In stellar terms, that is from 1.29 times Earth's sunlight to 0.39 times. I can say that the 'safe' habitable zone for an Earth-like planet extends from 0.88 AU to 1.6 AU, because after you go past 1.6 AU, any Earth-like planet that tries to warm up or is warm already will cool down and then will experience an runaway cooling effect due to increasing albedo. If the Earth does have other mechanisms such as adjusting CO2 levels, we may never know until later on. However, recent 3D models (my data is from this model: /doi/full/10.1002/2013GL058376#:~:text=A%20moist%20greenhouse%20is%20a,owing%20to%20water%20vapor%20photolysis) have disproven that, saying that even at 0.93 AU (1.15 times the normal sunlight), the Earth only reaches ~39 C. Some say that if you move the Earth to 0.99 AU from 1 AU, it would result in a rapid greenhouse effect. Now it has been argued about what the habitable zone for an Earth-like planet would be. This causes further heating, and the rest is history (moist greenhouse leads to runaway greenhouse, and yeah, you get a Venus). However, once you reach ~55 C, planets now favor a steam atmosphere, resulting in much greater heat trapping, but without reflection. This is one way that the Earth 'copes' with rapid heating.

As temperature increases, more evaporation means more clouds and rain, resulting in a cooling of the atmosphere due to reflection of solar says. This moist greenhouse temperature limit increases as you increase planet mass.Īnother factor that comes into play with an Earth-like planet is the favorability of a steam atmosphere vs a cloud atmosphere. However, after the 68 C mark, a hohger water mixing ratio in the stratosphere would lose the effectivity of the cold trap, resulting a rapid evaporation of water from the planet, loss of water, and rapid heating. Normally, the stratosphere is a 'cold trap' that keeps water vapor from rising any further. Earth-like planets experience this effect called the moist greenhouse effect at 68 C. The upper blund for a desert planet would be, perhaps, 0.95-1.05 AU. This may be okay, since deserts actually have an albedo about 0.4. However, desert planet wouldn't have that greenhouse effect, resulting in temperatures just a few degrees warmer than the effective temperature. While this would result in average temperatures of over 95 C for an Earth-like planet, and therefore would lead to a rapid moist greenhouse effect which would lead to a runaway green house effect soon after. Also, according to my tests on universe sandbox 2, it is possible for a tidally locked planet to see temperatures up to 20-25 C at 2.35 AU at its hottest parts, thereby being habitable.Īt 0.6 AU, you get ~2.7 times the amount of sunlight as Earth, resulting in an effective temperature (which is basically temperature without atmosphere) of ~50-80 C (according to Universe Sandbox 2, which accurately calculates that) assuiming an albedo between 0.2 and 0.4. The habitable zone extends from ~0.6 AU for desert planets with almost no CO2 to ~2.3-2.5 AU for larger planets up to 7-10 Earth masses. For all that you know, it may be habitable. While it is true that Proxima Centauri b receieves 400 times the amount of gamma radiation as the Earth, and it may have a weaker magnetic field due to the tidal locking, we still do not know whether life of any type can adapt to it. Morphyms1817 lichess user, who is currently taking a break from chess, did some calculations:

Universe sandbox 2 life likelihood