Planetary classifications

Since ancient times, no formal scientific definition of the word “planet” existed. In 2006, professional astronomers, for the first time, developed a formal definition of a planet. The new definition removed Pluto, previously labeled a planet for seven decades, from the list of planets. Pluto, however, was later reinstated as a dwarf planet, though debate rages on as to whether Pluto is a major planet in the solar system.

Overview

The word “planet” comes from the ancient Greek for “wanderer.” From prehistoric times, people have looked up and seen stars. Five of those stars seemed to wander from constellation to constellation. The Sun and Moon also moved among the heavens. Thus, seven bodies were historically classified as planets. They were the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The seven days of the week became associated with these planets. Other objects were seen to move in the sky, such as comets and meteors, but they did not move in predictable paths, and astronomers even debated whether they were celestial or atmospheric phenomena.

Nicolaus Copernicus’s heliocentric model, though, made the Sun the center of the solar system, with Earth, Mercury, Venus, Mars, Jupiter, and Saturn orbiting the Sun. The Moon orbits Earth. Proponents of the heliocentric model began listing planets as the bodies orbiting the Sun. Thus, six planets were recognized as orbiting the Sun. When Galileo Galilei discovered four satellites of Jupiter, they, too, were at first listed as planets. Soon, however, it was decided that bodies orbiting other planets are not planets themselves, so the bodies that Galileo found were, by acclamation, declared to be moons, or satellites, of Jupiter. Eventually, astronomers discovered Uranus and Neptune orbiting the Sun, bringing the list of planets to eight.

In the late eighteenth century, astronomers Johann Titius and Johann Bode suggested that the gap between Mars and Jupiter was disproportionately large and that an unseen planet may lurk between the two. Then, on January 1, 1801, Giuseppe Piazzi found an object orbiting the Sun in this gap between Mars and Jupiter. The body was named Ceres, and it was declared to be a planet. In the next few years, though, astronomers also found Pallas, Vesta, and Juno, all orbiting between Mars and Jupiter. Each of these bodies was classified as a planet. As discoveries continued to mount, however, astronomers realized that it had become unwieldy to consider all of these bodies planets. Since the bodies between Mars and Jupiter were all so small as to not even show a disk in the best of telescopes, they were deemed too small to be considered planets and thus were unofficially declared to be asteroids. For most of the twentieth century, these bodies were called minor planets by professional astronomers.

Astronomers also came to recognize comets as bodies orbiting the Sun. By the twentieth century, comets were known to be icy bodies that originated in the outer solar system. When they are disturbed into highly elliptical orbits that come too close to the Sun, the ice and frozen gases that make up the comet sublimate and boil off into space along with dust to form the tail of the comet. A more modern understanding of comets is that they may be similar to asteroids, only with a far larger percentage of ice and frozen gas.

In 1930, Clyde Tombaugh discovered Pluto. The planet Neptune had been found by observations of a perturbation in the orbit of Uranus by Neptune’s gravity. Early in the twentieth century, several astronomers mistakenly thought that Neptune’s orbit was also perturbed by another planet even farther from the Sun. Tombaugh was one of many astronomers searching for that planet when he discovered Pluto. Therefore, when Pluto was found, there was a predisposition to believe that it was quite large. Furthermore, at that time, astronomers knew of only two types of planetary bodies: small rocky worlds and gas giants shrouded in clouds. Pluto was clearly not a gas giant. If it were rocky, though, then it would have to be fairly large to appear as bright as it does since rocky worlds tend to reflect only a small percentage of sunlight that hits them. Within a few years, however, Pluto was found to be icy in nature. Ice is far more reflective than rock, so an icy Pluto could be much smaller than a rocky Pluto.

By the 1970s, astronomers realized that Pluto was tiny. It has since been found to be only about one-fifth the diameter of Earth and about 0.002 percent (one five-hundredth) the mass of Earth. Pluto is even smaller than Earth’s Moon. That makes Pluto much smaller than any other planet. Astronomers began debating as early as the 1980s whether Pluto should really remain on the list of planets, given that it is so small. Furthermore, Pluto’s composition is more like that of comets, which also originate in the outer solar system, than that of the other planets.

However, serious proposals were put forth to remove Pluto from the list of planets only after other icy bodies, very much like Pluto, only smaller, were discovered beyond Neptune. In the early years of the twenty-first century, objects were found that were only slightly smaller than Pluto, making the classification of Pluto as a planet, while not considering objects only slightly smaller to be planets, problematic. Then, object 2003 UB313 was discovered. This object, eventually named Eris, was somewhat larger than Pluto. Clearly, if Pluto were a planet, this object should also be a planet. The discovery of Eris, then, simply added to the arguments about what constituted a planet.

