Gliese 581d: A Habitable World After All?

by Paul Gilster on December 13, 2007

Gliese 581 is back in the news with a flourish. Astronomy & Astrophysics is publishing two independent studies of the system asserting that at least one of the inner planets is indeed located within the habitable zone of that star. Gliese 581 c and d are noteworthy every time they’re mentioned. Of five and eight Earth masses respectively, they are the first exoplanets ever considered serious candidates for habitability.

M dwarfs have inherent problems in terms of habitability, not the least of which is the tidal lock that planets in the HZ of such stars presumably experience, keeping one side perpetually dark. But models of atmospheric circulation exist that overcome that obstacle, and the intense magnetic activity of early M star life (producing dangerous flares) is no longer considered a necessary disqualifier for all forms of life. It’s going to be a while before we have any definitive answers, but current thinking is that habitable M dwarf planets are very much in the picture, with the inner two Gliese 581 worlds due for a look from future missions like Darwin.

So how big is the actual habitable zone around Gl 581? Franck Selsis (CRAL and LAB, France) and team look at the HZ in terms of liquid water at the surface, coping with the difficult problem of cloud modeling that makes precise determinations so difficult. Werner von Bloh (PIK, Germany) and team examine a more truncated HZ where photosynthesis could be possible. Interestingly, von Bloh’s work depends on the age of the planet, looking at the balance between sources of atmospheric CO2, released through ridges and volcanoes, and CO2 sinks, which consume the gas through weathering processes. Too old a planet might not release enough CO2 to keep the surface above the freezing point of water.

Here’s von Bloh’s conclusion, after ruling out Gliese 581c for habitability as being too close to its star. The team has looked at various climate models for Gl 581d, finding two of them promising:

A planet with eight Earth masses has more volatiles than an Earth size planet to build up such a dense atmosphere. This prevents the atmosphere from freezing out due to tidal locking. In case of an eccentric orbit of Gl 581d (e = 0.2), the planet is habitable for the entire luminosity range considered in this study, even if the maximum CO2 pressure is assumed as low as 5 bar. In conclusion, one might expect that life may have originated on Gl 581d. The appearance of complex life, however, is unlikely due to the rather adverse environmental conditions. To get an ultimate answer to the profound question of life on Gl 581d, we have to await future space missions such as the TPF/Darwin. They will allow for the first time to attempt the detection of biomarkers… in the atmospheres of the two super-Earths around Gl 581.

And this from the Selsis paper:

Darwin/TPF-I and TPF-C could eventually reveal what the actual properties of the atmosphere of Gl 581c and Gl 581d are. From their thermal light curves we could infer if a thick atmosphere is making the climate more or less uniform on both the day and night hemispheres of these planets, despite a (nearly?) synchronized rotation… Visible and mid-IR water vapor bands could be searched in the atmosphere of Gl 581d to confirm its habitability. Mid-IR spectra of this planet could also reveal other greenhouse gases at work. Spectral observations of Gl 581c could potentially distinguish between a Venus-like atmosphere dominated by CO2 or an H2O-rich atmosphere. The detection of O2 on this planet would generate a fascinating debate about its possible origin: as either a leftover of H2O photolysis and H escape or a biological release. There is certainly no doubt that Gl 581c and Gl 581d are prime targets for exoplanet characterization missions.

Habitable zones compared

Image (click to enlarge): Illustration of the habitable zone (HZ) boundaries as obtained by the two teams. The upper part of the figure shows the HZ of the Sun (at its present age). The red curve shows only the most extreme outer limit of the HZ. The actual outer boundary is indeed located somewhere between 1.7 and 2.4 AU. The green limits show the boundaries of the photosynthetic zone as computed with the model by von Bloh et al. The middle part of the figure shows the limits of the HZ of Gliese 581 computed with the atmospheric models from Selsis et al. The lower part illustrates the boundaries of the photosynthetic zone computed with the geophysical models from von Bloh et al. The boundaries are shown for several possible ages (5, 7, and 9 Gyr-old) of the Gliese 581 planetary system. Following the latest estimation, Gliese 581 would be 7 Gyr-old. The purple bars surrounding planets Gliese 581 c and d illustrate the variable distance to the star caused by the eccentricity of the orbits. Credit: Astronomy & Astrophysics.

