gliese 581d gets the nod for habitability

May 18, 2011

At the rate we’re going, it seems that the first target for one of our future interstellar spacecraft will just have to be the Gliese 581 system. Beyond the initial hype generated by the announcement of planet 581g and a very deflating set of calculations showing that it may have just been a mirage, there were still planets with a little potential for life as we can understand it. This is why when discussing the practicality of colonizing the solar system in question, I brought up two worlds which seem to have faded from our collective memories. Now, it seems that one of these worlds, 581d, may actually be the terrestrial, habitable planet for which astronomers have been searching. Originally thought too cold for liquid water, the planet was put on the back shelf until an advanced climate model ran by French scientists showed that such a world could actually have oceans, rain, and stay warm enough to avoid having its night side frozen solid if it were tidally locked in its orbit. How? Well, as it turns out, it all comes down to sunlight, or rather to the right wavelength of sunlight. For a big planet with an atmosphere rich in carbon dioxide, a red star may actually be an enabler of a dynamic, warm climate…

When astronomers first considered Gliese 581d as a candidate for habitability, they thought that much of the light from its parent star would be reflected back into space as happens on our world. However, the light we get from the Sun has a shorter wavelength and gets scattered when it meets air molecules and other small, fine particles in our atmosphere. This phenomenon called Rayleigh scattering and it’s what gives our skies a bluish tint. When dealing with longer wavelengths, like you’d find coming from oh say a red dwarf star, the red light scatters less and more of it reaches deeper into the kind of thick carbon dioxide atmospheres that seem common for large, rocky planets. As a result, the skies of Gliese 581d would have a murky reddish glow and the conditions would support a water cycle, which we know to be a key for enabling life. If this model is right, it would not only mean that a planet just over 20 light years away is habitable, but that it’s probably inhabited by something since it’s difficult to imagine a watery world without life. After all, once upon a time Earth’s air had a lot of carbon dioxide and other gases we consider noxious and deadly today while its oceans were home to a countless variety of bacterial colonies thriving for billions of years, reproducing and growing away while much of the planet was erupting away with vast lava flows and toxic plumes that made the surface unlivable.

Whether 581d is tidally locked or not could determine what could live there and where we could find traces of those living things. Having a perpetual day side could allow for constant photosynthesis for organisms which would form the base of food chains in which the lit hemisphere is the central hub of all activity. Life could exist on the night side as well, but if photosynthesis didn’t evolve on 581d, the base of the food chains would be an assortment of bacteria feeding off thermal vents on ocean floors. Were the planet to rotate around its axis and have an actual night and day cycle, we could expect living things to be more widespread as more habitats will be available to them. Either way, the entire planet would be warmed thanks to wind circulation, but with direct sunlight across 581d a greater variety of organisms may have chances to establish new footholds to escape predation or to take full advantage of abundant resources. This is after all how we think all our forests started out, as primitive plants which grew with no competition, feeding on sunlight and carbon dioxide. If 581d were to repeat this evolutionary step, we could find traces of some sort of biological activity in its atmosphere. This is all conjecture, of course, and further observations will need to be made to figure out more about this world, but since we now know its potential, we should devote some time and resources to study it.

Again, it should be noted that as far as interstellar objects go, Gliese 581 is very close by, close enough to be of interest to some sort of mission at some point in the future. Should our hunches turn out to be right, there’s a very strong case to be made for at least trying to send a probe there. It may take a very long time to come up with the required technology to make the trip quickly enough, but with a very reasonable target in sight, it may just motivate enough space agencies, scientists, and engineers to rise to the challenge and create the brand new generations of computers, engines, reactors, and materials required for the journey. Maybe discovering and confirming that 581d can support life is exactly what we need to motivate future space explorers. Though we also need to be very cautious not to get too attached to the idea that 581d must be habitable because the models may be wrong and we don’t want to let confirmation bias take over the astronomers’ observations. It’s fun to speculate and the models seem very promising thus far, but at the end of the day, real data will need to have the last word on whether Gliese 581d is really habitable or not.

Wordsworth, R., et al. (2011). Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone The Astrophysical Journal, 733 (2) DOI: 10.1088/2041-8205/733/2/L48

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  • John Bledford

    SETI. Listen and ask Gliese 581 how it is doing. It’s only 20.3 lightyears distant. Just in case… have some Nightsun 30 million candlepower UV 1600 Watt xenon short arc lamps ready to go to greet hostile visitors. Blinded by the light…

  • DDeden

    As I mentioned at New Papyrus magazine, the best beginning of spatial colonization starts at home… Earth 1 (here), Earth 2 (Moon at the sunlit-dark peripheral boundary), Earth 3 (Mars at the sunlit-dark peripheral boundary)… only then start focusing on planets at 20+ light years.

    2 mylar-like layers of laminated “balloon” with thick artificial atmosphere between them would block out lots of otherwise dangerous particles and rays. Another earth-normal atmosphere enclosed within a 3rd smaller balloon/geodesic dome would provide lots of terrestrial lifespace, these could be “strung” along the most optimal ring of sunlight/shade.

  • David Marjanović

    Well, when will Zefram Cochrane be born? In 2042? For the time being, “only” doesn’t apply to “20.3 lightyears”.

    Moon at the sunlit-dark peripheral boundary

    There is no such thing. The moon is tidally locked with the Earth, but not with the sun.

    Mars at the sunlit-dark peripheral boundary

    There is no such thing. Mars isn’t tidally locked with anything.

  • Brett

    If it’s life-bearing and tidally locked, I suspect that the majority of life would be located at the terminator area between the “day” and “night” sides. There was a National Geographic documentary that looked at what a tidally locked red dwarf-orbiting world would be like.

    The world they came up with had a permanent hurricane centered around the area most directly “beneath” the sun, and an ice cap on the “night” centered around the point farthest from it. You might also get the migration of ocean waters all towards the “day” side, with the “night” side being dry, cold, and barren (when it’s not directly covered by ice).

    In any case, Gliese 581d has a minimum estimated mass 5.6 times that of Earth, meaning that it could actually be larger. I wouldn’t be surprised if it was more of a mini-gas giant than anything resembling a terrestrial world.

  • Greg Fish

    “The world they came up with had a permanent hurricane centered around the area most directly ‘beneath’ the sun…”

    That would only work if there was a large body of water at the planet’s equator. Throw in a little bit of land and the hurricane either can’t form or can’t last forever. Likewise, if you have some circulation of air (which 581d should have), it would be too warm for an ice cap on the equator of the night side.

  • Brett

    That would only work if there was a large body of water at the planet’s equator. Throw in a little bit of land and the hurricane either can’t form or can’t last forever.

    This is a planet with a minimum mass of 5.6 that of Earth. Assuming it isn’t more massive than that (in which case it would more or less be a small gas giant), it’s still very likely going to have a deep layer of water/liquid over its solid crust.

  • Greg Fish

    Gas giants and rocky worlds are fundamentally different and made by very different sets of processes. It’s why gas giants can be hundreds of times more massive than Earth but rocky words will implode if they exceed 7 Earth masses. Liquid is not a given for super-Earths, more of a nice to have.

  • MaDeR

    I would be suprised if this planet would be closest potentially habitable world (excluding our solar system).

  • Hank

    Liquid is not a given, but atmosphere pretty much is. With higher gravity comes the ability to retain more atmosphere, at higher pressure. Substances that can exist as liquids would have a larger temperature range to be liquid at, so it seems likely.