Discovery and Classification in Astronomy: Scientific and Philosophical Challenges and the Importance of a Comprehensive and Consistent Classification System

Throughout history, the definition of “class” and the construction of astronomical classification systems has been a deep scientific and philosophical problem: scientific because facts such as physical composition ideally need to be known for proper classification but often are not, philosophical because astronomers need to understand the philosophical assumptions behind their attempts at classification, and because different philosophical ideas such as “natural kinds” often guide classification, even if unconsciously. The primary lesson of history is that the most useful classifications of celestial objects are optimally based on their physical nature. The second lesson is that because discovery is an extended process consisting of detection, interpretation, and understanding, initial classifications may be phenomenological, based on characteristics that may be useful in early “detection” stages of extended discovery. By contrast, final classifications of “the thing itself,” is achieved only after the “understanding” stage of discovery and must have a physical basis. A third lesson is that class status is best determined within a comprehensive classification system in order to determine taxon level, e.g., class, type, subtype. Such a system, encompassing all astronomical objects, illustrates the problems of class and classification, problems that may be applied to exoplanet discoveries.