29 May 2015

Tidal Locking, Flares and Photosynthesis on planets of Red Dwarfs (Geek Alert)

A short article about how the physics of atmospheres may act to prevent planets close to Red Dwarf stars from becoming tidally locked with respect to the star (like the Moon to the Earth or most of the giant planets' satellites with respect to them) (From Physicsworld):
(Link below)

This is all well and good, and I understand, of course, the interest in discovering any circumstances which may make the existence of habitable planets in M-V class (Red Dwarf) systems (which, after all, are something like 80% of all star systems) more likely.
However, an even bigger problem for the existence of habitable planets in such systems is the prevalence of powerful, and really quite lethal, stellar flares. You see, the following facts are well established:
  • 1. Solar (stellar) flares occur in approximately the same dimensions and power on medium to large size main sequence stars or on small, dwarf type stars. Proportionally, a flare may only be 1% increase in brightness on a G type main sequence star, but a similar flare, on a red dwarf, might briefly increase the star's brightness (and lethal hard radiation output) by 50% or even more.
  • 2. The habitable zone of such small stars is very close; within a few million miles, where proximity to such flares would be almost certainly lethal to any complex life residing on the surface of any planets located in those zones.
  • 3. Class M stars tend to have such flares quite frequently; in fact one group of such stars is referred to as "flare stars" because they occur as frequently as every few days.
The inescapable conclusion is that 80% of the stars, those in the M class, are probably ruled out for habitable worlds because of flares alone, regardless of whether they are rotationally stopped.
HOWEVER, one thing is sure. Humanity's investigations into these issues is in its infancy, and we really do not know the answers to these questions yet.

I will leave aside the issue that the wavelength of light from such stars is believed to be insufficient, on pure physics principles, for effective photosynthesis by organisms, although of course that is hardly encouraging either. The authors of the book «Revolutions that Made the Earth» (Lenton & Watson, Oxford 2010) were quite adamant that the photosynthesis reactions simply do not occur in with photons where the peak radiation is in the far red, which is what you have there, and that even tweaking the chemistry would probably not work because the problem is the energy of the each photon it takes to break a water molecule bond, which is higher than the energy of the red photons at issue. (If the energy of the photons isn't high enough, no matter what the flux of photons is, the reactions just don't take place). No substitution of alternative chemistry can change that fundamental physical parameter. I also leave to the side the tacit assumption that oxygenating (water-molecule breaking) photosynthesis, which evolved on Earth only the one time (the evolution of cyanobacteria, the common ancestor of ALL terrestrial oxygenic photosynthesizers), is absolutely necessary for the evolution of complex life, although I do believe that case is very, very strong.

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