The habitability of Earth-like (exo)planets: modelling and limitations.

Quick take: We investigate the conditions behind exoplanetary habitability. We compare how different models (complex physics-based vs. parameterized evolution) estimate the climate of Earth-like planets. We identify which planetary properties are critical to assess habitable conditions, and how that impacts the reliability of parameterized modeling.

Estimating whether an exoplanet is habitable is a complex question that goes far beyond calculating its host star Habitable Zone. In addition to incoming radiation from the star, atmosphere composition, planetary rotation, topography, and ocean/continent layout can all affect surface conditions spatial distribution. Simple parameterized models of those exoplanets allow for testing a large parameter space quickly, while physics-based models are more complex and much more time consuming, only allowing for the modelling of more restricted cases. We wish to test how the limitations of both approaches affect our capacity to assess planetary habitability, given the limited characterization available for exoplanets at present and for the foreseeable future.

We use Earth as a reference case, as the only planet where data is available regarding surface conditions evolution. We present new modeling results from the 3D climate General Circulation Model (GCM) ROCKE3D applied to Earth-like planets, based on atmospheric compositions derived from internal thermal histories and outgassing evolution scenarios consistent with Earth observation. We also compare atmospheric compositions and interior/atmosphere evolution scenarios obtained in a parameterized interior approach to the results of the 2D/3D Earth mantle dynamics model StagYY.

The main properties that we have investigated are variations of length of day, continental vs. oceanic coverage, topography and diverse atmospheric compositions consistent with recorded constraints on the Earth.

We compare average surface temperatures, albedos, precipitations, ice and clouds coverage obtained in both simulations. We then evaluate precipitations, sea surface level, and ice coverage obtained in GCM simulations and compare them to the usual criteria for habitability (such as average temperatures above 273-258 K). Finally, we assess the reasons for discrepancies between the models.

The trend of the variations of average temperature through time (and CO₂ abundances) is consistent in parameterized vs. GCM models, making parameterized approaches generally efficient for a broad estimate of average surface conditions. However, perturbations around the reference model result in stronger temperature variations in the GCM due to albedo feedback. The albedo variations can be significant in 3D simulations and are not considered in the parameterized approach. Additionally, spatial variations of local surface conditions are found to be large and dependent on properties that cannot be resolved by parameterized models nor observed for exoplanets. Supercontinent setups result in markedly dryer land than the present-day Earth continental layout. Even models with average temperatures below 273-258 K have significant ice-free ground in all continental setups.