Fritz Zwicky s Extraordinary Vision. New York Times. <http://www. nytimes.com/2011/10/07/opinion/the-universe-dark-energy-and-us. html> (accessed October 17, 2011). Kirshner, Robert P., The Universe, Dark Energy, and Us. American Museum of Natural History. <http://www.amnh.org/education/ resources/rfl/web/essaybooks/cosmic/p_zwicky.html> (accessed October 17, 2011). Dwarf Planets The classification of celestial objects as dwarf planets is relatively new. The International Astronomical Union (IAU) the preeminent association of professional astronomers recognized as the authority for classifying and naming natural objects in space created this category of object in 2006. As of October 2011, the IAU recognizes five dwarf planets in our solar system: Pluto, Eris, Ceres, Haumea, and Makemake. The new guidelines adopted by the IAU in 2006 changed Pluto s status to that of a dwarf planet. It had previously been considered the solar system s ninth and smallest planet. Astronomers predict that as astronomical observations improve, many more objects in the far reaches of the solar system will be categorized as dwarf planets. Defining Dwarf Planets The IAU laid out the following criteria for a celestial body to be recognized as a dwarf planet: It must be orbiting the Sun The mass of the object must be great enough so that its own gravity pulls it into a round shape (technically, the object is said to be in hydrostatic equilibrium) The object has failed to clear the neighborhood around its orbital path of other celestial objects The object is not itself a satellite of another object; for instance, it is not a moon orbiting a planet These rules have led to some relatively small celestial objects being classified as dwarf planets, while much larger objects in the solar system are not considered planets neither as a full-fledged planet like Earth or Mercury, nor even a dwarf planet. Under these guidelines, for instance, the asteroid Ceres is a dwarf planet: its gravity has pulled it into a round shape; it is not a satellite of a planet or other celestial object; it directly orbits the Sun; and finally, it has failed to clear Space Sciences, 2 nd Edition 65
* elliptical having an oval shape its orbital neighborhood in the solar system because all sorts of other asteroids come close to it. Now compare Ceres, with a diameter a bit under 1,000 kilometers (620 miles), to Titan, which is much larger (and heavier) with a diameter of nearly 2,600 kilometers (around 1,600 miles). Titan is round in shape, and even has a thick atmosphere and lakes consisting of hydrocarbon liquids. However, in spite of its large size (it s larger even than Mercury or Pluto), Titan is considered neither a planet, nor a dwarf planet, since it is a moon of the planet Saturn that is, it does not orbit the Sun directly. Besides the classification of dwarf planets, the IAU recognizes the eight planets of our solar system, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (from closest to farthest from the Sun). If a natural (not man-made) object fails to meet the requirements to be either a planet or a dwarf planet, and it is not itself a satellite (such as one of the many moons of Jupiter or Saturn), then it is classified as being a small solar system body. Indeed, most of the comets and asteroids in the solar system are considered small solar system bodies. The IAU s creation of the dwarf planet category, as well as some of its other solar system classifications, was met with enthusiasm by many professional and amateur astronomers; but others dislike at least some aspects of the new groupings of solar system objects. Opinions are particularly strong when it comes to the demotion (as some see it) of Pluto, from being considered the ninth planet of our solar system for around 75 years, to being reclassified as a dwarf planet, lumped into the same category as the asteroid Ceres. At the time of the announcement that changed Pluto s status from planet to dwarf planet, many people in the general public also objected to the change, especially schoolchildren who had previously been taught that Pluto was the ninth planet. Our Changing View of the Solar System The ancient Greeks noticed that a few of the stars in the night sky seemed to slowly change their positions relative to the vast majority of stars, which appeared to be fixed in certain patterns, or constellations. These few meandering stars they called planets, which means wandering star. Astronomers such as Nicolaus Copernicus (1473 1543), Johannes Kepler (1571 1630), and Galileo Galilei (1564 1642) ushered in a new paradigm in astronomical thinking, by showing that these planets, and also the Earth, are in elliptical* orbits around the Sun (instead of the previous Earth-centered view of the cosmos). The five classical planets observable with the naked eye and known since ancient times had their number increased for the first time when astronomer William Herschel (1738 1822) announced the discovery of the planet Uranus in 1781. In 1801, Giuseppe Piazzi discovered Ceres, which was celebrated at the time as another new planet discovery. Ceres orbits the Sun between the orbits of Mars and Jupiter. However, many other objects were soon found to possess orbits between Mars and Jupiter. 66 Space Sciences, 2 nd Edition
