Abstract
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 (0.3 MJ), 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 absorbers for energy partitioning and the generation of a temperature inversion; (iv) the favoured photochemical pathways and expectations for minor species (e.g. saturated hydrocarbons and nitriles); (v) the unexpected presence of molecules owing to vertical mixing of species above their quench levels; and (vi) methods for energy and material redistribution throughout the atmosphere (e.g. away from the highly irradiated daysides of close-in giants). Finally, we discuss the benefits and potential flaws of retrieval techniques for establishing a family of atmospheric solutions that reproduce the available data, and the requirements for future spectroscopic characterization of a set of Jupiter-class objects to test our physical and chemical understanding of these planets.
Highlights
Of the 900+ confirmed exoplanets discovered since 1995 for which we have constraints on their mass, 75% have masses larger than Saturn (0.3 MJ), 53% are more massive than Jupiter and 67% are within 1 AU of their host stars
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
Giant exoplanets (i.e. Neptune-sized and larger) appear to be commonplace: from a catalogue of confirmed exoplanets with mass determinations, 75% of all planets discovered to date have masses larger than Saturn (0.3 MJ), 53% are more massive than Jupiter and 67% are within 1 AU of their parent stars
Summary
The high-temperature hydrogen-rich atmospheres of extrasolar giant planets (EGPs) in close orbits around their parent stars have made them ideal candidates for preliminary spectroscopic characterization. Vertical and horizontal mixing by eddy diffusion, convection and wave propagation would be responsible for moving energy and material from place to place, causing the composition to deviate from the expectations of equilibrium [24,25,26] Each of these processes could provide additional dimensions to a classification scheme for EGPs. Of particular note is the recent twodimensional scheme devised by [13,23], which uses both the stellar irradiance and the chemical dependence on the C/O ratio, described later. A poor proxy for the true T(p), this quantity does at least permit preliminary categorizations [12,13,15] and will be used as a guide in the text that follows
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Schedule a callMore From: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
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