THE starting-point of science is collecting: animals, plants, minerals, elements, even stars. Then, once a collection is large enough, patterns begin to emerge. Animals and plants fall into phylogenetic trees, minerals into crystal groups, elements into the periodic table, stars into the Hertzsprung-Russell diagram. Those patterns both require and suggest explanation. Thus, the theory of evolution, the science of crystallography, an understanding of the chemical bond and a description of how stars shine over their lifetimes have all emerged from the classification of collections. Now, it appears, something similar is happening to planets.
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