Gliese 581: Right System, Wrong Planet?

by Paul Gilster on May 28, 2007

New work on Gliese 581′s interesting planetary system may prove dismaying for those hoping for a planet in the habitable zone. With two ‘super-Earths’ and a Neptune class world, this is a system that cries out for close analysis. The Geneva team that detected the super-Earths had calculated surface temperatures on Gliese 581 c at roughly 20 degrees C. What they left out was the likely greenhouse effect of the atmosphere.

For habitability — defined here as the presence of liquid water at the surface — is not dependent on the central star alone, but also on the properties of the planets circling it. Werner von Bloh (Potsdam Institute for Climate Impact Research) and team tackle the habitability question in terms of atmosphere. From their paper:

…habitability is linked to the photosynthetic activity of the planet, which in turn depends on the planetary atmospheric CO2 concentration, and is thus strongly influenced by the planetary dynamics. In principle, this leads to additional spatial and temporal limitations of habitability, as the stellar HZ (defined for a specific type of planet) becomes narrower with time due to the persistent decrease of the planetary atmospheric CO2 concentration.

From this, the team studies habitable zone limitations for super-Earths, using a thermal evolution model that takes into account atmospheric carbon dioxide and varying ratios between water and land surfaces. The results: Both super-Earths in the Gliese 581 system are inside the tidal locking radius, keeping one face turned toward the star at all times. Gliese 581 c, the source of so much press speculation about terrestrial worlds, turns out to be far too hot to support life. Even scaling for stellar luminosity, it’s closer to its star than Venus is to ours.

But we’re not quite through. Interestingly, Gliese 581 d, a super-Earth of roughly eight Earth masses, could well have built up a dense atmosphere. With at least some of the climate models and carbon dioxide pressures assumed, the planet nudges inside the habitable zone and could be habitable for a period as long as 7.2 billion years. From the paper:

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. Williams & Pollard (2002) concluded that a planet with a suffciently dense atmosphere could harbour life even if its orbit is temporarily outside the HZ. In conclusion, one might expect that life may have originated on Gl 581d.

The authors go on to add this caveat: Complex life is unlikely due to what they describe as ‘rather adverse environmental conditions.’ But of course we won’t know until we can get a look at potential biomarkers in the atmospheres of both these super-Earths. That will have to await later space missions like ESA’s Darwin and whatever mission grows out of the Terrestrial Planet Finder research. Given uncertainties of funding and ongoing technological reassessment, we may not have a definitive answer on the Gliese system for quite some time.

Thanks to Andy for the pointer to this one, and congratulations as well. Read through the comments on our first story on Gliese 581 c and you’ll see that Andy came up with remarkably similar conclusions not long after the story broke. Nice work indeed! The paper is von Bloh et al., “The habitability of super-Earths in Gliese 581,” available online.

Adam May 28, 2007 at 20:32

Hi Paul

I don’t think Gl581c is a total write-off yet, but Gl581d is definitely the nicer of the two, but both are rather nasty – “c” is very hot, probably a wet Greenhouse; “d” is warmed by a massive CO2 atmosphere. Considering the probable over supply of water they’re both covered in deep oceans and layers of Ice VII – maybe. We won’t know until we go look for ourselves, or until super-sized interferometers and coronagraphs get built out past Jupiter and the Zodiacal dust.

Administrator May 29, 2007 at 7:05

You’re right, Adam. From the current look of things, neither of these places looks to be optimal for life, and the ‘terrestrial’ tag is fading fast. Now we can speculate on what the next candidate for an Earth-like world will be. Given how fast things are happening, I suspect we’ll have another interesting possibility soon.

frege May 29, 2007 at 18:26

With all the Jupiters and Neptunes we’ve found, does anyone have a guess how many large moons reside in their systems habitable zone? I’ve seen the subject discussed before, but can’t remember seeing a guesstimate (because obviously we’d be guessing based on our best understandings of planet formation).

Administrator May 29, 2007 at 19:58

That’s a great question, and I’ve never seen an estimate on it — it may be there are just too many imponderables. Maybe someone else here knows of a reference.

