Small Worlds - The Neighborhood

Getting to Know the Neighborhood

As the technology of telescopes and the capabilities of spacecraft improve, new observations change our perceptions of the bodies that orbit our Sun. In recent years many distant icy worlds have been discovered in the vast cold region beyond Neptune. Some of these discoveries have prompted a passionate and very public discussion about how these objects should be classified. New revelations that challenge our assumptions are an exciting part of the process of exploration.


As we learn more about these primordial building blocks of our solar system, the distinction between asteroids, comets and dwarf planets has become increasingly difficult to make. But even with the shuffling of names and categories in the past several years, we can divide the small worlds into three main categories and share some of their general characteristics.




 - Ceres -
A Big Space Rock No Matter What We Call It

Ceres is a great example of how scientists’ thinking evolves regarding classification of objects in the solar

system.  Ceres was discovered in 1801 by Giuseppe Piazzi.  He initially reported it as a comet, but with more observations noticed its movement was slow and uniform.  It was ultimately classified as the eighth planet.  However, half a century later, as other objects were discovered in the area, scientists realized Ceres represented the first of a class of many similar objects that were called “asteroids,” meaning “star-like.” Ceres retained its comfortable position as asteroid #1, largest and first discovered, for about 150 years. 

Keck telescopes
 At 590 miles across, Ceres is the largest object in the asteroid belt.


With continually improving technologies, more bodies are being discovered all the time.  How to designate them is a matter of great scientific debate. In 2006, the International Astronomical Union (IAU), an organization of Ph.D. level professional astronomers that assigns designations to celestial bodies, met and set about to adopt a formal definition of “planet.” The definition for planet that was accepted by the members at the meeting is a celestial body that 1) is in orbit around the Sun, 2) is nearly round in shape, 3) has cleared the neighborhood around its orbit, and 4) is not a satellite.


Ceres meets three of the four criteria, but is not a planet because it shares its orbit with the thousands of other objects in the main asteroid belt. Instead it was classified as a dwarf planet (along with Pluto, Eris, Makemake, and Haumea). Is Ceres still an asteroid?  Being a dwarf planet does not preclude having other designations.  Its closest friends and neighbors are asteroids.  Whatever we call it is not nearly as exciting as viewing it up close for the first time will be in February 2015 when the Dawn spacecraft goes into orbit around Ceres.




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