The new investigation was called for because Gliese 581d is suspected to be tidally locked. If so, this would create a permanent night side on the planet. On this side, the temperatures would be significantly lower and gasses such as CO2 and H2O may find themselves in a region where they could no longer remain gaseous, freezing into ice crystals on the surface. Since that surface would never see the light of day, they could not be heated and released back into the atmosphere, thereby depleting the planet of greenhouse gasses necessary to warm the planet, causing what astronomers call an “atmospheric collapse.”.
A new study indicates that planets that are tidally locked to their stars may not be habitable due to a permanent night side. The atmosphere would freeze out on the coldest parts of the night side causing a collapse of the atmosphere. The gas molecule that fall out could not be reheated back into the atmosphere, trapping them permanently.
This has implications for the estimated number of habitable planets.