Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Galactic Habitable Zone

  1. Jan 16, 2004 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    A recent paper by three Australian astronomers

    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."
    Last edited: Jan 16, 2004
  2. jcsd
  3. Jan 16, 2004 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Lineweaver and friends Figure 3

    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
    Last edited: Jan 16, 2004
  4. Jan 20, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook