Exploring the Diversity of Jupiter-Class Planets

Contribution to a Royal Society Theo Murphy Discussion Meeting (2013) ‘Characterizing exoplanets: detec-tion, formation, interiors, atmospheres and habitability’. Of the 900+ confirmed exoplanets discovered since 1995 for which we have constraints on their mass (i.e., not including Kepler candidates), 75 % have masses larger than Saturn (0.3MJ), 53 % are more massive than Jupiter, and 67 % are within 1 AU of their host stars. When Kepler candidates are included, Neptune-sized giant planets could form the majority of the planetary population. And yet the term ‘hot Jupiter ’ fails to account for the incredible diversity of this class of astrophysical object, which exists on a continuum of giant planets from the cool jovians of our own solar system to the highly-irradiated, tidally-locked hot roasters. We review theoretical expectations for the temperatures, molecular composition and cloud properties of hydrogen-dominated Jupiter-class objects under a variety of different conditions. We discuss the classification schemes for these Jupiter-class planets proposed to date, including the implications for our own Solar System giant planets and the pitfalls associated with compositional classification at this early stage of exoplanetary spectroscopy. We discuss the range of planetary types described by previous authors, accounting for: (i) thermochemical equilibrium expectations for cloud condensation and favoured chemical stability fields; (ii) the metallicity and formation mechanism for these giant planets; (iii) the importance of optical absorber