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How’s the weather on Gl 581 c?

April 27th, 2007 by Hasenkopf · 10 Comments

A preprint letter to appear in a future Astronomy and Astrophysics detailing the discovery of the most Earth-like planet to date, Gl 581 c, was posted online today. You’ve probably already seen a few mass-media articles in the press about Gl 581 c (if not, check some out here or here or here). Absolutely fascinating! What’s so Earth-like about this planet? Well, for starters the star system in which the planet resides is ~4.3 billion years old – very comparable to our own Solar System. The planet itself is about 5 times as massive as Earth (bear in mind this is a very rough estimate and that the actual number is dependent upon the planet’s as of yet unknown composition), it is likely rocky and not completely gaseous, and it is thought to be in the habitable zone of its parent’s star despite the planet being only an estimated 7.3% of the distance the Earth is to the Sun. I gleaned the image below from wikipedia:


How is it that Gl 581 c is so close to its sun yet still considered in a habitable zone? The star Gl 581 is an M-type star which means it is much less luminous than our G-type sun, so it puts out a lot less energy-about 100 times less. The article reports planet surface temperatures ranging from 0 to 40 degrees C, which is very rough estimate, as well, since no compositional features of the surface or atmosphere (if one exists at all!) are known.

So what’s the weather like on Gl 581 c? Uh, sort of hard to say since we don’t even know if an atmosphere exists in the first place; however, if one imagined there were one, it would obviously be dramatically different than Earth’s. Since Gl 581 c has such a close orbit to its parent star, it is likely tidally locked to it, like the moon is to the Earth or Mercury nearly so to the Sun. This means that the planet may only receive radiation from its parent star on the same hemisphere and the other half would be in constant darkness. Clearly, the typical atmospheric circulation patterns that happen in our atmosphere would not apply here. What would they be like? Maybe all of the volatiles exposed on the hot side would evaporate and eventually condense and freeze out on the cold side, so any atmosphere that initially existed would quickly disappear? Maybe with the right amount of greenhouse gases, you could maintain an atmosphere throughout the planet? Who knows? But it’s fun to think about and it pushes our understanding of fundamental atmospheric dynamics to a new paradigm when we consider such exotic scenarios. Anyone care to speculate?

Tags: general interest

10 responses so far ↓

  • 1 Jamie // Apr 27, 2007 at 4:16 pm


    Is there some formula for calculating the time scale for the tidal locking of a planet with its star?

    I’m guessing it would depend on things like the moment of inertia, the initial angular momentum about the rotational axis and the asphericity of the planet’s density distribution.

    Just thought it’d be fun to know,


  • 2 Jamie // Apr 27, 2007 at 4:26 pm


    Sorry, I misspelled your name. Oops! JAS

  • 3 Chris McKay // Apr 27, 2007 at 4:47 pm

    Christa & Jamie (and any other water-based life forms)

    It almost certainly true that this new planet is tidally locked to its primary. But this is not necessarily inconsistent with habitable conditions. See the paper: Joshi, M.M., R. M. Haberle, and R. T. Reynolds. “Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability,” Icarus, 129, 450–465, 1997. They conclude that the atmosphere will transport heat from the light side to the dark side even for a 150 mbar atmosphere. For a thicker atmosphere it gets even better. Venus is a good example of how a thick atmosphere smooths out the temperature distribution on a very slowly rotating planet.

    So the temperature might be roughly the same on the light and dark sides.

    Obviously light levels would be different and any photosynthesizers would have to live on the light side.

    A question would be what kind of life would live on the dark side? Could there be an ecosystem there or would it be just barren.

    I don’t know if the atmospheric models are sophisticated enough to predict the distribution of clouds and rain on such a world. My guess: a rainy zone at the substellar point (like the tropics on Earth), a doughnut of desert about 30 degrees away then a zone of temperatre rain…


  • 4 Lab Lemming // Apr 27, 2007 at 9:23 pm

    If the star mostly radiates in the 0.5-3 micron wavelengths, won’t the atmosphere absorb most of the light before it reaches the surface?

    If so, it would most likely have venus-like weather, with a very stable lower atmosphere.

  • 5 Jim Kasting // Apr 30, 2007 at 7:03 am

    The new 5.5 Earth-mass planet, Gliese 581c, is
    inside the inner edge of the habitable zone(HZ), according to the estimates in my 1993 Icarus paper:

    Kasting, J.F., D.P. Whitmire, and R.T. Reynolds. Habitable zones around main sequence stars. Icarus 101, 108-128 (1993).

