Mike Brown, discovered an SDO at about 97 AU from the Sun, now called Eris. When a satellite was discovered in 2005, the mass of Eris was found to be 27 percent larger than that of Pluto, and a few astronomers began to refer to it as the tenth planet. Most astronomers, however, recognized that many TNOs might be larger than Pluto and that either they would also have to be classified as planets or Pluto would have to be reclassified to distinguish such objects from the traditional planets.

The definition of a planet was placed on the agenda for the General Assembly of the International Astronomical Union (IAU) meeting in August of 2006. An initial draft proposal recommended that Pluto be retained as a planet and that Ceres, Charon, and Eris be added to the list of planets. This recommendation was made by astronomers who viewed both Pluto and its satellite Charon as planets in a double-planet system, since each body rotates about a point located between the two. After many objections, an alternate proposal was offered by the Uruguayan astronomer Julio Fernández, who suggested an intermediate category for objects like Pluto, which are large enough to be nearly round but too small to clear their orbits of other planetesimals. The IAU accepted this proposal, and by unanimous vote it was agreed to call these intermediate objects "dwarf planets," with smaller objects to be called "small solar-system bodies." By further vote, it was agreed that Pluto is a dwarf planet.

These definitions as voted in Resolution 5A by the Twenty-sixth General Assembly of the IAU are as follows:

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" is 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, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is 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 neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "small solar-system bodies."

In three footnotes, this IAU resolution agreed that the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. It also agreed to establish a process for assigning objects to the category dwarf planet or another status. It also suggested that small solar-system bodies include most solar system asteroids, most TNOs, comets, and other small bodies. In the same meeting, the IAU announced only three members of the dwarf planet category: Ceres, Pluto and Eris.

Knowledge Gained
The new definitions of planets, dwarf planets, and small solar system bodies have helped clarify both the nature of these objects and the structure of the solar system, as well as stimulating new research about them. The new definitions have led to new searches for dwarf planets and new research on criteria for hydrostatic equilibrium shape (nearly round) and orbital dominance (clearing the neighborhood).

The IAU maintains a dwarf planet watch-list of about a dozen candidates, which keeps changing as new candidates are found and as more is learned about the physics of existing candidates. Current candidates include the plutinos Orcus and Ixion, cubewanos Quaoar and Varuna, and the SDO Sedna, all of which are similar in size to or larger than Ceres (975 kilometers, or just over 3 miles in diameter) but are not yet established as round. Observations indicate that icy bodies of more than about 400 kilometers reach hydrostatic equilibrium, but rocky objects with more rigid interiors might require at least 800 kilometers. The only other asteroid candidate seems to be Vesta, the second largest at 530 kilometers, which appears to be round except for a large impact crater. The Dawn space probe, scheduled to orbit Vesta by 2011, may resolve its status. Estimates range from forty to two hundred candidates in the Kuiper belt and more beyond it.

Context
The new definitions of "planet" and "dwarf planet" highlight the increasingly complex nature of the solar system as more is discovered about it. The definitions have also, however, introduced many ambiguities and criticisms.

The new definitions do incorporate accepted theories for the evolution of the solar system and appeal to observational criteria. As planets formed from the dust and planetesimals of the solar disk, their gravity attracted more matter and they eventually dominated their orbits. However, if planetesimals were sufficiently disturbed by gravitational forces, such as those from nearby Jupiter, they never formed planets and remained as asteroids. Although no planets have completely cleared their orbital neighborhoods, even Mars, as the least dominant planet, has collected more than five thousand times as much material as that which remained in its orbit, while Ceres is only 0.33 times larger and Pluto only 0.07 times larger than the remaining material in their orbits.

Critics complained, however, about that the new definitions were arbitrary, since no planet has completely cleared its orbit, and that the round shape of hydrostatic equilibrium is ambiguous, since there are various degrees of roundness. Others voiced concerns about the demotion of Pluto from its longtime status as a planet. Although the National Aeronautics and Space Administration (NASA) decided to accept the new definitions, many respected astronomers, including the director of the New Horizon mission to Pluto, Alan Stern, remained opposed, and his team continued to refer to Pluto as a planet. The discussions and debates would continue at later meetings of the IAU and as more was learned about solar-system objects and their physics.

Joseph L. Spradley

Further Reading
Hartmann, William K. Moons and Planets. 5th ed. New York: Brooks/Cole-Thomson Learning, 2005. An excellent introductory college text on planetary science by one of the leaders in the field. It has good chapters on the formation of the solar system and on asteroids and other small solar-system bodies.

Kaufmann, William J., III. Universe. 8th ed. New York: W. H. Freeman, 2007. College-level introductory text covering the field of astronomy. Contains descriptions of astrophysical questions and their relationships.

Serge, Brunier. Solar System Voyage. Translated by Storm Dunlop. New York: Cambridge University Press, 2000. This well-illustrated book describes the solar system and discusses issues related to the definition of planets.

Sobel, Dava. The Planets. New York: Viking, 2005. A very readable account by a popular science writer of the nature and history of planets and asteroids, and of the scientists who study them.

Soter, Steven. "What Is a Planet?" Scientific American 132, no. 6 (January, 2007): 2513-2519. A planetary scientist discusses the controversy over the revised definition of a planet, including both its flaws and the scientific advantages of the concept of a dwarf planet.

Weintraub, David A. Is Pluto a Planet? A Historical Journey Through the Solar System. Princeton, N.J.: Princeton University Press, 2006. This book traces the concept of a planet from antiquity to the present day, providing the historical and astronomical context for deciding if Pluto is a planet.

See Also
Extrasolar Planet Detection Methods; Extrasolar Planetary Systems; Interplanetary Environment; Kuiper Belt; Nemesis and Planet X; Oort Cloud; Planetary Classifications.