Mass-Radius Relations of Giant Planets: The Radius Anomaly and Interior Models

Abstract

Thousands of worlds are now known, and hundreds of giant planets (albeit mostly with short orbital periods) have been assessed with accurate measurements of their masses and radii. As a consequence, a large range of planetary bulk densities have been charted, giving important clues to the compositions, the interior structures, and the geophysical processes that typify extrasolar planets. Moreover, two decades of investigations – both observational and theoretical – have generated a compound of important insights and enduring mysteries. Taken broadly, the observations and models indicate that most giant planets have an inhomogeneous structure consisting of a heavy element core and a hydrogen-helium envelope which is itself divided into a liquid metallic inner component and a molecular outer component. This basic architecture is consistent with the core-accretion theory of giant planet formation. Simultaneously, however, many short-period giant planets exhibit anomalously large radii, which are commonly interpreted as indicating the existence of a structurally important source (or sources) of interior heating. We review the range of physical mechanisms that can potentially generate these inflated radii, and we discuss the directions by which progress can potentially be made with future research.