In 2005, therefore, the International Astronomical Union (IAU) established a committee to decide on a definition of what constitutes a planet. The committee made a recommendation that any body orbiting the Sun whose gravity is sufficient to form the body into an essentially spherical shape should be declared a planet. Such a definition retained Pluto’s status as a planet, added Eris to the list of planets, and returned Ceres to planetary status. However, this definition also made about a dozen other bodies possible planets; their planetary status would be unknown until astronomers were able to determine their shapes.

The committee’s proposal was rejected by the IAU. In 2006, after some deliberation, the IAU general assembly adopted a different definition of planet. That definition stated that a “planet” is a body orbiting the Sun large enough to form itself essentially into a sphere and sufficiently large for its gravity to clear the neighborhood of its orbit. A new category, called a “dwarf planet,” was defined as a body large enough to form itself into a spherical shape but not sufficiently massive to clear its orbital neighborhood. All other objects were to be designated as “small solar system bodies.” Since Pluto is only one of many objects of similar size and has not cleared its orbit, it was reassigned as a dwarf planet. Likewise, Ceres, Eris, Makemake, and Haumea are dwarf planets. The term “asteroid,” never an official term, was not mentioned, nor was the term “comet.” The term "plutoid" was designated for dwarf planets that orbit the Sun beyond Neptune. All comets, asteroids, and trans-Neptunian objects not large enough for their gravity to pull themselves into roughly spherical shapes are lumped together in the category of small solar system bodies.

Knowledge Gained

Scientists classify objects to aid their study of them. Astronomers are no different from other scientists in that respect. Classifying celestial bodies helps to organize knowledge about those bodies. Knowing that a newly discovered body is a comet or an asteroid gives a mental picture of its physical characteristics. However, the classification of objects as planets, comets, asteroids, and other entities was never officially defined before 2006. Astronomers had simply agreed informally on their definitions, and as a result, some bodies did not obviously fall into one category or another. With the discovery of more and more objects that fell between classifications—such as icy asteroids and not-so-icy comets—distinctions became more difficult. Eventually, objects such as Pluto, Eris, and similar-sized icy bodies forced a formal definition. However, both the original proposed definition and the one finally approved by the IAU were criticized as flawed, and with future discoveries and increasing data and insights, the issue continued to be debated.

There are multiple ways to classify objects in the solar system. Objects can be classified by size, method of formation, and physical composition. If size is the primary criterion for defining a planet, the eight bodies now defined as major planets do indeed meet that criterion since they are substantially larger than the other solar system bodies. If the method of formation is the most important factor, then Jupiter and Saturn would be seen differently since they formed in a manner different from that of the other bodies. Indeed, the inner rocky planets formed from components different from those of the outer planets; most astronomers consider the inner planets to be formed from smaller bodies such as the asteroids, which are likely leftovers from the formation of the solar system. The outer planets Uranus and Neptune are believed to have been formed from objects more like Pluto and the other trans-Neptunian bodies, making those objects in the outer solar system analogous to the asteroids in the inner solar system.

A major criticism of the 2006 IAU definition of a planet is that it does not define what is meant by a planet “clearing the neighborhood of its orbit.” For example, Jupiter shares its orbit with a number of asteroids, but those asteroids’ orbits are dominated and determined by Jupiter’s gravity. Asteroids in elliptical orbits cross the orbits of Earth, Mars, and Venus, yet those bodies are considered planets. The asteroids that cross those planetary orbits are unstable and cannot continue to cross those orbits forever. However, the orbits of the inner planets have not been completely cleared yet. Furthermore, the degree to which a body must be deformed toward a spherical shape to become a dwarf planet is not well defined. There are many bodies that are somewhat, but not quite, spherical. It is unclear if they are to be considered dwarf planets or small solar system bodies.

Context

Classification of bodies in the solar system is important to textbook authors and students, who often have as their first introduction to the solar system memorization of the planets and the types of objects in the solar system. There are different naming schemes for the different types of bodies, too, so knowing the classification for an object determines what sort of name is permitted. Planets are named one way, comets another way, and asteroids in yet a third way.

However, the particular classification of a body such as Pluto does not in any way change how professional astronomers view it. For many years, even before its reclassification as a “dwarf planet,” astronomers regarded Pluto as more like a comet or asteroid than like a planet. With the discovery that other trans-Neptunian objects were similar in size and composition to Pluto, astronomers began thinking of Pluto as simply one of the largest of those bodies. How it is classified in textbooks does not change how they regard it.

Making a rational definition of a planet is important to scientists in properly grouping bodies together that have similar characteristics to facilitate the study of those objects. When the scientific understanding of a body changes, then it sometimes needs to be reclassified. This reclassification is based on scientific principles, not public opinion. In the aftermath of the 2006 IAU definition, an attempt to mollify public affection for Pluto as the “ninth planet” led to another proposal for a definition of “planet” that did little to clarify the issue in the end. The final definition was crafted hastily and without sufficient forethought, making it, too, an incomplete definition. Therefore, the definitions of planets and other bodies may well be revisited in the near future.

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