PDA

View Full Version : Is there a "Habitable Zone" in the Galaxy?



ExJWWithQuestions
2010-Jun-20, 02:46 PM
Greetings! A little bit about myself. I'm a former Jehovah's Witness currently working on a blog project to debunk the religion's latest creationist brochure "Was Life Created?" I'm currently working on the first section, which cites a lot of astronomy "facts" to prove that Earth is in the absolute perfect position to support life. I've heard the argument that Earth's position in the solar system proves an intelligent hand in its placement, any closer or farther away and we couldn't be here, etc. etc. I know the counter-argument to that, but this brochure puts forth an argument I've never heard before, namely that the Solar System's placement in the Galaxy is also proof of "intelligent placement." Here's the text of the claim:


To begin with, our "city, " or solar sys-
tem, is located in the ideal region of the
Milky Way galaxy-not too close to the
center and not too far from it. This "hab-
itable zone," as scientists call it. contains
just the right concentrations of the chem-
ical elements needed to support life. Far-
ther out, those elements are too scarce;
rarther in, the neighborhood is too dan-
gerous because of the greater abundance
of potentially lethal radiation and other
factors. "We live in prime real estate,"
says Scientific American magazine.

The source for the SA issue is
1. Scientific American, Special Issue 2008 enti--
tied " Majestic Universe, p. 11.

I'm going to try the library to look this up. Somehow I doubt the quote is referring to the position of the solar system in the galaxy.

Anyway, I'm sorry for bringing the whole evolution/creation debate into an astronomy forum. I know it has nothing to do with biology, but since this particular claim is attacking it from the astronomy angle first, I'd just like to get your thoughts on it.

PetersCreek
2010-Jun-20, 05:04 PM
Welcome to the BAUT forums, ExJWWithQuestions. While this topic fits squarely within our rules about discussions of religion, I want to remind you...and respondents...to keep it within those narrow bounds. If you haven't already done so, please take some time to read our rules, posted in my signature line below. Enjoy!

Nereid
2010-Jun-20, 06:15 PM
A galactic habitability zone is certainly something you'll find discussed in the peer-reviewed literature.

However, that such a zone truly exists is by no means fully accepted; see this article (http://www.universetoday.com/2010/02/08/if-the-earth-is-rare-we-may-not-hear-from-et/), for example, from Universe Today. If you're interested, I'd be happy to show you how you can search for most papers like the one referenced in the Universe Today article.

grant hutchison
2010-Jun-20, 06:33 PM
The original proposal came from Gonzalez, Brownlee & Ward, Icarus (2001) 52: 185-200. There's a preprint version here (http://cdsweb.cern.ch/record/490517/files/0103165.pdf) (150KB pdf). The arxiv server seems to be down as I write, but this link (arxiv.org/abs/astro-ph/0103165) should take you to the arxiv version.

Grant Hutchison

ExJWWithQuestions
2010-Jun-20, 07:30 PM
Thanks for your replies. I ended up finding a few answers. Here's my rebuttal:


The idea of a GHZ (Galactic Habitable Zone) is the subject of much debate. It was first presented in Science Magazine by Charles H. Lineweaver, Yeshe Fenner, and Brad K. Gibson. The abstract reads as follows:
We modeled the evolution of the Milky Way Galaxy 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. - Science 2 January 2004:Vol. 303. no. 5654, pp. 59 - 62
In layman's terms, the proposed "habitable zone" is 25,000 light years from the galactic center and is an expanding region of approximately 6,000 light years across. This works out to be a little over 8% of the galaxy falling in the GHZ. Thus, it would seem there was a 12.5% chance of any star in the galaxy being in this location. These odds are far from "astronomically" improbable, and considering the sheer physical size of the galaxy, the GHZ would contain at least millions, of not billions, of stars. Millions, perhaps billions of chances to form earth-like planets.

