Sunday, October 3, 2010

Habitability of Gliese 581g

As you can see from my previous posts, I think life is fairly common in this cosmos, though life of our intelligence and technology level is very rare. The discovery of Gliese 581e definitely can make one question whether we’re underestimating the number of star systems containing life-bearing planets, or even planets with civilized life.  I understand the excitement about such a discovery, but that does not mean we should let our excitement cloud our judgment.  In that spirit, I want pass on some information about the star system itself.

I’ll go into greater detail below, but the short-and-skinny is this:  Gliese 581g definitely is the most promising candidate for extraterrestrial life of the Earth-type we’ve encountered. It (a) has a star is of sufficient age,(b) is massive enough to sustain the volcanos and plate tectonics on which earth-life ultimately depends (possibly even better than Earth), and (c) located at the right distance from the star to make liquid water quite possible if other factors are present.  

Nevertheless, the planet has as many factors inhibiting life as we know it as it does potentially fostering it – all of them center around one thing: rotation lock, when one side of a body permanently faces a body it orbits due to the larger body’s gravity.  The same thing happened to the moon with regard to Earth, forcing the moon to have one side permanently facing away from earth.  Likewise, the ‘g’ planet likely is close enough to Gliese 581 so that star locks one side of the planet permanently toward it.  The rotation lock does two things to the planet: potentially limit life to the terminator (the dusk-or-dawn boundary) and, probably more importantly, rob the planet of a sufficient magnetic field that protects life from excessive washes of cosmic rays, solar radiation flares, and similar phenomena that can prevent life from forming.

What do we know about the Gliese 581 system so far?

First, the star is a “red dwarf main sequence star”, compared to our sun being a “yellow dwarf main sequence star”.  Main sequence stars are stars that power themselves by fusing hydrogen into helium in their cores.  Stars that do not do so are on what is called the “asymtoptic giant branch”, meaning they start fusing helium into carbon and oxygen, or those atoms into heavier ones like neon, sulfur, calcium, and so forth. In the process, they swell to many times their original size (as in ‘volume’), though their mass may be less than earlier in their lives. Such stars are old and near the end of their lives, and therefore are poor candidates for life.  For this reason, the search for extrasolar planets focuses on the younger main sequence stars.  Gliese 581 is definitely such a star.

Secondly, the star’s mass is about 1/3 that of our sun.  You’d think that smaller mass means a shorter lifespan, but the opposite is actually true.  Red dwarfs may have smaller mass than our sun, but their gravity and core temperatures are much less.  So this star goes through its fuel much more slowly than larger stars, even if it starts off with less actual fuel.  Think of a Hummer with 100 liters of gasoline/petrol versus an economy car with 30 liters; which one will have to stop to refuel sooner, assuming both are traveling the same speed? 

Therefore, Gliese 581 is destined to last much longer than our own sun. Indeed, it’s estimated at 7 to 11 billion years old and shows no signs of old age at all.  Our sun will be in that age range starting in 2 billion years, by which time it will go into the transitional “sub-giant” phase, where the star is still basically main sequence but is in the first stages of switching its primary fuel from hydrogen to the helium “ash”, along with the consequent swelling (which will spell the doom of earth-life as we know it).  In fact, decades of observing stars shows that the mass-lifetime relationship is so strong we can make a good mathematical estimate of how long a star will “live”.  1/Ms^2.5 =Ls, where

Ms = Solar masses (the sun’s mass =1, half the sun’s mass =0.5, 50% greater than the sun’s mass is 1.5, etc)

Ls = Lifetime of the sun in solar lifetimes (where Ls=1 is the total expected lifetime of the sun [ about 10 billion years], Ls=3 is 3 times the sun’s lifetime, etc.)

We know Gliese 581’s mass is 0.32 solar masses, so all we have to do is raise 0.32 to the 2.5th power, then get its reciprocal. This reciprocal will tell us how much life the star has compared to our own.

