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Darrrius
2007-Mar-02, 04:21 PM
I have a question regarding HZ's. I understand the concept of HZ's within a solar system - being the region around the star that would allow liquid water to exist on a planets surface (although this type of HZ is becoming less meaningful as we discover more and more extremophiles capable of inhabiting hostile environments)

But I do not understand the concept of Galactic HZ's. How is an HZ defined within a galaxy? Where, for example, is the HZ for the Milky way? How large an area is this?

Are there some galaxy's that are considered un-inhabitable - if so why, and how could we know...?

antoniseb
2007-Mar-02, 04:43 PM
Usually, when discussing galactic habitable zones there are a number of factors considered:

- Metalicity of the stars there (and presumable from this planet-size and composition) - The core and outer halo are presumed less habitable for this reason.
- Radiation - An area with lots of nearby supernovae or an AGN is not likely to be very habitable.
- Age - It may take a billion years or more for the hot isotopes in a Solar System to die down enough that processes like DNA replication can be reliable enough for life.

I'm not defending these positions, as there is some grey area there, but that is what the term refers to.

Darrrius
2007-Mar-02, 04:46 PM
Excellent - Thanks for the answer!

RalofTyr
2007-Mar-02, 10:31 PM
What about the Andromeda galaxy? There are speculations that is doesn't have a habitable zone due to core radiation.

Kaptain K
2007-Mar-03, 06:08 AM
What about the Andromeda galaxy? There are speculations that is doesn't have a habitable zone due to core radiation.
Citation? I have not heard this before.

RalofTyr
2007-Mar-03, 06:30 AM
Something I probably saw on one of the discovery channels. I'll have to hunt for links.

Ronald Brak
2007-Mar-03, 07:00 AM
Some assumptions about radiation and habitable zones seem a little excessive to me. It would take a lot of radiation to make a planet uninhabitable as there are plenty of organisms on earth that have demonstrated an ability to adapt to high levels of it. However, if a nearby supernova melted the surface of your planet, that would be a bit of a downer.

Romanus
2007-Mar-03, 02:18 PM
Count me in as another doubter of the validity of the GHZ; IMO, there's far too little we know about exoplanets and HZs around stars--period--for us to extrapolate to the whole galaxy.

astromark
2007-Mar-04, 10:12 AM
While looking at this subject I noted that recently in Sky and Tel., it was discussed and revealed that even Alpha Cent., A and B might have planets in the zone. We can not yet rule them out. Your post asks relating to galactic zones. You can understand that if stars are grouped closer together the level of radiation could be a difficulty for life forms to have made the long term evolutionary steps required. Close in to the galactic core orbital stability might be compromised by fast moving larger massed objects. The denser areas of the spiral arms might have a similar problem. I do not see any reason why planets in the zone could not be formed right out on the edge of the galaxy's most distant stars.
Remembering that improbability does not disqualify chance. Just makes it less likely.

Kaptain K
2007-Mar-04, 10:52 AM
As far as habitability goes, there are several factors that make life in the core unlikely.
1) Low metallicity. Stars in the core are (mostly) first generation. Not much there to make rocky planets out of.
2) High density. With so many stars in such a (relatively) small volume, long term orbital stability might be questionable.
3) The "ogre" in the middle. The SMBH in the core occasionally eats stars for lunch. The resulting blast of radiation would definitely be unhealthy for any life in the core.