Boomer May 30, 2007 at 1:01

Any guesses on what these super-Earths or mini-Neptunes actually look like? Titan, Venus, and Uranus are all bland, and I don’t imagine that a thick envelope of supercritical water would add visual distinctiveness. Wouldn’t it just be tragic if we can finally get images and most of what we find are featureless ink blots of varying hues? We want to see continents, oceans, storms, and exotic colors, but even a habitable planet may look like a ball of smoke. Titan is a ball of orange smoke; Venus a ball of white smoke; Uranus a ball of cyan smoke; maybe we’ll find red, green, yellow, purple, pink, or other smokeballs, but that may begin to wear thin. I hope terrestrial or super-terrestrial planets with largely transparent atmospheres aren’t the exception.

Adam May 30, 2007 at 7:12

Hi Paul & frege

The formation of large moons isn’t very well understood – there are plenty of competing theories and only three regular satellite systems to provide data. Most cosmogonists assume that the giant planets formed much like the Sun and had disks of material in orbit about them which accreted into their present moons. Interestingly the moon systems all mass about 1/5000 of their primaries. This might preclude Earth-sized moons, but as Io, Europa and Ganymede have shown us moons just a bit heavier than our own can have magnetic fields and active geology/hydrology in the right circumstances.

Also there’s plenty of reasons to think that in the Trojan points of Jovian planets large planet-like objects might form and migrate into an orbit around the forming gas-giant. I personally suspect that’s the origin of Titan, maybe Triton, and the impactor that made the Moon. Thus a Jovian might capture an Earth-sized moon, but it probably won’t form one. At least around planets with our Solar System’s metallicity – but things are different in other systems. At least one hot Jupiter has a vast amount of heavier elements – if it had parked in an orbit further out it might have formed a huge moon system.

Based on our system’s admittedly meagre data I think we’d be mostly right to bet on about 4-5 large moons around Jovian planets in fairly distant unperturbed orbits. Hot Jovians are too close to their primaries for stable orbits for moons.

Sky & Telescope had an article on the issues involved a few years ago…

http://skytonight.com/resources/seti/3304591.html

…well worth a look.

Ronald May 30, 2007 at 17:19

With regard to moons of giant planets: but what are the possibilities of detecting them? Using radial velocity (doppler shift) they will not be distinguished from the primary and even using direct imaging (in the future) they will be overwhelmed by the primary.

ljk May 30, 2007 at 19:19

Determination of the size, mass, and density of “exomoons” from photometric transit timing variations

Authors: A. Simon, K. Szatmary, G.M. Szabo

(Submitted on 8 May 2007)

Abstract: Precise photometric measurements of the upcoming space missions allow the size, mass, and density of satellites of exoplanets to be determined.

Here we present such an analysis using the photometric transit timing variation ($TTV_p$). We examined the light curve effects of both the transiting planet and its satellite. We define the photometric central time of the transit that is equivalent to the transit of a fixed photocenter. This point orbits the barycenter, and leads to the photometric transit timing variations. The exact value of $TTV_p$ depends on the ratio of the density, the mass, and the size of the satellite and the planet. Since two of those parameters are independent, a reliable estimation of the density ratio leads to an estimation of the size and the mass of the exomoon. Upper estimations of the parameters are possible in the case when an upper limit of $TTV_p$ is known. In case the density ratio cannot be estimated reliably, we propose an approximation with assuming equal densities. The presented photocenter $TTV_p$ analysis predicts the size of the satellite better than the mass. We simulated transits of the Earth-Moon system in front of the Sun. The estimated size and mass of the Moon are 0.020 Earth-mass and 0.274 Earth-size if equal densities are assumed. This result is comparable to the real values within a factor of 2. If we include the real density ratio (about 0.6), the results are 0.010 Earth-Mass and 0.253 Earth-size, which agree with the real values within 20%.

Comments:

6 pages, 5 figures, to appear in Astronomy and Astrophysics

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0705.1046v1 [astro-ph]

Submission history

From: Gyula Szabo [view email]

[v1] Tue, 8 May 2007 08:36:53 GMT (190kb)

http://arxiv.org/abs/0705.1046

ljk May 31, 2007 at 12:22

Space artists Don Dixon has created several pieces about
the Gliese 581 system here:

http://www.cosmographica.com/gallery/extrasolar/Gliese581c/index.html

ljk June 18, 2007 at 11:58

Hopes Dashed for Life on Distant Planet

http://bcast1.imaginova.com/t?r=2&ctl=14559:4A48D

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