Climates and Habitability of the Eccentric Super-Earth HD 20794 d: A Multi-parametric Investigation

Abstract HD 20794 d is one of the best examples of a super-Earth planet in the habitable zone of a Sun-like star. Notably, due to the high eccentricity ( e = 0.45 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) it partially lies in the conservative habitable zone, suggesting a dynamically variable climate. The system’s proximity ( d = 6.0414 ± 0.0028 pc) makes it a high-priority target for future atmospheric characterization via direct imaging and positions HD 20794 d as an ideal benchmark for understanding the potential climate states of eccentric exoplanets. Assuming a rocky composition for HD 20794 d, we employ a seasonal-latitudinal energy balance model, EOS-ESTM, to explore the impact on surface temperature and habitability of climate factors unconstrained by observational data. To do so, first, we narrow ranges of surface pressures and CO 2 fractions that enable potentially habitable conditions. Then, we vary the planetary rotation period, axial tilt, and orbital eccentricity. Finally, we evaluate the impact of the stellar UV radiation field on atmospheric stability and prebiotic chemistry. Our simulations indicate that habitable conditions are generally favored in regimes characterized by high-CO 2 concentrations and elevated atmospheric pressure. Habitability also increases with higher axial obliquity (up to the point where an equatorial ice belt forms) and with longer rotation periods. We conclude that HD 20794 d can potentially maintain temperate surface conditions with modest seasonal temperature variations over a wide variety of planetary, orbital, and atmospheric conditions. Although no transits have yet been detected, our results underscore the importance of pursuing further observations of this benchmark system.