Proxima Centauri b

[radtech497]

Running a few numbers on this planet, I find the ranges of uncertainty are probably not conducive to making Proxima b a "Gaian" world, even stretching the definition to its limits. For one thing, Proxima Centauri was much more active in its youth than it is now, and even now emits X-rays at levels comparable to Sol. It has an activity period of about fourteen months (compared to about 11 years for Sol) and randomly emits flares that brighten the star by perhaps an order of magnitude. The main effect of this activity, with regard to Proxima b, is that after its primary atmosphere (mostly hydrogen and helium) had been stripped away, the intense stellar bombardment might have impeded the development of a secondary atmosphere. Beyond that, the uncertainties connected with the planet's orbital eccentricity (up to 0.35) mean that it may only transit the "habitable zone" rather than reside there, and have both a periastron and an apoastron that lay outside that zone. Third, the planet became tidally locked to its star within five millennia of its formation. Finally, the values assumed for the planet's bulk density (5515 kg/m^3, the same as Earth) and its albedo (0.30, about the same as Earth), may be entirely without merit. Even the planet's mass, ranging from a minimum of 1.27 Earth masses to a 90% probability it is not more than three Earth masses, gives quite a range,

Could such a world evolve life? Possibly, though it would probably be very different from life found on Earth (or even elsewhere in the Solar System). If it were to emerge, I would expect it to reside either in the deeper parts of whatever oceans/seas/lakes as may exist and/or deep underground, as a defense against radiation. This would have the effect of leaving the surface largely unpopulated and barren in aspect. Those few lifeforms that did evolve for life on the surface would need to be very hardy, seeing as they would need radiation protection and possibly thermal protection as well. I suppose they could remain dormant until temperatures were conducive to having their biosolvent in a liquid state, though this might limit them to only a few days out of every "year" (11.186 days); they might lead a sort of punctuated existence, spending their time between periods of frenzied activity and inert dormancy. Any plant life on the surface would most likely be black, in order to maximize the longer wavelengths of light emitted by Proxima Centauri. A more robust genome would be necessary, or at least one that is highly fault-tolerant. Variable atmospheric pressures might also be a factor, depending on whether the atmosphere (if any) has a sufficient quantity of carbon dioxide; if not, the atmosphere might collapse on the "dark side" only to thaw again when and if the planet librated. Surface bodies of liquid water may then be problematic, though subsurface deposits might not be affected as much. It is possible, even likely, that orbital surveys of Proxima b might show a dry, lifeless world not unlike the first images of Mars. But images can be deceiving.

Radtech497