0.325^2.5 = 0.0579 ->  1/0.579 = 17.2633, meaning Gliese 581’s total life is about likely to be 17.2633 times as long as our sun’s.  With a total solar lifetime of 10 billion years, then that means Gliese 581 will live for over 170 billion years!  With our universe only 13.7 billion years old, Gl 581’s still a toddler under any reasonable scenario and therefore unlikely to have changed substantially in the past several billion years.  Lack of change over the last several billion years is the critical factor here, for earth-like ecologies need a reasonably stable temperature regime in order to thrive. Gl 581 apparently passes this with flying colors.
Even better, the star does not seem prone to energetic surface flaring like many stars its size (flares release intense X-rays that can damage life). Nor does it seem “variable” (vary in brightness abruptly, thus creating dangerous changes in temperature life cannot adapt to).  Therefore, we see nothing about the star actual behavior that can inhibit life’s formation on a planet in the “goldilocks zone”. 

The Planet

Unfortunately, its very longevity and coolness renders its habitable zone inside its “rotation lock zone” (discussed above).  This can truly reek havoc on an otherwise quite suitable planet by stopping its rotation.  On Earth, our magnetic fields block out the worst of the various cosmic radiations that would otherwise quickly kill us. This magnetic field exists because our planet is rotating rapidly enough to create electromagnetic fields in the core that extend well beyond our planet’s surface.  Therefore, if any life exists on Gliese 581g, it will have to be deep underground, deep in the oceans, or have an exceptionally competent means of self-repairing radiation damage (very likely the only way life can survive on its surface). 

 On the other hand, atmospheres can also block out some of the cosmic radiation hitting a planet, so ‘g’ just might have a thick enough atmosphere to block out the cosmic radiation that would otherwise hit it. This is quite possible, given that ‘g’ is only 3 times the mass of earth (and hence very likely a rocky-metallic world). Planets that massive definitely have enough internal heat to sustain volcanism.  Add an ocean to this world and it almost certainly will sustain plate tectonics (and hence a “carbon cycle”, which draws carbon dioxide from the atmosphere into rocks, with the said rocks being drawn back into the mantle at oceanic trenches, thereby preventing excessive C02 buildup that creates a Venus-like planet).

Whether there actually are any oceans at all on ‘g’ is anyone’s guess. Regardless, 'g' almost certainly has volcanoes, for the larger a world, the more internal heat it will have at any given moment in time. This translates into more internal heat the planet will inevitably release, namely in the form of volcanoes (and possibly plate tectonics if there is sufficient ocean cover on its surface). The volcanoes would release water into the atmosphere, the source of oceans.  Still, for all we know, ‘g’s crust might be so thick as to inhibit all but the most explosive vulcanisms, which works against plate tectonics because the thicker the crust, the more difficult it is to form trenches that subduct the plates into the planet.  This would favor a continued buildup of CO2 to potentially Venus-like levels; not exactly a biologically friendly environment to say the least.

There’s also the possibility that ‘g’ is a water world, covered completely or almost so by oceans.  This would almost certainly give ‘g’ plate tectonics, but all the continents would remain under water, even if they did turn out to be full of life.  This would be fascinating for biologists but disappointing for those hoping for land-based species of our type, and likely even civilization itself.  Civilization requires an ability to work materials into different forms.  That means changing the temperature of the materials, which certainly means fire.  Because fire cannot exist under water, that severely limits a species ability to move beyond the hunter-gatherer stage of development.


Gliese 581g will go down as one of astronomy’s great discoveries. Even if the planet proves unfavorable for life after all, it still suggests that there are likely other planets meeting the necessary preconditions for life, and more importantly that actual life-bearing worlds may not be as rare as once thought.   There’s still a long way to go before we discover evidence of alien civilizations (currently existing or vanished), but there is a real chance there are plenty of planetary Seringetis or Jurassic Parks awaiting us next door.  If a planet does show all relevant chemical signatures indicating life, a probe will no doubt eventually be launched toward that planet.  The discoveries and samplings of that alien biosphere will provide new insights of the potential variability life can take in this cosmos, and in doing so help us appreciate our own place in the universe.

No comments: