Earth’s climate has been shown by a wide range of climate models to be bistable, which means that it can exist in both a frozen state or a warm, ice-free state with the same amount of solar energy. Earth today resides in a warm state with small ice caps, but geologic evidence suggests that the Neoproterozoic Earth, about 500 Myr ago, may have been frozen all the way down to the equator.
Other terrestrial planets orbiting other stars should behave similarly, although the transition point between climate states may differ for stars that are brighter and dimmer than the sun. In a recent paper published in The Astrophysical Journal, titled “Constraints on climate and habitability for Earth-like exoplanets determined from a general circulation model,” my co-authors and I analyze three-dimensional climate simulations of planets orbiting a range of stars. We show that planets can exist around such stars as a frozen snowball, partial melt with an equatorial waterbelt, temperature conditions everywhere, and a hothouse with gradual water loss. Stars slightly cooler than the sun could maintain habitable conditions for longer periods of time, due to the slower rate of water loss.