Fig. 4. Planet core formation and atmospheres.

Rocky planets in a wide range of masses are commonly expected to usually have atmospheres dominated by N₂ and CO₂ when their orbits fall within the broader habitable zone (aka liquid-water belt) of their host stars and after some iron has been removed from the mantle by forming a core—the full transition takes 10⁷ to 10⁸ years and is illustrated by the first and second cutouts for the case of Earth (60, 61). A highly reducing atmosphere (as shown in the first cutout) is photochemically unstable on rocky planets with surface oceans and vapor H₂O in the atmosphere because of the fast escape of H to space after photolysis of CH₄ and NH₃. In general, outgassed carbon would be converted into CO₂, and an N₂-CO₂ atmosphere would be generated on a time scale of about 10⁵ years (64–66).