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A Physical Classification of Exoplanet Thermal Environments: Stellar Irradiation versus Tidal Heating

Daniel Fadrique Barbero

Abstract

In this study, we introduce a physical framework to analyse and classify the thermal regimes governed by tidal heating and stellar irradiation. Although all planetary systems are exposed to stellar radiation, this source is not always the dominant energy mechanism. This study is motivated by the lack of a physical framework that examines tidal heating in cases where this phenomenon dominates over stellar irradiation. We develop a reproducible physical approach that allows us to classify the relative contribution of both fluxes in a population of exoplanets, identifying the most relevant physical mechanisms that determine the thermal regime. We apply this method to a population of approximately 2000 exoplanets. This framework is centred on the dimensionless parameter \(Λ=F_{\mathrm{abs}}/F_{\mathrm{tide}}\), which quantifies the relative contribution of each flux. Our results show that most planetary thermal environments are dominated by \(F_{\mathrm{abs}}\), although there is a significant fraction of systems in which the tidal flux dominates. We identify a physical boundary at \(Λ=1\) that defines a regime in which both fluxes are comparable. We identify the semi-major axis \(a\) and the eccentricity \(e\) are the parameters that most influence the tidal flux. This framework provides a transparent and physically motivated tool for characterising planetary thermal environments and exploring the physical trends governing exoplanet populations.

A Physical Classification of Exoplanet Thermal Environments: Stellar Irradiation versus Tidal Heating

Abstract

In this study, we introduce a physical framework to analyse and classify the thermal regimes governed by tidal heating and stellar irradiation. Although all planetary systems are exposed to stellar radiation, this source is not always the dominant energy mechanism. This study is motivated by the lack of a physical framework that examines tidal heating in cases where this phenomenon dominates over stellar irradiation. We develop a reproducible physical approach that allows us to classify the relative contribution of both fluxes in a population of exoplanets, identifying the most relevant physical mechanisms that determine the thermal regime. We apply this method to a population of approximately 2000 exoplanets. This framework is centred on the dimensionless parameter , which quantifies the relative contribution of each flux. Our results show that most planetary thermal environments are dominated by , although there is a significant fraction of systems in which the tidal flux dominates. We identify a physical boundary at that defines a regime in which both fluxes are comparable. We identify the semi-major axis and the eccentricity are the parameters that most influence the tidal flux. This framework provides a transparent and physically motivated tool for characterising planetary thermal environments and exploring the physical trends governing exoplanet populations.
Paper Structure (22 sections, 11 equations, 9 figures)

This paper contains 22 sections, 11 equations, 9 figures.

Table of Contents

  1. Introduction
  2. Theoretical Framework
  3. The Thermal Environment of Exoplanets
  4. The Incident Stellar Flux
  5. The Tidal Heating Flux
  6. The Coexistence of Both Flows on the Same Exoplanet
  7. $\Lambda$: Physical Parameter for the Classification of Thermal Regimes
  8. Methodology
  9. Results
  10. Transition between Tidal and Stellar Irradiation Regimes as a function of Orbital Period
  11. Exoplanet Thermal Regimes Defined by the Flux Ratio $\Lambda$
  12. Dominant Parameters Governing Tidal Heating in Exoplanets
  13. Two-dimensional plot of $F_{\mathrm{tide}}$ versus $F_{\mathrm{abs}}$
  14. Tidal Heating Landscape in the ($a$, $e$) Parameter Space
  15. Discussion
  16. ...and 7 more sections

Figures (9)

  • Figure 1: Energy fluxes as a function of orbital period for an irradiation-dominated case.
  • Figure 2: Energy fluxes for a tidally dominated case.
  • Figure 3: Thermal regimes of exoplanets defined by the flux ratio $\Lambda$ as a function of orbital period. The blue points correspond to planets where radiation dominates ($\Lambda>1$), whilst the red points indicate that tidal heating dominates ($\Lambda<1$). The dashed line marks the transition between the two regimes ($\Lambda=1$).
  • Figure 4: Tidal heating flux $F_{\mathrm{tide}}$ as a function of eccentricity $e$ (cases with $e=0$ are not shown in log scale).
  • Figure 5: Dependence of tidal heating flux $F_{\mathrm{tide}}$ on semi-major axis $a$.
  • ...and 4 more figures