Reconsidering Gliese 581

by Paul Gilster on October 31, 2007

Gliese 581 continues to occupy the attention, and understandably so. At least three planets orbit this M-dwarf, one of which sprang into the public consciousness with the announcement that it might be in its star’s habitable zone. But both Gl 581c and d are interesting from the habitability standpoint, even if subsequent discussions have pointed out just how problematic it is to make such judgments on insufficient data.

Ponder how tricky the call can be. For being in a circumstellar habitable zone only means that a terrestrial-size planet can have liquid water on its surface. A new paper by Franck Selsis (Centre de Recherche Astrophysique de Lyon), James Kasting (Pennsylvania State) and colleagues wades right into this morass, pointing out how many other factors could make such a planet remain uninhabitable:

  • Water may not be available
  • A high impact rate may prevent the emergence of life
  • The thus far unknown minimum ingredients for life’s formation may not be present
  • Gravity may be too low to retain a dense atmosphere
  • The planet may have formed an atmosphere that keeps the surface pressure too high for water to remain liquid

And so on. The point being that the habitable zone is no guarantee of life. For that matter, we have a long way to go in understanding how to put all these factors together. The Selsis/Kasting paper, already under discussion in various messages here, sees Gl 581c as definitely problematic but not completely out of the life-forming picture. While it’s unlikely to have liquid water, the uncertainties involved in its cloud properties and cloud cover leave the question at least slightly open. From the paper:

…Gl 581c would be habitable only if clouds with the highest reflectivity covered most of the daytime hemisphere. A 50% cloud cover is not sufficient to prevent a runaway greenhouse effect on Gl 581c, which receives 30% more energy than Venus today. This problem is exacerbated by the fact that Venus has a much higher albedo than the expected value for a habitable planet at the orbital distance of Gl 581c. The composition of the atmosphere of Gl 581c depends on the mass of the initial water reservoir on the planet, and on the efficiency of the gravitational escape of H.

The paper goes on to describe possible scenarios, including one involving water forming “…a mantle of hot and high-pressure ice underneath a fluid envelope of supercritical H2O,” as well as a Venus-like scenario leaving a CO2 rich atmosphere with surface temperatures as high as 1000 K. But we’re not through — change the models around and things get much iffier. The authors note, for example, how much depends on the carbonate-silicate cycle, which stabilizes surface temperature and the amount of CO2 in the atmosphere. Even at 1 AU, Earth itself would not be close enough to the Sun to maintain water above the freezing point without enough atmospheric CO2.

What happens if we do away with the greenhouse effect on Gl 581c, or take albedo to the extreme? How about geochemical processes and their effect on whatever atmosphere exists there? Dig into this paper for the speculative details. With so many questions, it’s clear how much we need by way of further data before we can make any calls. The authors put it this way:

Because of the uncertainties in the precise location of the HZ boundaries, planets at the edge of what is thought to be the HZ are crucial targets for future observatories able to characterize their atmosphere. At the moment, our theory of habitability is only confirmed by the divergent fates of Venus and the Earth. We will have to confront our models with actual observations to better understand what makes a planet habitable. The current diversity of exoplanets (planets around pulsars, hot Jupiters, hot Neptunes, super-Earths…) has already taught us that Nature has a lot more imagination when building a variety of worlds than we expected from our former models inspired by the Solar System.

Amen to that. And as for Gl 581d, it’s conceivable that we’re looking at an early Mars scenario, a situation thought to have involved plentiful water at the surface. Add a greenhouse effect from CO2 ice clouds and planet d might just be the best bet for habitability in this odd and fascinating system. But here again, we don’t know enough about the geochemical processes involved that could stabilize such an atmosphere. A mission like Darwin or whatever emerges from the US equivalent could tell us much about the atmospheres of both worlds, searching in particular for water vapor bands in the atmosphere of Gl 581d, and on Gl 581c distinguishing between a CO2 atmosphere and an H2O rich alternative.

Prime targets for follow-up work with next generation space technology? You bet. The paper is Selsis et al., “Habitable planets around the star Gl 581?” accepted for publication in Astronomy and Astrophysics, available online in preprint form and, as one of our readers has already noted, as absorbing and thorough a take on these distant worlds as we’ve yet seen.

andy October 31, 2007 at 10:50

Since I’ve gone on record on this blog of predicting the non-habitability of Gliese 581 c, this paper is an interesting read. Firstly it seems it was right to question the “naive” temperature estimate used to claim habitability in the first place, even if it turns out that the planet is habitable. In addition, the scenarios for saving the habitability of Gliese 581 c lead to further questions: if clouds and other effects can save Gliese 581 c from runaway greenhouse, why this didn’t work on the less-insolated Venus?

