Habitability on exoplanets in eccentric orbits: the case of Gl 514 b and HD 20794 d

Introduction: High-eccentricity planets are not rare among confirmed exoplanets. Despite their dramatic seasonal changes in insolation between periastron and apoastron, several studies agree on assuming such worlds are habitable ([1,2,3,4,5,6]). However, they also agree that planets located near the outer regions of the habitable zone may enter a globally frozen ‘snowball’ state, posing a threat to their ability to support water-based life [2]. Therefore, detailed climate studies of highly eccentric planets are essential for testing these predictions. In this context, Gl 514b [7,8] and HD 20794d [9,10] offer the best chance for such investigations because, among the confirmed exoplanets orbiting around M-dwarfs and Sun-like stars, they have the highest eccentricity, e ∼ 0.45. In the present work, we used a seasonal-latitudinal energy balance model, EOS-ESTM [11], to explore the potential impact of both constrained and unconstrained planetary, orbital, and atmospheric parameters on their habitability, mapped in terms of surface temperature.

Results: To explore the surface habitability, we calculated a temperature-dependent habitability index, h, which represents the fraction of planetary surface with temperature within the liquid-water range. The climate simulations were constrained using measured quantities (e.g., insolation and planet mass) and parametrizing unknown planetary (e.g., geography, rotation period, axis obliquity), orbital (e.g., eccentricity, argument of periastron), and atmospheric (e.g., surface pressure, chemical composition) quantities. Since measurements of the radius are not available for the two planets, we assumed an internal composition similar to that of Earth.
Regarding the planetary atmosphere, in the case of Gl 514b, we tested three sets of CO2-dominated atmospheres, each with its own CH4 concentration (xCH4: 0, 0.1, and 1 percent), varying the total surface pressure in the range ptot ∈ (1, 13) bar. In contrast, for HD 20794d, we narrowed the ranges of surface pressures and CO2 fractions that enable potentially habitable conditions (Figure 1).
As a general trend, the higher the global coverage of oceans is, the more habitable the planet is (Figure 2). This behaviour is due to the combination of two factors (i) the land has a lower thermal capacity than the water and (ii) oceans are darker than bare soil.

When the obliquity increases, the planet experiences stronger seasonal excursions of surface temperature. This means a larger fraction of polar regions undergo periods of high daily-averaged insolation, reducing the ice caps and increasing habitability (Figure 3). However, the impact of higher obliquity tends to disappear as surface pressure increases due to the high efficiency of horizontal energy transport.

In the range of orbital eccentricity consistent with the observations (e=0.30-0.60), the impact of the eccentricity on habitability is important. The higher e, the wider the range of atmospheric pressure favourable to habitability becomes, down to a moderate pressure (ptot ∼1 bar). We find that the impact on habitability of eccentricity variations is higher than that induced by variations of other key planetary quantities, such as obliquity.
More in general, we underline that remarkable differences exist between the low- and high-concentration of CO2 and CH4, as well as between the low- and high-pressure regimes. These results are due to the higher greenhouse effect of the thick, CO2/CH4-rich atmospheres and to the higher efficiency of the hori-zontal transport at high atmospheric pressure.

Figure 1. Predicted values of the average surface temperature as a function of CO2 and ptot for an aquaplanet scenario. We adopt ε = 0◦, Prot = 1 day, e = 0.45 and ωperi = 0◦. The dashed areas indicate the parameter space in which atmospheric CO2 condensates (oblique bars) and H2O on the surface evaporates (horizontal bars). Yellow and red contour lines highlight the regions of the parameter space for which pure water can be maintained in liquid form and the biological limit, respectively. Dashed lines represent the average temperature along the orbit whilst solid lines represent the maximum temperature. Credits: [10].

Figure 2. Predicted values of h as a function of the ocean cover fraction and total surface pressure for an atmospheric composition with CO2+1 per cent CH4. We adopt ε=23.44◦, Prot=1 d and ωperi = 0◦. Credits: [8].

Figure 3. Seasonal and latitudinal maps of surface temperature obtained by extracting the results of case with 1% CH4 at constant values of axis obliquity (from ε = 20◦ to 60◦) and total pressure (from ptot = 4641 mbar to1668 mbar). The solid line indicate the limit within which water can be maintained in liquid form. Credits: [8].

Future perspectives: Future observations may help constrain the actual range of stellar, orbital, and planetary properties that affect the habitability of Gl 514b and HD 20794d. Asteroseismology obtained through extensive monitoring of nearby bright stars with PLATO may help measure stellar ages and internal structures. The large uncertainty in eccentricity can be reduced by a long-term sequence of radial velocity measurements.
Regarding Gl 514b, searches for transits might be performed with CHEOPS and PLATO. Moreover, high-contrast imaging is expected to become feasible with the ELT [7].
Concerning HD 20794d, high-contrast imaging with next-generation facilities (ELT) and dedicated missions like LIFE and HWO will enable direct atmospheric characterization in both the thermal and visible/near-infrared regimes. Given HD 20794’s proximity (6.04 pc) and its inclusion in target lists for PLATO and HWO, HD 20794d is poised to become a flagship object in our quest to understand the complex interplay between orbital dynamics, atmospheric processes, and habitability in super-Earths.

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