Dwarf planets

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 nine planets in the solar system, but 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, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Latin names for the days of the week were based on these seven celestial deities. In Greek thought, the planets were believed to orbit 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 discovered with 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 (though later reinstated as a dwarf planet). The asteroids are believed to be remnants of 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 could 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 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 twenty years of its 248-year period. It was also much smaller than the outer gas giant planets and consisted mainly of icy materials.

In 1977, Charles Kowal discovered a small, icy planetoid orbiting the Sun between Jupiter and Uranus, later named Chiron. In the 1990s, several similar, comet-like objects were found between Jupiter and Neptune and are 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 only 0.2 percent of Earth’s mass. That is much less than even Mercury at 5.5 percent of the mass of Earth. Pluto’s mass was too small to have produced deviations in Neptune’s orbit, which were then found negligible.

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 the Kuiper Belt region. Dutch American astronomer Gerard Kuiper had predicted the existence of such a region. 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 in the asteroid belt. The Kuiper Belt extends from the orbit of Neptune, between thirty and about fifty-five astronomical units (AU), and is believed to contain thousands of objects larger than one hundred kilometers in diameter. The New Horizons space mission began in January 2006, and following its investigation of Pluto, it reached the Kuiper Belt, where it remained through the early 2020s.

More than 130 Kuiper Belt objects (KBOs) have been found within nearly the same 248-year period as Pluto at about forty AU from the Sun. These plutinos complete their orbits twice during the 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 fifty-five AU and with 330-year periods. Some objects have been found beyond fifty-five AU but are believed to have been scattered from the Kuiper Belt into a region called the scattered disk, which contains scattered disk objects (SDOs). Planetesimal objects in these latter regions (KBOs and SDOs) are called trans-Neptunian objects (TNOs).

Astronomers began to view Pluto as the most prominent 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 the Palomar Observatory in north San Diego County and led by astronomer Mike Brown, discovered an SDO at about 97 AU from the Sun called Eris. When a satellite was discovered in 2005, the mass of Eris was 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 they would have to be classified as planets, or Pluto needed 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 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 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, the Uruguayan astronomer Julio Fernández offered an alternate proposal, 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 a majority 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, state that "planets and other bodies in our Solar System, except satellites, be defined into three distinct categories" and were based on the IAU's statement that "recent discoveries lead us to create a new definition [of the word planet], which we can make using currently available scientific information." The resolution then provides three footnotes defining a planet, a dwarf planet, and all other objects, thereby confirming that the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The process for assigning objects to the category dwarf planet or another status was established, and the suggestion was made 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. In 2008, Haumea, discovered in 2004, and Makemake, discovered in 2005, were recognized as dwarf planets, for a total of five officially recognized dwarf planets. In October 2019, scientists took the first high-definition picture of the asteroid Hygiea, showing that the object is spherical. This discovery made Hygiea a contender to be reclassified as the sixth dwarf planet.

Knowledge Gained

The revised 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 research about them. The revised definitions have led to searches for dwarf planets and research on criteria for hydrostatic equilibrium shape (nearly round) and orbital dominance (clearing the neighborhood around its orbit).

The IAU maintains a dwarf planet watch list, which regularly changes as new candidates are found and as more is learned about the physics of existing candidates, including the plutinos Orcus and Ixion, cubewanos Quaoar and Varuna, and the SDO Sedna. These candidates are similar in size or larger than Ceres (approximately 950 kilometers in diameter) but are not yet established as round. Observations indicate that icy bodies of more than 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 object in the asteroid belt at 530 kilometers, which appears to be round except for a large impact crater. The Dawn space probe, which orbited Vesta in 2011 and 2012, may resolve its status in time. Estimates range from forty to two hundred dwarf-planet candidates in the Kuiper Belt and beyond.

Context

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

The revised definitions incorporate accepted theories for the solar system's evolution 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 like those from nearby Jupiter, they never formed as planets and remained as asteroids. Although no planets have completely cleared their orbital neighborhoods, even Mars, the least dominant planet, has collected more than five thousand times as much material as what remained in its orbit. Ceres is only 0.33 times larger, and Pluto is only 0.07 times larger than the remaining material in their orbits.

Critics complained that the revised definitions were arbitrary. Many have stated that no planet has completely cleared its orbit, and hydrostatic equilibrium's round shape is ambiguous because there are varying degrees of roundness. Others have voiced concern about the demotion of Pluto from its longtime status as a planet. Although the National Aeronautics and Space Administration (NASA) decided to accept the revised 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.

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