The Galactic Habitable Zone

[marcus]

A recent paper by three Australian astronomers
http://arxiv.org./abs/astro-ph/0401024

It has a bearing on Fermi's Question "Where are they"

-----quote from the abstract-------

"We modeled the evolution of the Milky Way to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun."

[marcus]

Figure 3 on page 5:
Plotting distance from Galactic center on the X-axis and
age of star on Y-axis

they found that the habitable region was bounded by four other regions
which were inhospitable for various reasons---
too metal-poor
too many supernovas
too metal-rich
too little time

The article was published in the journal Science
2 January 2004 issue

[Nereid]

Including your much-admired Lineweaver I see.

Nice read, I'd encourage PF guests and members to take a peek. Oh to be able to live for another 200 years or so! How quaint it will all seem then [:((]

Just a few comments on why the 'GHZ' may be bigger:
- if the bulk of the Earth's biomass lies in the rocks (the volume occupied vastly overcomes the low cubic density of life), then the SNe constraint may be much, much weakened
- ditto, re life around black smokers (they aren't affected in the least by nearby SNe, except if they're powerful enough to boil all the oceans) - and that's 'complex life', cf life among the rocks
- ditto, if there's complex life in the Europan ocean (and Ganymede, and Callisto?)
- if life on Europa (etc), then metallicity constraints are considerably weakened too (Gallilean satellites may form in a very wide range of metallicities, and it'll take a good century or so before we know the likelihood of their formation, by metallicity)
- lonely planets (wandering the Milky Way without a Sun): we have no idea how common they are (the lensing data to date constrains their space density only very weakly; solar system formation models aren't good enough yet to model most stars in real environments, e.g. even moderately dense star clusters), nor what proportion have Europas
- meta-Europas, e.g. Titan
- the role of canibalisation, mergers and acquisitions (hostile or otherwise) - there are huge clouds of gas coming and going, dwarf galaxies and globulars being shredded, and so on - Lineweaver et al didn't consider the effects of these dramas

... and I'm sure others will add comments on why the 'GHZ' may be smaller ...

In a way, Lineweaver et al's GHZ is like that in a star cluster - too near the core and planets won't form (the young OBs boil off the proto-planetary disks; close encounters scramble the tender young'uns); too far and there's not enough material to form Sun-sized systems; better to be in the suburbs. Also like galaxy clusters - too near the core and collisions create too many starbursts (deep-fried life); too far and there's no oxygen.

So sad that I'll not be here to see the data come in and the debate unfold [must resist urge to say bad things about Bush and Hu]