Planets around M dwarf stars have generated a lot of interest lately—especially with the recent discovery of an earth-mass planet in the region where liquid water could exist on the surface. Since 75% of stars are M dwarfs, this number greatly expands the potential life-sites in the universe if habitable planets can form around these stars. We discussed some constraints on any such planets in a previous TNRTB.
An article published recently in the Astrophysical Journal identifies another problem for getting liquid water on any earth-sized planets around M dwarfs. As you might expect, planets closer to the star form in a hotter environment than planets farther away. During the planetary accretion phase, the temperature in the habitable zone around an M dwarf is significantly higher than it is around a G-class star like our sun. Additionally, the amount of volatile material, like water, that a planet accretes during formation sensitively depends on the temperature of the surrounding environment.
The author of the article paints two scenarios of planet formation around an M dwarf. Any planet that can accrete enough water to make an ocean forms far outside the habitable zone where liquid water could exist on the surface. Alternatively, any planet forming in the habitable zone does not accrete enough water during formation to make an ocean. Either way, these results add to the growing body of evidence that the abundant M dwarf stars fail to meet the most basic requirements for supporting life. I find these results exciting because they continue to buttress the biblical inference that Earth is unique in its capacity to support advanced life like human beings.