    The star’s luminosity is 0.013 times that of our Sun. Taking into account the albedo correction caused by the longer wavelength radiation from the cooler red dwarf, the HZ inner edge should be at about 0.09 AU. The semi-major axis of Gliese 581c is 0.73 AU, putting it well inside the inner edge. Put another way, this planet would absorb about 2.5 times as much sunlight as does Earth, if it had Earth’s atmosphere. That’s too much! Consequently, this planet is probably more like a super-Venus than a super-Earth. Note also that 5.5 Earth masses is only a lower limit. The
    actual planetary mass could be higher if its orbit is highly inclined to our line of sight.

    Interestingly, the other new planet discovered in that same system, Gliese 581d, is very close to the outer edge of the HZ. So, it may have a better chance of being habitable than does the innermost one. However, the lower limit on its mass is about 8 Earth masses. So, given the uncertain inclination, this one may be above the ~10 Earth-mass limit for becoming a gas giant.

  • 6 Christa // Apr 30, 2007 at 7:48 am

    Hey Jamie,

    I think you got the main parameters dictating the tidal-locking timescale. I think of the timescale as depending on (1) how large the differential (tidal) forces on the object are – this is dependent on the mass of the two objects in the system, the distance between each other, and the size/shape of the objects. These factors dictate a rate of how energy will be dissipated from the object in question’s rotation. And then (2) the timescale depends on how much initial angular momentum the system has, as you said, or, put another way, just the initial rotation speed of the object being tidally locked.

    I found this as a rough estimate online (where T is the timescale):

    “Satellites” (U of Az Press), edited by Burns & Mathews:

    T = 16 rho omega a^6 (Q/k2) / ( 45 G M^2 )

    rho = density of body being despun [kg/m^3]

    omega = inital rotation rate of body being de-spun [rad/s] = 2 pi / P, where P is the inital rotation rate [s]

    a = semi-major axis of orbit [m]

    Q/k2 = dissipation function divided by the 2nd order Love #

    M = mass of body doing the despinning [kg]


  • 7 Christa // Apr 30, 2007 at 8:06 am

    Chris’ and Lab’s comments reminded me of an interesting discussion on the irradiance requirements of photosynthetic organisms on Earth in Astrobiology by Raven & Cockell:

    Influence on Photosynthesis of Starlight, Moonlight, Planetlight, and Light Pollution (Reflections on Photosynthetically Active Radiation in the Universe)
    Aug 2006, Vol. 6, No. 4 : 668 -675

    Obviously, life that could possibly develop around a star that peaks at lower wavelengths, gives off lower total amounts of radiation, and inhabits a planet that possibly absorbs a lot of this radiation would be drastically different than the 400-700nm photosynthetic organisms on Earth….but it’s still fun to check out.

  • 8 William // May 1, 2007 at 7:36 pm

    Take in mind im 17 and do not have a very vivid range of knowledge in this type of exploration.

    But according to your graph of habitateability,
    wouldn’t it be more likely to find a more possibly habitateable planet north east, then aposed to
    GL-581 c, Which is south west according to the chart?


  • 9 Christa // May 1, 2007 at 8:27 pm

    Hi William,

    I see your point – you would think that since the habitability zone is wider for more massive stars than the one Gl 581 c orbits, why wouldn’t astronomers look around those stars for Earth-like planets instead? The answer is that the star this planet orbits is a very, very common type of star called a red dwarf. In fact, of the 100 closest stars to us, 80 of them are red dwarfs! So if we can find “Earth-like” planets going around a common star-billions of which exist in our Galaxy- then you can interpret that to mean Earth-like planets are prevalent, as well. Also, the way that this planet was found was through its gravitational influence on its parent star. This influence is more pronounced the less massive the star is, so it is easier to detect smaller mass (aka Earth-like)planets that orbit around smaller stars.

    This space.com article talks a little bit about the basic technique used to find this planet (though keep in mind they are NOT talking about the most recent planet found in the Gl 581 system, but a Neptune-sized one found a few years ago).


  • 10 becca ashbacher // May 10, 2007 at 10:12 am

    can you send me something that says what the necessary factors to allow humans to live on this planet would be. Thank youuu!

    Becca Ashbacher
    Kennedy high school
    foundations of science
    May 10, 07

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