Note that the Selsis paper does not rule out GL 581c’s possible habitability, although noting that the world receives thirty percent more energy from its star than Venus does from the Sun. Uncertainties in cloud properties make the final call problematic. Like von Bloh’s team, however, Selsis leans toward the possibility of CO2 ice clouds creating much better conditions for life on GL 581d, with the potential for greenhouse gases to maintain habitable conditions over long periods of time.

The papers are von Bloh et al., “The habitability of super-Earths in Gliese 581″ (abstract) and Selsis et al., “Habitable planets around the star Gliese 581?” (abstract), appearing in Astronomy & Astrophysics Vol 476-3 (2007), pp. 1365-1387. Note that a third paper, to be published in 2008 in the same journal, will examine the dynamical stability of the Gl 581 planetary system. That paper, by Hervé Beust (LAOG France) and team, will conclude that the GL 581 system appears dynamically stable, thus rendering the climate on both these worlds potentially stable as well.

AGeek December 13, 2007 at 16:38

I find statements like “The appearance of complex life, however, is unlikely due to the rather adverse environmental conditions.” pretty strange. Strong winds = no complex organisms? If there is enough water for lakes or oceans, a few meters below the surface that’s a non-argument, and there’s nothing preventing the existence of things like fish — and smart octopi hunting them. On land, stronger gravity and winds would favor organisms with a low center of gravity, maybe many legs, maybe living in crevices (or valleys) or digging their way through the dirt. No complexity? I can’t imagine why not.

andy December 13, 2007 at 18:22

I too am sceptical that we can conclude anything about the probabilities of complex life. An interesting question is how we could tell whether complex but nontechnological life has emerged on a planet from a distance of 20 light years.

And it depends what is meant by “complex”: what would be classed as simple life on Earth? Do we draw the line at multicellular life, or eukaryotic life, or what?

Christopher L. Bennett December 14, 2007 at 10:48

I’m finding it very hard to interpret the illustration, since it’s reproduced too small for most of the text to be legible. Can someone provide a link to a larger version?

Administrator December 14, 2007 at 11:51

Christopher, you’re right — I couldn’t link earlier to the larger image for various reasons, but I’ve now set up the image as ‘click to enlarge.’ Seems to be working, but let me know if you have further problems.

Dave Moore December 14, 2007 at 19:55

I noticed that both the papers talking about the habitability of Gliese 581d refered to it as tidally locked. However, Gliese 581d’s ecenticity and rotational period are an almost exact match for Mercury’s (The semi major axis is less because the primary is only 1/3 of the mass of the sun.)
This would almost certainly mean that the planet is in a 3/2 rotational resonance with its orbit, which would give it day length of aproximately 170 Earth days followed by a night of similar length. While this would lead to large temperature fluctuations, it would not lead the freezing out of a thin atmosphere.

Question: could an all gasious/liquid planet maintain a 3/2 orbital resonance or would tidal effects circularize the planet”s orbit? Does a planet need to have an asymetric (solid) mass to maintain a 3/2 resonance orbit?

Dave.

andy December 15, 2007 at 10:04

Dave Moore: as I understand it, Mercury’s current orbital eccentricity isn’t high enough to make capture into the 3/2 resonance likely. However thanks to perturbations from the other planets, the eccentricity can get much higher than the current value, making capture more likely. From the Beust et al. paper, it doesn’t look like Gliese 581d’s eccentricity gets anything like as high as Mercury’s may do.

Dave Moore December 15, 2007 at 16:51

Andy, how Mercury got into its 3/2 rotational/orbital period coupling, I don’t know, but it does have its rotational period 58.65 days is exactly 2/3 of its siderial period 87.97 days.
Mercury’s orbital eccentricity is 0.206. I have Gliese 581d’s as 0.2 from the extrasolar encyclopeadia site, which is within the error bars.
Whenever I come across a planet with an eccentric orbit that is closer to its primary than 0.5 au, I check for the following eccentricites. 0.2, 0.41, 0.58, 0.73 & 0.87. These are the eccentricities than generate stable, 3:2, 2:1, 5:2, 3:1 and 7:2 rotational/orbital period couplings (Worlds Without End by John S. Lewis p150-151)

Dave

andy December 15, 2007 at 20:06

Dave Moore: see this abstract:

For example, we find that the synchronous state is the end-state for e < 0.2366, while the 2/3 resonance becomes the tidal equilibrium for 0.2366 < e < 0.3672. The 1/2 state dominates for 0.3672 < e < 0.4522, the 2/5 state for 0.4522 < e < 0.5132, the 1/3 state for 0.5132 < e < 0.5596, and so on.

So Mercury’s current eccentricity perhaps isn’t high enough to get the planet captured into the current rotation state, though during times of higher eccentricity capture is more likely.

Dave Moore December 16, 2007 at 15:53

I would have to read Dobrovolskis’ paper before I could make any further comment on it, but I would like to point out that his concluding line: “It is unlikely for a planet to spin slower than its tidal end-state,” implies the figures quoted are the minimum’s for spin-orbital resonance.

Mercury’s current configuration is a minimum energy state in terms of tidal friction. From a higher rotational rate, it may well need to start from a point of much higher eccentricty and fall into its current configuration, but the same could be said for Gliese 581d

Dave.

ljk December 20, 2007 at 0:49

Habitability of Super-Earths: Gliese 581c and 581d

Authors: W. von Bloh, C. Bounama, M. Cuntz, S. Franck

(Submitted on 19 Dec 2007)

Abstract: The unexpected diversity of exoplanets includes a growing number of super-Earth planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogical composition as Earth.

We present a thermal evolution model for super-Earth planets to identify the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply our model to calculate the habitability of the two super-Earths in the Gliese 581 system.

The super-Earth Gl 581c is clearly outside the pHZ, while Gl 581d is at the outer edge of the pHZ, and therefore could at least harbor some primitive forms of life.

Comments: 3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.3219v1 [astro-ph]

Submission history

From: Manfred Cuntz [view email]

[v1] Wed, 19 Dec 2007 20:14:26 GMT (136kb)

http://arxiv.org/abs/0712.3219

graywyvern December 26, 2007 at 0:59

wouldn’t a 2/3 resonance result in a solar day exactly twice the orbital period, thus not a “day length of aproximately 170 Earth days followed by a night of similar length” but one of 86 days of sunlight & 86 of darkness, by the formula

S = (rot/rev) / (1- (rot/rev))…?

m.

andy March 12, 2008 at 7:59

Tidal Heating of Extra-Solar Planets – prediction is the energy flux from tidal heating on Gliese 581c is more than twice that of Io. The prediction for Gliese 876d on the other hand is insane… potentially hundreds to thousands of times that of Io, which would mean that the planet is likely not a solid rocky planet. Lava oceans anyone?

Administrator March 12, 2008 at 9:47

andy, you’ve got a broken link in that message. Is this the Jackson/Greenberg paper? If so, it’s:

http://arxiv.org/abs/0803.0026

It’s also in queue here; what a take on Gl 581′s most interesting planets!

andy March 12, 2008 at 12:20

Hmmm… don’t know what happened there, seems like the clipboard got scrambled or something – but yes, that’s the paper I meant.

paul odell April 16, 2011 at 13:13

i dont know much of the science hocus pocus so can someone explain to me if humans can live on the gliese planets or not by saying yes or no and if possible can someone tell me if the chances of animals, plants, fish ,etc is likely it would help

Paul Gilster April 16, 2011 at 14:19

The answer is, it’s unlikely that any of the planets in the Gliese system are habitable by our standards, but we do not yet know for sure. The planet recently thought to be in the habitable zone there, Gl 581 g, is controversial and many people think it is not there at all. Gl 581 c is probably more like Venus, and 581 d is marginal at best. We need more data to know something definitive.

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