Figuring out planetary habitable zones gets a little less theoretical when we start talking about known systems. And when that system is Gliese 581, the interest level rises considerably. After the initial announcements of a possibly habitable planet around that star, Gliese 581c was later analyzed (in a paper by Werner von Bloh and team) as being too close to its star for liquid water to exist. But another planet, the more distant GL 581d, seemed to hold distinct promise of being in the habitable zone.
Now a new paper tackles the question with intriguing results. Petr Chylek and Mario Pérez (Los Alamos National Laboratory) find some reason to think that both inner planets in this system may, under special but feasible conditions, have become suitable for life. The thinking here depends upon analyzing planetary environments as they evolve, with reference to our own Solar System in terms of that evolution.
Start with this: Early on, Venus, Earth and Mars lost their original, hydrogen-rich atmospheres, to produce secondary atmospheres that resulted from the release of water vapor and carbon dioxide from within the planets themselves. The widely divergent atmospheres we see today are, Chylek and Perez argue, the result of surface temperature, gravity, planetary motion and geological composition. Planetary systems that evolve like this may not support a simplified notion of habitable zones. In fact, we may find that habitable planets can exist well outside the zones we theorize.
With that in mind, the researchers look at the planets of GL 581 to determine their effective surface temperatures during the time after they lost their primitive atmospheres and before formation of their secondary atmospheres via outgassing from the planets’ interiors. Under conditions like those of our own Solar System, neither planet develops an atmosphere allowing liquid water on the surface. One is too hot, the other too cold, assuming processes like those on Venus, Earth and Mars.
But if we look at a different set of conditions, things change. Examining the greenhouse effect and cloud formation in the context of planetary albedo, the team concludes that if Gliese 581d developed an atmosphere producing a larger greenhouse effect than found on Earth, it could fit well within the habitable zone. Note this from the paper:
We can assume that a medium strength (comparable to Earth) atmospheric greenhouse effect could provide about 35K warming. The accompanying cloudiness would increase the planetary albedo to about 0.30, which would produce a cooling of about 11K. Thus the result of Earth-like atmosphere would produce a net surface warming of about 24K, which is not sufficient to bring the planet into the range of habitability. However, a considerably denser atmosphere that would lead to a planet completely covered by clouds, producing a planetary albedo of about 0.6, and warming of around 100K (about three times the greenhouse effect of the Earth’s atmosphere) would be sufficient to bring the planet d into a HZ. No objectionable mechanism is apparent that would prohibit the formation of such an atmosphere.
But what about Gliese 581c, the planet that kicked off all the fuss in the first place? Even a moderate greenhouse effect there should create temperatures too high for liquid water on the surface. Even here, though, interesting mechanisms may be at work to offset this apparent problem.
For we know little about the atmospheric effects of tidal lock, through which both these planets presumably keep one faced turned forever toward their star. In what may be their most controversial contribution, the authors argue that atmospheric circulation could transport excess heat from the illuminated to the dark side of the planet, forming a region where liquid water could exist. “Perhaps tropical forests on the hot side can provide a sufficient amount of oxygen for the whole planet and near the illuminated/shadowed boundary a habitable region may occur.”
Clearly, we need to know more about atmospheric movement in a tidally-locked context. Its effect on our notion of a habitable zone could be profound:
We found that a gravitationally locked-in planet can have liquid water on its surface even if it is outside a HZ, deﬁned traditionally by the parameters and distance to the sustaining star… Since half of the planet is permanently in “sunshine” an atmospheric circulation can develop between the illuminated and shadow parts and create a HZ with a possibility of surface liquid water in a planet that, otherwise, would be unsuitable. Thus the auxiliary deﬁnition of a HZ should be extended to include the rotational and atmospheric characteristics of planets…
The paper is Chylek and Perez, “Considerations for the habitable zone of super-Earth planets in Gliese 581,” available online.