I want to work a new paper on red dwarf habitability in here because it fits in so well with yesterday’s discussion of the super-Earth GJ1214b. The latter orbits an M-dwarf in Ophiuchus that yields a hefty 1.4 percent transit depth, meaning scientists have a strong lightcurve to work with as they examine this potential ‘waterworld.’ In transit terms, red dwarfs, much smaller and cooler than the Sun, are compelling exoplanet hosts because any habitable worlds around them would orbit close to their star, making transits frequent.
When I first wrote about red dwarfs and habitability in my Centauri Dreams book, it was in connection with the possibilities around Proxima Centauri, but of course we can extend the discussion to M-dwarfs anywhere, this being the most common type of star in the galaxy (leaving brown dwarfs out of the equation until we have a better idea of their prevalence). Manoj Joshi and Robert Haberle had published a paper in 1997 that described their simulations for tidally locked planets orbiting red dwarf stars, findings that held open the possibility of atmospheric circulation moderating temperatures on the planet’s dark side. There seemed at least some possibility for extraterrestrial life on such a world, although the prospect remains controversial.
Image: X-ray observations of Proxima Centauri, the nearest star to the Sun, have shown that its surface is in a state of turmoil. Flares, or explosive outbursts occur almost continually. This behavior can be traced to the star’s low mass, about a tenth that of the Sun. In the cores of low mass stars, nuclear fusion reactions that convert hydrogen to helium proceed very slowly, and create a turbulent, convective motion throughout their interiors. This motion stores up magnetic energy which is often released explosively in the star’s upper atmosphere where it produces flares in X-rays and other forms of light. X-rays from Proxima Centauri are consistent with a point-like source. The extended X-ray glow is an instrumental effect. The nature of the two dots above the image is unknown – they could be background sources. Credit: NASA/CXC/SAO.
A lot of work has been done on M-dwarfs and habitability in the years since, and we also have the problem of this class of stars emitting flares of X-ray or ultraviolet radiation, making the prospects for life still uncertain. 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. Joshi, now studying planetary atmospheric models at the University of East Anglia, has recently published a new paper with 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 finding has no bearing on the inner edge of the habitable zone, as the paper notes:
The effect considered here should not move the inner edge of the habitable zone, usually considered as the locus of orbits where loss rates of water become significant to dry a planet on geological timescales (Kasting et al 1993), away from the parent M-star. This is because when a planet is at the inner edge of the habitable zone, surface temperatures should be high enough to ensure that ice cover is small. For a tidally locked planet, this implies that ice is confined to the dark side that perpetually faces away from the parent star; such ice receives no stellar radiation, which renders albedo effects unimportant.
The dependence of ice and snow albedo on wavelength is small for wavelengths shorter than 1 μm, which is where the Sun emits most of its energy, so we see little effect on our own climate. But the longer wavelengths emitted by red dwarfs could keep snow and ice-covered worlds warmer than we once thought. How various atmospheric models would affect the absorption of the star’s light is something that will need more detailed work, say the authors, but they consider their extension of the outer edge of the habitable zone to be a robust conclusion.
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,” accepted by Astrobiology (preprint).