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.
The problem with the arguments presented in that paper is the energy transport and greenhouse effect of the atmosphere is assumed as a given. What is really needed is a study of the climate on Gliese 581 c with consideration of feedback loops (i.e. enhanced evaporation of water vapour over hot parts of the planet, etc.). If temperatures are reaching 350 K on the day side there may well be an awful lot of evaporation going on, dumping more greenhouse gas into the atmosphere. They even state in the paper that the basic climate model used has instability problems, so I don’t think this can be regarded as a definitive estimate of habitability.
Furthermore the analysis still seems to be assuming a terrestrial nature for the planets. A study of the habitability of ice (ocean) planets would perhaps be a more useful way to go.
Agreed, Andy, and especially re the instability problem, which seems profound. But I think it’s useful even when assigning arbitrary values to greenhouse effect and other variables just to work out a theoretical case for habitability. Not a definitive one, as you say, but the authors don’t imply that it could be.
yes and another argument which they again taken to be given is that the planets lost theit original H and He
atmospheres which were replaced by outgassing from the interiors. I don’t think we can call that a given since
that is close to the border where such atmospheres can be retained.
I meant their masses are close to the border where the original H and He atmosphere could be retained.
Indeed. See this paper: the critical mass can conceivably be as low as 5 Earth masses, which could mean all three planets of Gliese 581 have hydrogen atmospheres. (On the other hand, the study is for planets around solar-type stars, conditions in an M star disc would be different). I’m not sure whether atmospheric hydrogen would help or hinder formation of a liquid water ocean though.
Hi andy
Hydrogen retention might enable water ocean formation via locking up oxygen as water instead of as metal oxides in the mantle. Protoplanetary chemistry, as a field, is full of opinions on the end state of the chemical ferment that the planets formed in – I’m finding it hard to have an opinion that does justice to the varying lines of evidence.
Interesting reading I have yesterday. I was packing away very old “Astounding” magazines (the oldest I have is 1935) and read a fact article from 1941. The carbon dioxide, water and oxygen levels on Venus had just been measured, and the formaldehyde model of Venus’s clouds was proposed – Venus was dry and the clouds were formaldehyde polymer dust. This dry, desert model was as close as people got to the real Venus in the 1940s and 50s, until the microwave temperature was first measured in the late 50s. The case was argued in a very similar manner to current chemical evolution models – oxidation of surface, hydrogen escape and so on. No one expected sulphuric acid clouds, but that’s an obvious end-state in hindsight too.
Adam: interesting point there. I was thinking along the lines of these models of Neptune, where the planet is apparently too hot and dry to form oceans at present. It seems to suggest that if there is too much hydrogen, the water won’t condense.
It’s kind of ironic given that the bulk of Neptune is thought to be water, and it is so far from the Sun, yet it is too hot and dry to fulfil the criteria for liquid water.
Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
Authors: Philip Nutzman, David Charbonneau
(Submitted on 18 Sep 2007)
Abstract: By targeting nearby M dwarfs, a transit search using modest equipment is capable of discovering planets as small as 2 Earth radii in the habitable zones of their host stars. The MEarth Project, a future transit search, aims to employ a network of humble, ground-based robotic telescopes to monitor M dwarfs in the northern hemisphere with sufficient precision and cadence to detect such planets. Here we investigate the design requirements for the MEarth Project. We evaluate the optimal bandpass, and the necessary field of view, telescope aperture, and telescope time allocation on a star-by-star basis, as is possible for the well-characterized nearby M dwarfs. Through these considerations, 1,976 late M dwarfs (R less than 0.33 Rsun) emerge as favorable targets for transit monitoring. Based on an observational cadence and on total telescope time allocation tailored to recover 90% of transit signals from planets in habitable zone orbits, we find that a network of ten 30 cm telescopes could survey these 1,976 M dwarfs in less than 3 years. A null result from this survey would set an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets larger than 2 Earth radii in the habitable zones of late M dwarfs, and even stronger constraints for planets lying closer than the habitable zone. If the true occurrence rate of habitable planets is 10%, the expected yield would be 2.6 planets.
Comments: submitted to PASP
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0709.2879v1 [astro-ph]
Submission history
From: Philip Nutzman [view email]
[v1] Tue, 18 Sep 2007 16:59:57 GMT (177kb)
http://arxiv.org/abs/0709.2879
Habitable planets around the star Gl 581?
Authors: Franck Selsis (CRAL, LAB), J. F. Kasting, B. Levrard (CRAL, IMCCE), J. Paillet, I. Ribas, X. Delfosse (LAOG)
(Submitted on 28 Oct 2007)
Abstract: Radial velocity surveys are now able to detect terrestrial planets at habitable distance from M-type stars. Recently, two planets with minimum masses below 10 Earth masses were reported in a triple system around the M-type star Gliese 581. Using results from atmospheric models and constraints from the evolution of Venus and Mars, we assess the habitability of planets Gl 581c and Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ) boundaries determination. We provide simplified formulae to estimate the HZ limits that may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Planets Gl 581c and ‘d’ are near, but outside, what can be considered as the conservative HZ. Planet ‘c’ receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. Its habitability cannot however be positively ruled out by theoretical models due to uncertainties affecting cloud properties. Irradiation conditions of planet ‘d’ are comparable with those of early Mars. Thanks to the warming effect of CO2-ice clouds planet ‘d’ might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of various greenhouse gases could also maintain habitable conditions on this planet.
Comments: Astronomy and Astrophysics (2007) accepted for publication
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0710.5294v1 [astro-ph]
Submission history
From: Franck Selsis [view email] [via CCSD proxy]
[v1] Sun, 28 Oct 2007 15:37:38 GMT (389kb)
http://arxiv.org/abs/0710.5294
Hi Larry & all Centauri Dreamers
Good paper that one by Selsis et al. Covers the state of knowledge of the inner and outer boundaries of the Habitable Zone – for example, without CO2 Earth would be frozen over with a global temperature of around -40 (C or F), while 100% cloud cover would allow a planet to remain viable to within 0.5 AU of the Sun. Builds a lot on Jim Kasting’s early works, but covers the broad range of the current debate on plate tectonics and CO2 clouds etc.
As it’s available to all check it out and be informed.
Just read that paper now. Interestingly it does appear to cover the case of Gliese 581 c being an ocean planet and suggests the ocean would be supercritical (the large water reservoir scenario).
Seems current knowledge indicates that habitable conditions on either Gliese 581 c or d would be fairly tricky to arrange – it’s apparently quite easy to tip Gliese 581 c into runaway greenhouse and Gliese 581 d into a snowball scenario.