Home > Science > Rethinking the “Galactic Habitable Zone”

Rethinking the “Galactic Habitable Zone”

I’ve just read a very interesting paper reassessing the idea of a Galactic Habitable Zone.  This notion came along about a decade ago, and it was based on the idea that there were limiting factors on potential habitability based on a star system’s location in the galaxy.  The idea was that if you were too close to the galactic center, there’d be too many supernovae happening near your planet and they’d repeatedly sterilize it before complex life could form; whereas if you were too far from the galactic center, the metallicity of stars in that region would be too low for the formation of terrestrial planets large enough to support life.  Some scientists concluded that habitable worlds were only likely to be found in a narrow band a couple of thousand parsecs wide, with the Sun being just about in the middle of that range.

Now, I was always skeptical of this.  It seemed to me that all the science really showed was that habitable planets would be less likely or less common outside the “GHZ,” not that they’d be completely nonexistent.  I figured it would make more sense to call it a Galactic Temperate Zone rather than a Habitable Zone.

So I’m pleased with the findings of this new paper, “A Model of Habitability Within the Milky Way Galaxy” by Michael G. Gowanlock, David R. Patton, Sabine M. McConnell (arXiv:1107.1286v1).  The abstract reads:

We present a model of the Galactic Habitable Zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favour the development of complex life. The Milky Way galaxy is modelled using a computational approach by populating stars and their planetary systems on an individual basis using Monte-Carlo methods. We begin with well-established properties of the disk of the Milky Way, such as the stellar number density distribution, the initial mass function, the star formation history, and the metallicity gradient as a function of radial position and time. We vary some of these properties, creating four models to test the sensitivity of our assumptions. To assess habitability on the Galactic scale, we model supernova rates, planet formation, and the time required for complex life to evolve. Our study improves on other literature on the GHZ by populating stars on an individual basis and by modelling SNII and SNIa sterilizations by selecting their progenitors from within this preexisting stellar population. Furthermore, we consider habitability on tidally locked and non-tidally locked planets separately, and study habitability as a function of height above and below the Galactic midplane. In the model that most accurately reproduces the properties of the Galaxy, the results indicate that an individual SNIa is ~5.6 \times more lethal than an individual SNII on average. In addition, we predict that ~1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy. Of those stars with a habitable planet, ~75% of planets are predicted to be in a tidally locked configuration with their host star. The majority of these planets that may support complex life are found towards the inner Galaxy, distributed within, and significantly above and below, the Galactic midplane.

Basically, it’s saying two things: One, that the sterilizing effects of supernovae in the inner galaxy aren’t as pervasive as formerly claimed (this study actually modelled individual stars rather than going for a statistical aggregate), and two, that the sheer number of stars in the inner galaxy is so great in proportion to the outer galaxy that it more than cancels out the supernova effect.  That is, it’s true that a much higher percentage of habitable planets get sterilized the closer you get to the center of the galactic disk (and it only covers the disk, not the central bulge), but the number of stars is so much greater that said percentage still comes out to a much larger amount.  For instance, at a galactic radius of 2.5 thousand parsecs (kpc) you might have 10% of 7.5 million candidates = 750,000 habitable worlds, while at 12 kpc you might have more like 70% of 1 million candidates = 700,000 habitable worlds.   Although the numbers actually add up so that maybe half the life-bearing worlds in the model are between 2.5-4 kpc, partly because star formation began earlier in the inner galaxy so there’s been time for more habitable worlds to develop life (assuming a uniform rate of life emergence comparable to Earth’s, which is a huge and rather arbitrary assumption, but we don’t have any other examples yet).

Anyway, the exact numbers aren’t very meaningful due to all the assumptions and conjectures (and the paper shows the differing results of four separate galactic models it used), but the overall trend is what’s significant.  It suggests that inhabited worlds could be found just about anywhere in the galactic disk, much more broadly than past studies have suggested.  So the “Galactic Habitable Zone” is pretty much the whole disk.

So to get speculative here, what might it be like for intelligent life in the inner disk?  Well, neighboring intelligences would probably be closer together than out here.  They’d occupy a smaller percentage of the stars, but the stars would be packed more tightly.  And that might make it easier for them to expand and colonize, to hopscotch across the star systems and reach one another.  (After all, this paper only considers worlds where life evolved independently.  Once you throw colonization into the mix, all bets are off.)  So it might be easier to form a robust interstellar community in the inner galaxy.  Particularly since there’d be a much higher number of uninhabited worlds for every inhabited world — hence more resources and territory to go around, reducing the pressure for conflict.

Conversely, maybe the earliest species to develop would’ve had an easier time colonizing the more densely packed inner galaxy, settling other habitable worlds before they could develop their own indigenous sentience, or finding indigenous intelligences so far behind them that they could be easily dominated and crowded out.  So maybe the inner galaxy would be characterized by regions that were dominated by the descendants of a single species each.  No telling what might happen when two such regions impinged on one another.

In either case, though, the paper estimates that life in the inner disk has maybe a 2 billion year headstart on us, on the average.  So why haven’t they made their way out here?  That’s a trickier question to speculate about.  I think I’ll let it go for now.

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