However, the idea of a GHZ is not without professional criticism. Nikos Prantzos, of the Institut d'Astrophysique de Paris, says:
The concept of Galactic Habitable Zone (GHZ) was introduced a few years ago as an extension of the much older concept of Circumstellar Habitable Zone. However, the physical processes underlying the former concept are hard to identify and even harder to quantify. That difficulty does not allow us, at present, to draw any significant conclusions about the extent of the GHZ: it may well be that the entire Milky Way disk is suitable for complex life. - source (pdf)
Other criticisms of the GHZ hypothesis abound as well. Ian Stewart and Jack Cohen in their book Evolving the Alien point out two problems: the first is that the hypothesis assumes alien life has the same requirements as terrestrial life; the second is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone". For instance, Jupiter's moon Europa is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophiles (such as the tardigrades) on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan believed that life was also possible on the gas giants, such as Jupiter itself; Iain M. Banks's novel The Algebraist is based on the same idea. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditions when the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".

Others point out that the primordial atmosphere of Earth was not habitable as it was mainly carbon dioxide (much like Venus sans heat), but was transformed by early primitive plant life into a breathable atmosphere, so that any definition of a habitable zone for people has to include the presence of plant life and the possibility of photosynthesis. - source (pdf)

The idea of the GHZ is far from settled, and for the author of this article to present it as established fact shows his ignorance of the facts, if not his blatant dishonesty.

Obviously I'll have to correct my statement that it was "first proposed" by Lineweaver et. al. but I welcome any and all input you might have. I'm a complete layman at this so I'm hoping I didn't bite off more than I could chew.

Also, if you're interested, here's my blog with the rest of the post:
http://waslifecreatedrebuttal.blogspot.com/

Again, any and all criticism/correction is welcomed.

Van Rijn
2010-Jun-20, 11:43 PM
In layman's terms, the proposed "habitable zone" is 25,000 light years from the galactic center and is an expanding region of approximately 6,000 light years across. This works out to be a little over 8% of the galaxy falling in the GHZ. Thus, it would seem there was a 12.5% chance of any star in the galaxy being in this location. These odds are far from "astronomically" improbable, and considering the sheer physical size of the galaxy, the GHZ would contain at least millions, of not billions, of stars. Millions, perhaps billions of chances to form earth-like planets.

And this is just one galaxy among billions that would be expected to have similar regions, even given the GHZ idea.

WayneFrancis
2010-Jun-21, 04:32 AM
Thanks for your replies. I ended up finding a few answers. Here's my rebuttal:

...

Obviously I'll have to correct my statement that it was "first proposed" by Lineweaver et. al. but I welcome any and all input you might have. I'm a complete layman at this so I'm hoping I didn't bite off more than I could chew.

Also, if you're interested, here's my blog with the rest of the post:
http://waslifecreatedrebuttal.blogspot.com/

Again, any and all criticism/correction is welcomed.

Sounds good. Always good to see people eager to challenge the propaganda of the places like the Discovery Institute and others.

Jens
2010-Jun-21, 05:36 AM
Just generally speaking, it seems to me that a better way to rebut those arguments is to point out that it may be that life tried to get started in many different places, but it is only where life was viable that it took root. Because regardless of the argument about galactic inhabitable zones, it is crystal clear that there are uninhabitable zones. For example, life might be possible in a small number of places in the solar system, but it is fairly obvious that it is not viable in 99.9999% of the area, i.e. the places where there are no planets at all, which makes up the vast majority of the solar system. I don't think there's anything in that which is so special.

tnjrp
2010-Jun-21, 06:35 AM
I've heard the argument that Earth's position in the solar system proves an intelligent hand in its placement, any closer or farther away and we couldn't be here, etc. etc. I know the counter-argument to that, but this brochure puts forth an argument I've never heard before, namely that the Solar System's placement in the Galaxy is also proof of "intelligent placement."This is, of course, pure ****e: Earth's location in the galaxy does no such as thing as prove anything. Based on what we know, Earth is merely in a sufficiently good position to produce and maintain Earthlike life. End of.