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S. J. Dick (NASA HQ)
Discovery, classification and interpretation are problems common to biology and astronomy, constituting their ‘natural history.’ Unlike biology, and despite a long history of discovery and classification of numerous types of objects, astronomy has no comprehensive classification system. After reviewing the history of classification systems in chemistry, physics, biology and astronomy, a comprehensive classification system is proposed for astronomical objects. Like biology’s system, it is hierarchical in nature, beginning with astronomy’s three kingdoms: planets, stars and galaxies. The discovery of each of these kingdoms has a rich history.
Employing gravity’s way of organizing astronomical objects as a basis for taxonomy, a classification system is built around these three kingdoms, beginning with six Families of objects for each Kingdom. These Families are the proto-family (protoplanetary, protostellar, protogalactic), the central objects themselves of each Kingdom (planets, stars and galaxies), the circum-family (circumplanetary, circumstellar, circumgalactic), the sub-family (subplanetary, substellar, subgalactic), the inter-family (interplanetary, interstellar and intergalactic), and the systems family (planetary systems, stellar systems, galactic systems). Thus, there are 18 Families of astronomical objects. Under them come at least 73 classes of objects, incorporating a wide variety of classification systems that astronomers have already devised. Further divisions at lower levels of the hierarchy are possible. Such a comprehensive classification system has many uses, ranging from pedagogic to scientific. As in phylogenetics for biology, classification is important for understanding the natural evolutionary relationships of astronomical objects.
The intertwined history of the discovery, classification and interpretation of classes of astronomical objects is a rich subject for further research. It is especially relevant today in terms of the controversies over the definition of ‘planet,’ the dividing line between ‘planet’ and ‘star,’ and the discovery of new classes of objects. Some lessons of history are applied to the current controversies in light of the system here proposed.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.