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Title: The Intrinsic Temperature and Radiative-Convective Boundary Depth in the Atmospheres of Hot Jupiters
Authors: Daniel P. Thorngren, Peter Gao, Jonathan J. Fortney
First Author’s Institution: University of California, Santa Cruz
Status: Submitted to ApJL
Jupiter-sized gas-giant exoplanets in close orbits around their stars, commonly referred to as hot Jupiters, have been the prime targets for probing planetary atmospheres beyond our solar system. One of the many mystifying features of hot Jupiters — which, ironically, also makes them easier to detect and characterize — is their inflated radii. A good fraction of known hot Jupiters have sizes larger than those predicted by evolutionary models that take into account the properties of the system like temperature, age, and metallicity of the system. What could be causing these hot Jupiters to puff up?
A proposed mechanism to explain hot-Jupiter inflation is deposition of energy from stellar irradiation deep into the interiors of the planet. However, in addition to inflating the planet, energy from stellar flux heating up the planetary interiors can also radically alter the thermal structure (temperature variation with altitude) of its atmosphere which has direct consequences on its inferred atmospheric properties. Today’s paper attempts to draw a connection between the stellar irradiation of hot Jupiters and their intrinsic temperature, and how that ultimately affects the observations and our understanding of the atmospheres of these gas giants.
Structuring the Atmosphere of a Gas Giant
The vertical thermal structure — also referred to as the pressure–temperature profile — of a planetary atmosphere is directly related to change in the mode of heat transport (radiation or convection) within the atmosphere at different heights. You can think of this in the context of the Earth’s atmosphere: closer to the surface heat exchange occurs through convection, with hot parcels of air rising up and adiabatically expanding and cooling. This causes the temperature to steadily decrease as you go up until a certain altitude called the tropopause; above this you hit the stratosphere, where the air absorbs most of the heat from ultraviolet radiation from the Sun, causing the temperature to now increase with altitude. Even before this happens convection begins to weaken considerably and radiation takes over as the dominant mode of heat exchange. The altitude or the pressure level at which this happens is called the radiative-convective boundary (RCB; see Figure 1 for example). Such stratification of atmospheres is very commonly seen in planetary atmospheres in the solar system and has been studied extensively from measurements by probes like Galileo and Cassini-Huygens.

Figure 1: Pressure-temperature profiles for hot Jupiters at different distances from a Sun-like star, and hence different equilibrium temperatures (Teq). Note that on y-axis, the pressure decreases as you go up, corresponding to going higher up in the atmosphere. The thick parts of the profiles mark the regions of the atmosphere that are convective, and you can see how the radiative–convective equilibrium boundary moves to lower pressures for hotter planets. [Thorngren et al. 2019]
To answer this question, the authors calculate temperature–pressure profiles from thermal equilibrium atmospheric models of archetypal hot Jupiters with a range of Teq, and they then investigate how the height of the RCB changes with respect to different levels of stellar irradiation (see Figure 1 and 2).
Marking the Boundary
As is evident from Figure 1, the RCB moves to lower pressures (larger altitudes) with higher Teq, similar to how Tint increases with Teq. The surface gravity and metallicity of the planet also affect the RCB height, as seen in Figure 2.

Figure 2: The RCB pressure level with respect to the Teq of the planet, as calculated for different surface gravities and metallicities of the planet. Note that the RCB ends up at higher pressures for higher surface gravity and lower pressures for higher metallicity. [Thorngren et al. 2019]
With more exoplanet discoveries from TESS and exoplanet characterization opportunities from JWST on the horizon, we can hope to obtain a stronger constraint on atmospheric boundary conditions such as these, which would be important for accurate interpretations of exoplanet atmosphere observations.
About the author, Vatsal Panwar:
I am a PhD student at the Anton Pannekoek Institute for Astronomy, University of Amsterdam. I work on characterization of exoplanet atmospheres to understand the diversity and origins of planetary systems. I also enjoy yoga, Lindyhop, and pushing my culinary boundaries every weekend.