3Novices:Planets come in different species

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.

A quarter of a century ago only nine planets were known—those of the Solar System, a number subsequently reduced to eight by the demotion of Pluto. These nearby worlds have, however, now been joined by thousands more that orbit stars other than the sun. Many of those have been discovered or confirmed to exist by Kepler, an...Continue reading

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3Novices