Secondly, what is obvious is that the possibility that neither of the Gliese 581 planets is habitable is not an unlikely one. Furthermore the possibility that the planets may have accreted thick hydrogen/helium atmospheres (and thus are low-mass analogues of Uranus and Neptune, rather than ocean planets or superterrestrials) and have surface pressures above the critical pressure of water is another one that casts a shadow on habitability, particularly of the more massive Gliese 581 d.

It’s also nice to read that the scenario of a supercritical ocean world (which is the one I favoured from the start) is plausible for conditions on Gliese 581 c.

Definitely a system to keep an eye on here.

Adam October 31, 2007 at 16:10

Hi andy

Perhaps Gl581c is a real-life Tenebra – super-critical ocean, high gravity and so forth. If the planet is rotating the lower atmosphere might partially condense at night. Rain-drops metres wide!

dad2059 October 31, 2007 at 18:51

Fascinating scenarios to say the least.

Even if Dawn, or any other next generation planet detector/telescope finds that Gl 581d is an early Mars type world with minimal water, it can prove to be a valuable tool in our own study of Mars and how to re-aquify it possibly.

ljk November 1, 2007 at 13:52

Scientists Discover New Member of Exoplanet Family

Astronomers will announce new findings about a planetary
system similar to our own at a media teleconference Tuesday,
Nov. 6, at 1 p.m. EST. Funding for the study was provided by
NASA and the National Science Foundation.

The briefing participants are:

— Debra Fischer — astronomer, San Francisco State University
— Geoff Marcy — astronomer, University of California, Berkeley
— Jonathan Lunine — planetary scientist, University of Arizona, Tucson
— Zlatan Tsvetanov — program scientist, NASA Headquarters, Washington

Reporters should call the Media Relations Office at NASA’s Jet Propulsion
Laboratory at 1-818-354-5011 for dial-in participation information. NASA will
stream audio of the briefing live on the Web at:

http://www.nasa.gov/newsaudio

At the start of the briefing, NASA will post supporting images and graphics at:

http://www.nasa.gov/audience/formedia/telecon-20071106/

For information about NASA’s planet-hunting missions, visit:

http://planetquest.jpl.nasa.gov/

andy November 1, 2007 at 15:36

ljk – sounds very interesting… Steinn Sigurðsson mentioned in a previous blog posting a Jupiter-analogue around HD 154345, perhaps this is the public announcement.

If it’s something else, that will also be extremely interesting.

andy November 2, 2007 at 16:42

Adam: didn’t get the reference at first, I haven’t actually read any Hal Clement.

I would be surprised if there weren’t any liquid water in the atmosphere of Gliese 581 c, but I suspect that like the Venusian sulphuric acid rains, it never gets anywhere near the surface. If we want to dive headlong into irresponsible speculation, maybe the planet’s nightside has vast thunderstorms churning through the atmosphere (IIRC thunderstorms are associated with radio emission, which suggests that it might be possible to detect this kind of thing…)

Adam November 2, 2007 at 17:30

Hi andy

Cool idea, detecting storms via radio emissions. We can do that for Jupiter can’t we?

andy November 2, 2007 at 18:20

As far as I know it has been done for Saturn, but detecting lightning on an extrasolar planet might be very challenging, not sure if it is feasible (I did say I was diving into irresponsible speculation there!)

Nevertheless radio detection of extrasolar planets would be extremely useful: for a start, we would find out if extrasolar planets have magnetic fields and how strong they are. Secondly, it is by monitoring the radio emissions that we know the rotation rates of the gas giant planets, thirdly it would reveal the presence of systems similar to the Io-Jupiter flux tube (thus providing a method of finding extrasolar moons), and maybe (just maybe) might reveal lightning storms on the planets themselves.

ljk November 12, 2007 at 1:07

Evolution over Time of Magentic Dynamo Driven UV & X-ray Emissions of dG-M Stars and Effects on Hosted Planets

Authors: Edward F. Guinan, Scott G. Engle

(Submitted on 9 Nov 2007)

