Science is and always will be a dynamic process, characterized by questions, observations, examinations, and discoveries that change previous ideas about our understanding of the universe and our place within it.
Science also is dependent upon classification of celestial objects to better understand them. With this emerging understanding come new ideas, perspectives and reclassifications of objects over time.
Defining the term "planet" is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, the objects thought to be planets have changed. The ancient Greeks counted the Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. By the 17th century, astronomers realized that the Sun was the celestial object around which all the planets - including Earth - orbit, and that the Moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.
The small body Ceres was discovered between Mars and Jupiter in 1801, and was at first classified as a planet. Many more objects were found in the same region and also classified as planets. Eventually Ceres and the rest of the bodies in the asteroid belt were reclassified as minor planets, but they still retain the asteroid label.
Pluto was discovered in 1930 and identified as the ninth planet in our solar system. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Its huge satellite, Charon, is nearly half the size of Pluto and shares Pluto's orbit. As more became known about Pluto, some astronomers questioned whether it should really be called a planet. Pluto kept its planetary status until the early 1990's, when huge technological advances in telescopes led to better observations and improved detection of very small, very distant objects.
Astronomers began finding numerous icy worlds orbiting the Kuiper Belt. This doughnut-shaped region lies beyond the orbit of Neptune, closer to Pluto's realm. With the discovery of the Kuiper Belt and its trillions of icy bodies (known as Kuiper Belt objects (KBO's), also called Trans-neptunian objects (TNO's), it was more accurate to define Pluto as a large KBO instead of a planet. Then, in 2005, a team of astronomers announced that they had found the tenth planet - it
was a KBO even larger than Pluto. Now, the dynamic nature of science was in full force, changing perspectives and posing the question that would forever change the definition of Pluto and our understanding of these distant icy worlds.
What IS a planet, anyway?
The International Astronomical Union (IAU) a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris), and, after much debate, in 2006 passed a resolution that defined a planet and established a new category, dwarf planet.
Eris, Ceres, (up until then defined as an asteroid) Pluto, and two more recently discovered KBOs named Haumea and Makemake, are dwarf planets recognized by the IAU as of September 2009. Pluto, Eris, Haumea, and Makemake are also classified as KBO's.
What makes a planet, according to the definitions adopted by the IAU -
| Is in orbit around the Sun || Yes || Yes |
| Has sufficient mass to assume a nearly round shape || Yes || Yes |
| Is not a satellite (moon) || Yes || Yes |
| Has cleared the neighborhood around its orbit || Yes || No |
| Has not cleared the neighborhood around its orbit || No || Yes |
As our knowledge of this mysterious, fascinating area of our solar system deepens and expands, the more complex it becomes and raises more questions that strive to be answered through advanced, innovative exploration.
More Information about Dwarf Planets:
What is Pluto?
NASA's Hubble Space Telescope: Measuring the Mass of Eris
NASA's Dawn Mission
Ceres: A Dwarf Planet