Maximum greenhouse effect of CO₂ dominated atmospheres for planets orbiting different main-sequence stars. The figure shows the stellar insolation required to obtain a temperature of 273.15 K as a function of the CO₂ surface pressure. The stellar flux at the top of the atmosphere F_∗ is depicted in units of the solar constant S_⊙. The minima of the curves correspond to the maximum greenhouse effect and indicate the outer boundaries of the cloud-free habitable zone.