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 firstorder 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. 1. What is a massive planet? For the purposes of this discussion, a massive planet is a body that has been compressed to densities much higher than the normal (condensed) low-pressure state and yet is cold enough (i.e., degenerate) that there are no significant thermonuclear reactions. Roughly speaking, this places our consideration in the range 0.1 MJ < M < 80 MJ where MJ is the mass of Jupiter (0.001 of the mass of the Sun). The upper bound of 80 MJ is also the lower bound for stars on the main sequence, but it is common practice to think of bodies in the range of about 10 to 80 MJ (so-called brown dwarfs; cf. Stevenson 1991) as forming like stars. These bodies also have a small but non-negligible deuterium-burning epoch. Accordingly, I will concentrate on bodies less massive than about 10 MJ. The universe is mostly hydrogen and there are usually insufficient heavy elements around to make massive bodies as I have defined them out of material other than hydrogen. (The low-mass end of 0.1 MJ does, however, approach Uranus/Neptune bodies for which water is a likely major constituent; cf. Stevenson 1982.) For example, it is widely believed that the discs (gaseous nebulae) from which planets form around stars are typically limited to of order 5% of a stellar mass, and only a few per cent of that is not hydrogen or helium; this limits the total available heavy elements to less than a Jupiter mass and this material is widely disseminated. Moreover, large assemblies of ice and rock are nuclei for the unstable infall and aggregation of enormous amounts of gas. Accordingly, massive planets will be almost invariably hydrogen rich and I will concentrate my attention on bodies of this composition. † E-mail: [email protected]. 0953-8984/98/4911227+08$19.50 c © 1998 IOP Publishing Ltd 11227