The Galactic Habitable Zone

In the past, I've talked about the habitable zone around stars where water can exist as a liquid. This week, I'm going to talk about the galactic habitable zone which is the area in a galaxy where conditions are sufficiently hospitable for life to develop on planets which themselves are in an appropriate stellar habitable zone. Unlike the stellar habitable zone (also called circumstellar habitable zone), some aspects of the galactic habitable zone apply more broadly to theoretical life forms which might be completely different to the type of life we've encountered on Earth.

This post has been inspired by an article I came across on arXiv.org: A Model of Habitability Within the Milky Way Galaxy by Gowanlock, Patton & McConnell, which I will henceforth refer to as GPM. They constructed a few models of our galaxy and ran simulations to see which regions could be habitable.

What aspects should we care about?

The parent star
The sun of a potentially habitable planet orbits needs to be small enough that it survives for long enough for life to develop. Remember that more massive stars have shorter lifetimes and die explosive, sterilising deaths. The general consensus is that large, blue stars don't last long enough for complex life to develop. Even if a planet survives the actual supernova, its atmosphere would have been obliterated in the explosion and the corpse of the star—either a neutron star or a black hole, depending—wouldn't be very hospitable either.

Nearby stars
By a similar token, we don't want there to be a large short-lived star too close by either. A nearby star going supernova would also be quite bad for potential life harbouring planets. You don't have to be right next to a supernova for the gamma rays (and X-rays and cosmic rays) to do some serious molecule-killing sterilisation. However, if nearby supernovae happen early in the planet's history, there shouldn't be a problem with life developing later on (after the ozone layer heals).

GPM come to the conclusion that, depending on the type of supernova, it could sterilise planets within a range of 2-27 parsecs (6.5-88 light-years). The range is so broad because supernovae come in a variety if flavours from the dying stars I mentioned earlier, which can be of all different masses, to binary stars where the larger one throws off its outer layers first, turns into a white dwarf, cannibalises its partner and then explodes from over-eating. The latter are more bright and will on-average sterilise planets within 18 parsecs (59 light-years) whereas an average star-dying supernova will sterilise within 8 parsecs (26 light years). For a bit of perspective, the our galaxy is about 30 000 parsecs or 100 000 light-years in diameter.

As an aside, I should also mention that there is a theory that some past mass extinction events on Earth were caused by supernovae. Googling "extinction supernova" brought up a lot of hits for different extinctions. Here is one of the top hits, chosen a bit arbitrarily.

Finally, GPM conclude that there is no where outside of the central 2500 parsecs of the galaxy (which they didn't consider in detail) where there are always going to be too many supernovae for life to develop, where they've defined the time taken for complex life to develop as four billion years. That's four billion years either from the time the planet forms or from the time it gets sterilised by a nearby supernova.

Metallicity
This is sort of a less obvious one. Most of the universe is made of hydrogen and helium and a little bit of other rubbish. Although chemists define metals in a fairly specific way, astrophysicists tend to lump anything heavier than helium (that is, elements whose atoms have more than two protons in their nuclei) into the “metal” category.

Rocky planets are made out of, well, rocks rather than hydrogen or helium and if there are no heavier elements around, we'll only get gas giants forming. Heavier elements are produced when stars die, either in a supernova or in the more mundane red giant phase that our sun will eventually go through. Therefore, rocky planets can only form in areas where there have been enough stellar deaths to seed the interstellar medium with heavier elements. How big stars are depends mainly on how much gas there was around when they formed. Consequently, bigger stars are able to form closer to the centre of the galaxy (in the most dense environment) earlier, die explosively and leave metal-enriched dust behind. Then, when later generations of stars form, there is more chance of rocky planets forming around them.

The most metal-poor areas of the galaxy are the outer edge and the halo which is the spherical and sparsely populated area surrounding the disk of our galaxy. The spiral arms, where we are (if you're wondering, we're about two-thirds of the way out from the centre, close to the middle of the stellar disk). The other thing to note about metallicity is that it increases over the lifetime of the galaxy.

GPM looked at stars with lifetimes longer than four billion years whose planets escaped being irradiated by a nearby supernova for at least that period of time as well. Most of the habitable planets exist close to the centre of the galaxy, with half of them between 2500 parsecs and about 4000 parsecs, but life was still possible (though much sparser) up to the edge of the galaxy. (Remember, Earth is around 8500 parsecs from the centre.)

Other planets
I'm only going to cover this one briefly. The presence of other planets in any given system with a habitable planet could stuff around with our habitable planet. Our searches for extrasolar planets have found a lot of “hot Jupiters”—gas giants very close to the star—and our current theories of planetary formation suggest that these got there by migrating in after forming much further out. Such a migration would almost certainly spell terminal trouble for the previously habitable planet.

So where is this galactic habitable zone of which you speak?

Previous studies had defined the galactic habitable zone as an annulus (or flat doughnut, for those of you more culinarily and less mathematically inclined), with the inner rim defined by the radius at which there are too many hazards to life (for example from supernovae in the densely starred inner regions), and the outer rim determined by metallicity or lack thereof. In general, this region has in the past been calculated to be centred on our location in the galaxy, extending inwards and outwards by only 1000 or so parsecs.

On the other hand, GPM found that the whole galaxy (minus the inner region which they ignored but will study in a later paper) was habitable but the areas most amenable to life were close to the centre and a bit above and below the main concentration of stars in disk. The former for reasons of metallicity and the latter because those areas had the same metallicity as the main disk but there were fewer nearby stars to go boom and sterilise them. Our Earth, for comparison, is fairly close to the centre of the disk.

There is a bit more I'd like to say about galactic habitability, but I think I'll leave it for a future blog post. This post only covers habitability without our own galaxy, but stay tuned for more!