States of matter in massive planets

This brief article addresses the question: among the very large number of interesting condensed matter physics issues, which are particularly interesting from a planetary perspective? Following some definitions and background, it is argued that we need to understand relevant first-order phase transitions (especially the nature of the hydrogen phase diagram), the behaviour of the entropy (i.e., the Grüneisen parameter), the solubility and partitioning of minor elements (e.g. noble gases mixed with hydrogen), and microscopic transport properties, especially electrical and thermal conductivity. Examples are presented of how these issues influence current interpretations of the observations of Jupiter in particular. In the future, it may be possible to observe spectroscopically the compositions of extra-solar-system planets and brown dwarfs, and thereby learn more about the physics of these bodies.