The idea of a galactic habitable zone (GHZ) has a certain inevitability. After all, we talk about habitable zones around stars, so why not galaxies? A stellar habitable zone is usually considered to refer to those areas around the star where liquid water can exist on a planetary surface. Those who believe that confining habitable zones to regions like these carries an implicit bias — limiting them to life much like our own — miss the point. The habitable zone concept simply tells us where it makes the most sense to search for the kind of life we can most readily recognize, and as such, it hardly rules out other, more exotic forms of life.
But while liquid water takes precedence in a stellar habitable zone, a galactic HZ is still being defined. Charles Lineweaver and team have examined it, among other things, in terms of stellar metallicity (the elements heavier than hydrogen and helium found in the body of a star), concluding that there is a ring several kiloparsecs wide surrounding galactic center in which life would be most likely to be found. But the ring evolves, spreads outwards with time, leaving us to recognize that galactic habitable zones can vary over the eons.
Image: A computer simulation showing the development and evolution of the disk of a galaxy such as the Milky Way. Credit: Rok Roškar/University of Washington.
That evolution now gets a much closer look from a team at the University of Washington, which ran 100,000 hours of computer simulations to study how galactic disks evolve, beginning with conditions nine billion years ago. The resultant data show that the average star can migrate through the galaxy, thus skewing the results of any habitable zone based partly on the abundance of certain chemical elements necessary for life. UW graduate student Rok Roškar puts the case this way:
“Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy’s disk but not others. If stars migrate, then that zone can’t be a stationary place.”
Stellar migration comes in handy because our understanding of the relationship between age and metallicity across star populations is changing. The paper on this work explains that relationship in terms of galactic chemical evolution:
Stars of the same age in the same general region of the galaxy are… expected to have similar metallicities. Indeed, early determinations of the AMR [age-metallicity relationship] confirmed that the mean trend of stars in the solar neighborhood is toward lower metallicity with increasing age. Models, which assume that stars remain where they are born and return their nucleosynthetic yields to their local ISM [interstellar medium], typically successfully reproduce this trend.
All of which gets interesting when you consider that in the part of the galaxy in which the Sun resides, field stars and open clusters show a wide range of metallicities. The ‘scatter’ in these data implies that stars near the Sun may well have come from completely different parts of the galaxy. The findings from the simulated galaxy are clear:
Roughly 50% of all “solar neighborhood” stars have come from elsewhere, primarily from the disk interior. Interestingly, some metal poor stars have been scattered into the solar neighborhood from the outer part of the disk. Such migration has recently been inferred from observational data… Metal-rich stars, like our Sun, could have originated almost anywhere in the Galaxy.
We just looked at the question of the Sun’s siblings, and whether or not systems forming in the same early cluster as the Sun might have exchanged life-bearing materials. Tracing stellar movements back in time to reveal which stars comprise those siblings may be a tricky matter. Stars encountering a galactic spiral arm seem to retain the circularity of their orbits after such an encounter, but their orbits may change considerably in size, meaning the Sun could have been in a much different position in relation to galactic center when it formed than it is today.
Bear in mind that galactic habitable zones also factor in supernovae explosions, which could cause exterminations on nearby worlds. We have much to learn in the study of supernovae, but the UW simulations suggest that the GHZ may be a more flexible area than we have previously considered. The paper is Roškar et al., “Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks,” accepted for publication in Astrophysical Journal Letters (abstract). The earlier Charles Lineweaver paper is “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science Vol. 303, No. 5654 (2 January 2004), pp. 59-62, with abstract here.
Ok, first I’ll admit I haven’t read the paper. I still see that there is some hope for a GHZ.
Stars that migrate outward from closer to the galaxy’s center must be longer-lived stars. A blue giant, as a prime example, would go nova long before it left the core region. This might increase incidence of sterilizing events there because of the increased stellar density there. That is, novas would tend to be nearby. Conversely, novas out here may be more beneficial since they are closer to fewer other stars, so damage potential is lower, yet they disperse their metals into the environment.
Stars from out this way that migrate inward would be exposed to a more hostile environment, though only for as long as they are in that region. They will then continue outward once more. I haven’t looked at their model, but I would expect some fraction of stars out here, whether a large or small percentage, do not migrate inward during their lifetimes at all. We could be one of those.
(And, Paul, welcome back.)
Hi Paul
Interesting result. Does it mean that the metallicity evolution of the Galaxy is quite different to the simple linear models of the old GHZ? If Sol formed further in, then does that mean the outer reaches are yet to become planetogenic and the whole timescale Lineweaver derived needs a rethink?
Perhaps we really are among the first?
100,000 hours of computer simulation?! Must have been a boring 11 years, sitting around waiting for the output. I hope this was a massively-parallel job!
As for GHZ being defined on the basis of star metallicity and location… I think that (eventually) our ancestors will find that ‘life’ will be much more diverse (physically and chemically) and ubiquitous than we can currently imagine.
Basically this seems to leave the GHZ concept as not particularly useful for finding habitable/inhabited planets, as these get scattered through the disc. It also might have interesting implications for various scenarios where extinction periodicities (if indeed these periodicities are valid) are related to the Sun’s trajectory through the galaxy.
gaak! I meant ‘descendants’ of course!
Hi Folks;
A really cool discussion!
When considering Galactic Habitable Zones, obviously suitable supplies of reactive atomic chemicals or molecules would seem necessary in order to provide the thermodynamic degrees of freedom for organisms to develop.
We can look at other planetary bodies within our solar system and note the seemingly rich soil chemistry of Mars, the perhaps briny sub-surface water oceans on the moon Europa, the lakes of liquid hydrocarbons on the moon Titan, and the Moon Enceladus for which evidence suggest the existence of salt water perhaps located beneath its frozen surface.
There are perhaps many possible types of life that do not rely on hydrocarbon based DNA. Some of these life forms might even be technological intellegent creations of advanced naturally developed and evolved hydrocarbon based life forms.
Thanks;
Jim