Fig. 1: The expected distribution of stable, Earth-like exoplanet climates from our habitable zone weathering model.
The horizontal axis shows incident flux, S, normalized to the solar constant (S⊕) and the corresponding orbital distance in Astronomical Units (AU) above the plot. The vertical axis shows the atmospheric CO2 partial pressure (pCO2) in bar. Each point represents a climate in steady state. The black labeled contours show the mean global surface temperature for the given pCO2 and incident flux. The white region below the 248 K contour is where our model assumption of a liquid ocean is no longer plausible so no planets are shown in that region. Above the 355 K contour, Earth-like planets are too hot to retain their liquid oceans for billions of years. Similar to the frozen planets, such hot planets are not considered habitable. Modern Earth and Mars are shown by black squares. The blue histogram at the bottom of the figure shows the number of stable planets in each incident flux bin. The color of each simulated planet shows the relative point density in the plot at that location. The color was calculated using a kernel-density estimate with Gaussian kernels and rescaled from 0 to 1. A color value of 0 represents the lowest relative point density, 1 the highest. The log-linear line of best fit between pCO2 and S is shown in red. The slope of the red, best fit line is 3.92 ± 0.24 (95%) with units −\({\mathrm{log}\,}_{10}\)(pCO2 [bar])/(S/S⊕). Our model predicts that atmospheric CO2 should increase with orbital distance in the HZ.
