Re: ET: Scientific American article on GHZ

Date: Mon Oct 01 2001 - 10:49:29 MDT

Daniel Ust writes:
> October 2001's _Scientific American_ has an article that consolidates on a
> popular level some of the explanations for the Fermi Paradox. See "Refuges
> for Life in a Hostile Universe" by Guillermo Gonzalez, Donald Brownlee and
> Peter D. Ward. (It's not online, but the issue contents are at

A preprint of a technical presentation on the same topic is at We discussed this some time
ago I think but I have not read it recently.

> There are few points they make, including that only certain conditions are
> right for forming earthlike planets, particle and electromagnetic radiation
> is too high in many of the places where they could be formed, and other
> factors in the galactic environment might make complex life less likely.
> Only a narrow region of the galaxy has these conditions. They call this the
> Galactic Habitable Zone (GHZ).

The article actually suggested that some aspects of the GHZ concept make the
Fermi paradox worse:

"One of the arguments proposed to avoid that conclusion [i.e. that
extraterrestrial life should have been here already] is that ETs may
have no motivation to leave their home world and scatter signs of their
presence through space. But if our ideas about the GHZ are correct,
we live within an especially comfortable region of the Milky Way.
Any civilization seeking a new world would, no doubt, place our solar
system on their home-shopping list. The GHZ theory also weakens the
argument that the galaxy is so big that interstellar explorers or
colonizers have passed us by. The GHZ may be large, but it is just a
part of the entire galaxy, and any galactic travelers would tend to roam
around the annulus rather than haphazardly through the galaxy."

The explanatory power claimed for the GHZ with respect to the Paradox
is that it reduces the time frame in which life could have evolved.
Until the last few billion years the galaxy was supposedly much more
dangerous, with supernovas and an active nucleus. Plus our Sun is
about 40 percent richer in heavy elements than most other stars formed
in the same place and time, making it more likely to develop a variety
of planets. This suggests that the Solar System may be near-optimal
for the first development of complex life in the galaxy.


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