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Gliese 581: Stable but No Transits

Those following the Gliese 581 story have been awaiting the results of the MOST observations with great interest. The Canadian mission put the red dwarf under study for six weeks after the recent flurry of speculation regarding a possible habitable planet, Gliese 581 c, in the system. If the planet made a transit, moving across the face of its star as seen from Earth, then we could learn more about its size and makeup.

The results are now in, and no transit occurred. But a second issue is a bit more satisfactory. During the observation period, Gliese 581 showed little change in brightness, indicating a level of stability that would prove beneficial to the growth of life, whether on Gliese 581 c or the more distant (and massive) Gl 581 d, which may orbit on the outer edge of the star’s habitable zone. Here’s Jaymie Matthews (University of British Columbia and a MOST mission scientist) on the matter:

“The climate there should not be a wild rollercoaster ride that would make it difficult for life to get a foothold. It also suggests the star is quite old, and settled in its ways, so that the planets around it have been around for billions of years. We know it took about three and a half billion years for life on Earth to reach the level of complexity that we call human, so it’s more encouraging for the prospects of complex life on any planet around Gliese 581 if it’s been around for at least as long.”

Note that phrase ‘any planet around Gliese 581.’ I assume it’s an oblique reference to the growing evidence that Gl 581 c is not the Earth-like world first suggested. In fact, recent work on the planet suggests it resembles Venus much more than Earth, a seething cauldron far too close to its primary to provide a reasonable foothold for life. But Gl 581 d may still be in the running, and it is encouraging in any case to know that a star like this can produce candidate planets near the needed limits. It won’t be long before we have another candidate, and a transiting one at that.

Bringing MOST to bear on this interesting system was useful in another regard. The observations rule out sunspots or other activity on Gliese 581’s surface that might have posed an alternative explanation for the 13-day cycle that has been interpreted as a planet. No such variations occur in the MOST findings, adding to our confidence that Gliese 581 c is indeed there. Whether it is habitable or not seems increasingly in doubt.

Comments on this entry are closed.

  • ljk September 12, 2007, 0:10

    Considerations for the habitable zone of super-Earth planets in Gliese 581

    Authors: P. Chylek, M.R. Perez

    (Submitted on 10 Sep 2007)

    Abstract: We assess the possibility that planets Gliese 581 c and d are within the habitable zone. In analogy with our solar system, we use an empirical definition of the habitable zone. We include assumptions such as planetary climates, and atmospheric circulation on gravitationally locked synchronous rotation. Based on the different scenarios, we argue that both planets in Gliese 581 could develop conditions for a habitable zone. In an Earth-like environment planet d could be within a habitable zone, if an atmosphere producing greenhouse effect of 100K could have developed. If the planets are gravitationally locked-in, planet c could develop atmospheric circulation that would allow it to reach temperatures consistent with the existence of surface liquid water, which in turn could support life.

    Comments: 4 pages, 3 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0709.1476v1 [astro-ph]

    Submission history

    From: Mario Perez [view email]

    [v1] Mon, 10 Sep 2007 20:11:07 GMT (278kb)


  • andy September 12, 2007, 3:43

    Interesting that one, but the argument for habitability on a tidally-locked Gliese 581c is somewhat simplistic (assume an amount of heat transfer to the dark side, figure out the temperatures that would result): I don’t think the consequences of having a 350 K region on a “habitable” planet are explored enough. At those temperatures you would get a lot of evaporation going on, which would surely alter the greenhouse properties of the atmosphere. Whether it would go runaway, I don’t know.

    Treating these planets as terrestrial is still somewhat dubious I think: an assessment of the habitability of the Gliese 581 planets treated as icy planets (“ocean planets”) would perhaps be more realistic.

  • Allan sneddon March 6, 2008, 6:43

    i think that there is some way you can live on an other planet because, we addapted as humans to live on this planet so who is to say that, if there is life on other planets that, they have not adapted to the diversity of there planet even if it does not have water. becuase we only know the human race its is hard to tell what you need. what we should do is chack every planet within a certain distance and then if we find nothing then go a bit futher , we should look on the surface of the planet. we need to build a bigger telescop the biggest one is a 10 meter mirror in it just make a giant one then we can see if there is life on the surface.