Ravi Kumar Kopparapu and colleagues [Habitable Zones Around Main-Sequence Stars: New Estimates] have revised the 20-year old work of Kasting, Whitmire and Reynolds [Habitable Zones Around Main-Sequence Stars], with two seemingly minor changes with surprising effects.
Firstly, they’ve extended the range covered by the model to include stars below the 3700 K limit of the previous work, towards the much cooler 2600 K realm of very low mass Red Dwarfs. The very coldest hydrogen-fusing stars are still white-hot – Red Dwarfs drop down to just ~2300 K at the H-fusing limit of 0.08 solar masses, and 2600 K is hit by 0.09 solar mass stars. Secondly, they’ve improved the modeling of the greenhouse effect created by CO2, which has produced some startling changes in results. Earth ends up near the inner edge of the Habitable Zone – instead of 0.95 AU, the inner edge for a Solar-like star is ~0.99 AU and is even further out for redder stars. Ravi has provided a calculator of the effects on his web-site…
…the default is the Sun, with an effective Temperature of 5780 K and 1 Solar Luminosity, and an Inner and Outer Edge of the Habitable Zone of 0.9928 and 1.6886 AU. Push the Calculator to 7200 K and the Edges become 0.9451 to 1.5285 AU. Drop the temperature to 2600 K and the range changes to 1.0883 to 2.1238 AU. So what’s going on? Why the changes? The peak radiation frequency of the spectrum is proportional to the effective temperature – hotter stars peak towards the ultraviolet, while cooler stars peak towards the infra-red. Planetary atmospheres are more effective scatterers of higher frequencies than lower frequencies – the cause of the blue daylight sky – and this means under a bluer spectrum, a planet is cooler, or warmer under a redder spectrum.
The Inner Edge is hit when the surface temperature runs away from our temperate ~288 K (15 C) and climbs towards 340 K (67 C), causing a wet stratosphere and eventual dessication of the planet via hydrogen loss. The Outer Edge is reached when no amount of extra carbon dioxide can further increase the surface temperature – adding more just reflects heat away. In fact at 35 bars the CO2 will condense into liquid at 273 K, but becomes opaque at lower pressures before then.
Interestingly if Venus was as far from the Sun as Mars most of its atmosphere (~90 bars CO2, 2 bars N2) would begin condensing rapidly. A turbulent atmosphere, full of convecting plumes, would carry heat away from the hot surface to space… rapidly. In a couple of decades, the place would probably find an equilibrium at something close to CO2’s critical point, about 304 K – just 31 C. An ocean of liquid carbon dioxide about 120 metres deep would form, though much of it would eventually percolate into the regolith creating a very exotic “ground-water”. Ultimately bright CO2 clouds might form and drive the temperatures lower, towards full condensation of the atmosphere…