Eventually, the newly-found bodies like Ceres were called minor planets in order to differentiate them from the much larger, well-known planets, including Earth. In 1846, Neptune was spotted out beyond the orbit of Uranus. Its discovery was due to mathematician Urbain Le Verrier s (1811 1877) calculations which showed that Uranus s orbit was being affected by another massive body farther out in the solar system. Similar calculations about Neptune s own orbit later shown to be incorrect implied there was a ninth massive body even farther out from the Sun. While searching for this Planet X, Clyde Tombaugh found Pluto in 1930. * eccentric the term that describes how oval the orbit of an astronomical body is * ecliptic the plane of Earth s orbit * astronomical unit the average distance between Earth and the Sun (152 million kilometers [93 million miles]) Misfit Pluto From almost the beginning of its discovery, Pluto stood out as different from the other planets in the solar system. Compared to the other planets, Pluto s orbit is highly eccentric*. It is also 17 degrees off of the ecliptic* plane. Most planets are within 3.5 degrees of the ecliptic, and the exception Mercury is only 7 degrees off. Furthermore, as the twentieth century progressed, better estimates of Pluto s mass were possible, and showed that it is about 30 times less massive than the next lightest planet in the solar system (Mercury). Taken together, the characteristics of Pluto seem quite different than those of the major planets. It did not appear to astronomers that Pluto formed in the same way as did the other planets. Moreover, some astronomers speculated that other objects similar to Pluto might exist even farther out from the Sun. Over the course of the twentieth century, the Kuiper Belt and the Oort Cloud were discovered. The Kuiper Belt is a disk that begins beyond the orbit of Neptune and stretches to about 50 astronomical units* (AU) from the Sun. Astronomers theorized that it contains a vast quantity of relatively small objects, left over from the cloud of particles that had condensed to form the Sun and the other planets. The Oort Cloud is a vast sphere enclosing the solar system at an even greater distance. The term trans-neptunian object (TNO) is used to discuss any object that on average is farther from the Sun than Neptune, and includes both of these bodies. Due to their great distance from the Sun the Kuiper Belt and Oort Cloud are difficult to observe directly. The Kuiper Belt was just a hypothesis until the first body besides Pluto was found there in 1992, with a second such object discovered six months later. These small objects remain formally unnamed (with only astronomical number-and-letter designations), but they demonstrated the existence of the Kuiper Belt. Thousands more small objects have since been found in the Kuiper Belt. The discovery of TNOs besides Pluto led some astronomers to suggest that Pluto should be reclassified. Then in 2005, another TNO was discovered that was apparently larger and more massive. (This new TNO, since named Eris, is now thought to be a little smaller than Pluto, but about one-quarter more massive.) Faced with this new discovery, and the Space Sciences, 2 nd Edition 67
An artist s conception of the dwarf planet Eris, with the Sun shown in the distance. NASA/JPL-Caltech.... * aphelion the point in an object s orbit that is farthest from the Sun prospect that many more such objects were waiting to be discovered, the IAU created the category of dwarf planet and reclassified Pluto, as well as Ceres and Eris. The Ongoing Search for Additional Dwarf Planets By the end of 2011, a couple of hundred TNOs had been identified in the Kuiper Belt that possessed a good probability or better of qualifying as dwarf planets once further measurements can be taken. And there were several hundred more TNOs which had the smaller but still distinct possibility of being dwarf planets. Taking into account the regions beyond the Kuiper Belt, such as the scattered disc and Oort Cloud, there could conceivably be several thousand objects in the solar system that could qualify as dwarf planets. The problem with finding and confirming TNOs as dwarf planets has to do with the vast distances involved, and the very faint sunlight such objects reflect, which makes determining their sizes very difficult with the current technology in place. Undoubtedly, however, more TNOs will be classified as dwarf planets, to join those already known. The five confirmed dwarf planets, as of late 2011, are listed below in their order of discovery: Ceres, discovered in 1801, and the only asteroid that is also a dwarf planet; it is the smallest of the five dwarfs and has a maximum distance from the Sun (aphelion*) of nearly 3 AU Pluto, discovered in 1930; it has four known moons and an aphelion of almost 50 AU 68 Space Sciences, 2 nd Edition
Haumea, discovered in 2004; it has two known moons and an aphelion of nearly 52 AU Eris, discovered in 2005; it is the most massive of the dwarf planets, possesses one known moon, and has a aphelion of slightly over 97 AU Makemake, discovered in 2005 (after Eris); no known moon, and an aphelion of approximately 53 AU The availability of advanced, extremely sensitive infrared space telescopes such as the Herschel Space Telescope of the European Space Agency, and the U.S. Spitzer Space Telescope, as well as ongoing use of the workhorse Hubble Space Telescope, should lead to the discovery of many more TNOs, including candidates for dwarf planet status. These space telescopes can image the infrared light emitted by the extremely distant, and therefore extremely cold, TNOs. See also Asteroids (Volume 2) Kuiper Belt (Volume 2) Oort Cloud (Volume 2) Planet X (Volume 2) Pluto (Volume 2) Small Bodies (Volume 2) Resources Books and Articles Brown, Mike. How I Killed Pluto and Why It Had It Coming. New York: Spiegel & Grau, 2010. Rivkin, Andrew S. Guide to the Universe: Asteroids, Comets, and Dwarf Planets. Santa Barbara, California: Greenwood Press, 2009. Schilling, Govert. The Hunt for Planet X: New Worlds and the Fate of Pluto. New York: Copernicus Books/Springer Science, 2009. Websites The Dwarf Planets. California Institute of Technology. <http://web.gps. caltech.edu/~mbrown/dwarfplanets/> (accessed October 14, 2011). Dwarf Planets: Overview. National Aeronautics and Space Administration. <http://solarsystem.nasa.gov/planets/profile. cfm?object=dwarf> (accessed October 14, 2011). Pluto and the Developing Landscape of Our Solar System. International Astronomical Union. <http://www.iau.org/public/pluto/> (accessed October 13, 2011). Space Sciences, 2 nd Edition 69