Abstract: The evolution over time of the magnetic activity and the resulting X-ray and UV coronal and chromospheric emissions of main-sequence dG, dK, and dM stars with widely different ages are discussed. Young cool stars spin rapidly and have correspondingly very robust magnetic dynamos and strong coronal and chromospheric X-ray – UV (XUV) emissions. However, these stars spin-down with time as they lose angular momentum via magnetized winds and their magnetic generated activity and emissions significantly decrease. Studies of dK-dM stars over a wide range of ages and rotations show similar (but not identical) behavior. Particular emphasis is given to discussing the effects that XUV emissions have on the atmospheres and evolution of solar system planets as well as the increasing number of extrasolar planets found hosted by dG-dM stars. The results from modeling the early atmospheres of Venus, Earth and Mars using recently determined XUV irradiances and inferred winds of the young Sun are also briefly discussed. For example, the loss of water from juvenile Venus and Mars can be explained by action of the strong XUV emissions and robust winds of the young Sun. We also examine the effects of strong X-ray and UV coronal and chromospheric emissions (and frequent flares) that dM stars have on possible planets orbiting within their shrunken habitable zones (HZs) – located close to the low luminosity host stars (HZ less than ~0.4 AU).

Dwarf M stars make interesting targets for further study because of their deep outer convective zones (CZs), efficient dynamos, frequent flares and strong XUV emissions. Furthermore, a large fraction of dM stars are very old (greater than 5 Gyr), which present intriguing possibilities for the development of highly advanced modes of intelligent life on planets that may orbit them.

Comments: 14 pages, 9 figures, Oral Contribution to IAU 26th General Assembly Joint Discussion 4 – The Ultraviolet Universe: Stars from Birth to Death

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0711.1530v1 [astro-ph]

Submission history

From: Scott Engle [view email]

[v1] Fri, 9 Nov 2007 19:30:58 GMT (2666kb)

http://arxiv.org/abs/0711.1530

ljk November 14, 2007 at 11:27

Characterizing the Near-UV Environment of M Dwarfs

Authors: Lucianne M. Walkowicz, Christopher M. Johns-Krull, Suzanne L. Hawley

(Submitted on 12 Nov 2007)

Abstract: We report the results of our HST snapshot survey with the ACS HRC PR200L prism, designed to measure the near-UV emission in a sample of nearby M dwarfs. 33 stars were observed, spanning the mass range from 0.1 – 0.6 solar masses (T_eff ~ 2200K – 4000K) where the UV energy distributions vary widely between active and inactive stars. These observations provide much-needed constraints on models of the habitability zone and the atmospheres of possible terrestrial planets orbiting M dwarf hosts, and will be useful in refining the target selection for future space missions such as TPF. We compare our data with a new generation of M dwarf atmospheric models and discuss their implication for the chromospheric energy budget. These NUV data will also be valuable in conjunction with existing optical, FUV and X-ray data to explore unanswered questions regarding the dynamo generation and magnetic heating in low-mass stars.

Comments: 21 pages, 17 figures; accepted for publication in ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0711.1861v1 [astro-ph]

Submission history

From: Lucianne Walkowicz [view email]

[v1] Mon, 12 Nov 2007 21:00:21 GMT (240kb)

http://arxiv.org/abs/0711.1861

ljk December 7, 2007 at 13:15

A depiction of Gliese 581c and its parent star at
Orion’s Arm here:

http://www.orionsarm.com/gallery/gliese581c.jpg

ljk December 13, 2007 at 10:30

Dynamical evolution of the Gliese 581 planetary system

Authors: Hervé Beust (LAOG), Xavier Bonfils (LAOG), Xavier Delfosse (LAOG, OSUG), Stephane Udry

(Submitted on 12 Dec 2007)

Abstract: We address the issue of the dynamical evolution of the Gliese 581 planetary system. It is crucial when considering the planets’ habitability because the secular evolution of the orbits may regulate their climate, even in the case where the system is stable. We have numerically integrated the planetary system over 10^8 yrs, starting from the present fitted solution. In all cases, the system appears dynamically stable, even in close to pole-on configurations. Only a limited range of inclinations can be excluded.

The climate on the planets is expected to be secularly stable, thus not precluding the development of life. Gl 581 remains the best candidate for a planetary system with planets that potentially bear primitive forms of life.

Comments: 7 pages. Astronomy & Astrophysics (2007) accepted

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.1907v1 [astro-ph]

Submission history

From: Herve Beust [view email] [via CCSD proxy]

[v1] Wed, 12 Dec 2007 10:49:59 GMT (209kb)

http://arxiv.org/abs/0712.1907

ljk December 13, 2007 at 13:55

An A&A press release on the above Arvix paper,
complete with a diagram showing the various
models for habitability zones with Gliese 581.

http://www.aanda.org/content/view/275/42/lang,en/

Apparently at least one planet should be in the
HZ, according to these models, at least enough
for simple life forms.

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