![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
rfmcdonaldRed dwarf stars, small and relatively cool and dim, much less notable than our yellow dwarf Sol, are the most common stars in the galaxy; the small amounts of mass that go into red dwarf stars gather together more readily than the larger amounts necessary to produce brighter stars. Because red dwarf stars are so common, the question of whether or not these stars could support Earth-like worlds has been hotly debated--Wikipedia even has a page dedicated to the potential habitability of worlds in red dwarf systems.
The biggest possible strike against red dwarf stars is that they are so dim that the odds a planet could exist in an orbit hospitable to an Earth-like biosphere may be very small, and that those worlds which did would be tidally locked--that is, they would not rotate relative to the star, leaving one side permanently exposed to its sun and the other side in permanent darkness. Secondary concerns include the question of whether or not the tendency of red dwarf stars to experience large stellar flares might make potentially Earth-like worlds un-Earth-like.
So far, theoretical studies suggest that tidal locking wouldn't necessarily be a problem, an early model suggesting that so long as a planetary atmosphere was a tenth as dense as Earth's (about twice that of Mars') the atmosphere would carry enough heat from the side facing the sun to the side facing away from the sun, preventing catastrophe. Stellar flares, similarly, needn't pose an existential threat to life on a world in a suitable orbit; at worst living organisms would be protected by a couple of metres' water, atmospheres and oceans need not experience significant erosion, and as red dwarf stars age their tendency to flare dminishes.
Now, via Centauri Dreams' post "M-Dwarfs: A New and Wider Habitable Zone", I've learned that a recent paper may mean that broadly Earth-like worlds could orbit red dwarf stars at greater distances than previous thought. Why? The relative abundance of dim, warm light produced by red dwarf stars has an impact on climate.
The effect of this would be to make it possible for worlds with Earth-like climates to orbit red dwarf stars at distances 10 and 30 percent greater than previously thought. Further details, the authors note, remain to be determined by detailed atmospheric models.
Already, there are speculations that there could be as many Earth-like worlds orbiting red dwarf stars as brighter stars, on the grounds that though red dwarf stars' habitable zones are substantially smaller than those of brighter stars, there are so many more red dwarfs that the two categories of stars host Earth-like planets in equal number. Could red dwarf stars host more Earth-like worlds than their brighter counterparts?
The paper is Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone"
The biggest possible strike against red dwarf stars is that they are so dim that the odds a planet could exist in an orbit hospitable to an Earth-like biosphere may be very small, and that those worlds which did would be tidally locked--that is, they would not rotate relative to the star, leaving one side permanently exposed to its sun and the other side in permanent darkness. Secondary concerns include the question of whether or not the tendency of red dwarf stars to experience large stellar flares might make potentially Earth-like worlds un-Earth-like.
So far, theoretical studies suggest that tidal locking wouldn't necessarily be a problem, an early model suggesting that so long as a planetary atmosphere was a tenth as dense as Earth's (about twice that of Mars') the atmosphere would carry enough heat from the side facing the sun to the side facing away from the sun, preventing catastrophe. Stellar flares, similarly, needn't pose an existential threat to life on a world in a suitable orbit; at worst living organisms would be protected by a couple of metres' water, atmospheres and oceans need not experience significant erosion, and as red dwarf stars age their tendency to flare dminishes.
Now, via Centauri Dreams' post "M-Dwarfs: A New and Wider Habitable Zone", I've learned that a recent paper may mean that broadly Earth-like worlds could orbit red dwarf stars at greater distances than previous thought. Why? The relative abundance of dim, warm light produced by red dwarf stars has an impact on climate.
It would be helpful, then, if we could find a way to back a planet off from its host star while still allowing it to be habitable. The flare problem would be partially mitigated, and tidal lock might not be a factor. [Manoj] Joshi, now studying planetary atmospheric models at the University of East Anglia, has recently published a new paper with [Robert] Haberle (University of Reading) arguing that the habitable zone around M-dwarfs may actually extend as much as 30 percent further out from the parent star than had been previously thought.
At issue is the reflectivity of ice and snow. M-dwarfs emit a much greater fraction of their radiation at wavelengths longer than 1 μm than the Sun does, a part of the spectrum where the reflectivity (albedo) of snow and ice is smaller than at visible light wavelengths. The upshot is that more of the long-wave radiation emitted by these stars will be absorbed by the planetary surface instead of being reflected from it, thus lowering the average albedo and keeping the planet warmer. Joshi and Haberle modeled the reflectivity of ice and snow on simulated planets around Gliese 436 and GJ 1214, finding both the snow and ice albedos to be significantly lower given these constraints.
The effect of this would be to make it possible for worlds with Earth-like climates to orbit red dwarf stars at distances 10 and 30 percent greater than previously thought. Further details, the authors note, remain to be determined by detailed atmospheric models.
Already, there are speculations that there could be as many Earth-like worlds orbiting red dwarf stars as brighter stars, on the grounds that though red dwarf stars' habitable zones are substantially smaller than those of brighter stars, there are so many more red dwarfs that the two categories of stars host Earth-like planets in equal number. Could red dwarf stars host more Earth-like worlds than their brighter counterparts?
The paper is Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone"
