Entering the Habitable Zone: An Earth-like Perspective

Whether a planet has liquid water at its surface depends on at least two energy sources: solar and internal. As a star ages, it brightens and delivers more energy to a planet at a given orbital distance. The planet ages, too. It produces heat within through radiogenic decay. One mode to eliminate internal energy is through mantle convection. By this process, CO2 and water cycle between the planet’s surface and interior based on how efficiently the planet eliminates internal energy. The efficiency of mantle convection on Earth remains debated, with multiple hypotheses describing the convective dynamics of Earth having been put forth. Geological proxy data allow us to validate these hypotheses over different parameter space regions with some level of certainty. We consider validated models Earth-like. That these different Earth-like models would lead to different volatile cycling patterns should not surprise us. It does raise the question of how each Earth-like model influences surface temperature histories. We find that the time Earth-like planets enter the habitable zone varies by billions of years. The timing depends on both the amount of solar energy absorbed from the star based on the planet’s orbital distance and albedo as well as how efficiently convective processes eliminate internal energy. How both energy sources evoled matters. For the same aged planet, whether it resides in the habitable zone may may depend on its efficiency at eliminating internal heat. We demonstrate this point for an Earth-like at different orbital distances and consider how an episodic convective regime may impact these probabilities.