A _Rare Earth_ review (was: Scientific American article on GHZ)

From: Amara Graps (amara@amara.com)
Date: Sun Oct 28 2001 - 05:11:03 MST

From: Don Klemencic (klemencc@sgi.net), Tue Oct 09 2001

>For anybody who missed the connection, Donald Brownlee and Peter D. Ward are
>the authors of the book, Rare Earth, previously discussed on this list. The
>book of course goes into these issues in much greater detail. The hardback
>edition has been remaindered and is selling at Amazon for $8.25. There
>doesn't seem to be a paperback edition, so it may disappear in a while.

You might be interested to know that some (or many) astronomers are
treating the Rare Earth book with a fair bit of criticism.

The following, attached review I saved from sci.astro 1 1/2 years ago.
I haven't finished reading the book yet, but I agree (so far) with
Joseph Lazio's comments below.

This part I can add to:

>Moreover, it's also worth noting
>that the Sun is currently inside a cavity of hot gas known as the
>Local Bubble. This cavity may have been formed by one or more
>supernovae within the (astronomically) recent past. Obviously, life
>on Earth survived.

There's been a lot of work on characterizing the local region around
the Sun. One should look up the work by Priscilla Frisch at U of Chicago.
Most likely several supernova-triggered star-forming epochs occurred
within 1500 light years of the Sun. The Sun has been passing through a
hot, very low-density region, the Local Bubble, for several million years,
and it is now embedded in a shell of warm, partly ionized material flowing
from the Scorpius-Centaurus star-forming region. Within this shell,
in a region of about 10 light years across, we have our "Local Interstellar
Cloud (LIC). Part of the LIC cuts across the Sun's path, where the we
know some characteristics:

atomic neutral hydrogen density: 0.22 cm^(-3)
temperature: 6900 K
relative Sun-LIC velocity of aobut 26 km-sec^(-1)

In our 10 l.y. LIC environment evidence of both low and high velocity
shocks (20 to 200 km-sec^(-1) are found. So Ward and Brownlee should
be aware that our own solar system formed within a very complex
enviroment, and here, intelligent life exists.. strange as it
seems sometimes.

One of our goals in the Heidelberg Dust group is to learn more about
the interstellar dust that we are detecting flowing through the
solar system (following up on the Ulysses discovery of interstellar
dust 9 years ago). We have a "Dust Telescope" project that is in a
study by ESA right now, that we are hoping to be funded.


Frisch P.C. 1998 Space Sci. Rev. 86, 107
Frisch P.C. 1999 ApJ, 525, 492


From: jlazio@patriot.net (T. Joseph W. Lazio)
Newsgroups: sci.astro.seti,sci.astro
Subject: Book Review: Rare Earth (correct version)
Date: 22 May 2000 19:42:07 -0400

                              Rare Earth
                   Peter D. Ward & Donald Brownlee
                           Copernicus Press
                            Copyright 2000
                          ISBN 0-387-98701-0

Written by a pair of professors at the University of Washington, _Rare
Earth_ is a polemic for the view that complex life, both animals and
higher planets, is rare in the Milky Way Galaxy and perhaps even in
the Universe. Thus, the authors contend that there could be many,
perhaps millions, of planets scattered throughout the Galaxy on which
single-celled microorganisms thrive. On only a handful of planets,
perhaps only one, would the air be filled with flying creatures and
the ground covered with creepy or crawly things.

Cast in terms of the Drake equation, the authors believe that f_l, the
probability that life will originate on a habitable planet, could
approach unity (1). However, they argue that f_h, the fraction of
planets that are habitable, and f_i, the probability that intelligent
life will develop on a planet on which life has emerged, are
individually or both probably quite close to 0. Thus, the total
number of technological civilizations in the Galaxy would be small,
perhaps only one. (However, they also mis-characterize the Drake
equation as assuming that "once life originates on a planet, it
evolves toward ever higher complexity." As the Drake equation
contains a factor f_i there is clearly no assumption in the Drake
equation itself that life "evolves toward ever higher complexity.")

The authors bring to bear a number of different aspects of the Earth's
astrophysical, geological, and biological history, some of them fairly
recently appreciated, to support their argument. Among these aspects
* A Galactic habitable zone, the possibility that habitable planets
can only exist in a restricted region of the Galaxy,
* A temporal habitable zone, based on the fact that a star's
luminosity gradually increases over time meaning that in 1 billion
years or so the Sun will boil away the Earth's oceans,
* The presence of a comet-slaying giant planet to reduce considerably
the number of comets that strike a habitable planet, like Jupiter in
our solar system,
* The "snowball" Earth hypothesis, that on at least two occasions the
Earth's oceans may have largely or entirely frozen over, and
* The time between the first appearance of life on this planet and the
Cambrian explosion when the diversity of life exploded and animal life
first appeared.

It has been this reviewer's experience that reading well-written
apologies of a position with which one disagrees can often make one
sharpen one's arguments. _Rare Earth_ cannot be put in this
category. I found it not only to be not convincing, but not
particularly thought provoking and in some places sloppy almost to the
point of being wrong.

Consider just a few examples. The authors suggest a Galactic
habitable zone. They contend that too close to the Galaxy's center a
planet would be fried by the intense radiation at the center, too far
from the center and Earth-like planets might never form because the
clouds of dust and gas at the edge of the Galaxy do not contain enough
"metals" to form terrestrial planets. In principle they are correct.
(I would add that too close to the Galaxy's center terrestrial planets
might never form or would be ejected quickly because of the frequent
close passages of other stars.) In practice, what is the size of this
Galactic habitable zone? They argue for an apparently quite small
zone, though they provide no firm quantitative estimate. I would
certainly agree that the inner 300 light years, maybe the inner 1000
light years, of the Galaxy are probably not too hospitable. They list
a number of possible hazards that life toward the Galactic center
might face---ionizing radiation (but how strong would the radiation
have to be before the atmosphere of a planet would not protect life on
its surface?), strongly magnetized neutron stars (which are how
frequent?), and supernovae. Their primary concern seems to be with
supernovae, but they also seem to neglect one favorable aspect of the
inner regions of the Galaxy. The interstellar gas density in the
inner regions of the Galaxy is higher than near the Sun (and many
supernovae occur in or near quite dense molecular clouds). This
higher gas density should also decrease the effects of nearby
supernovae on planets with life. Moreover, it's also worth noting
that the Sun is currently inside a cavity of hot gas known as the
Local Bubble. This cavity may have been formed by one or more
supernovae within the (astronomically) recent past. Obviously, life
on Earth survived. Similarly, they provide no outer radius for their
Galactic habitable zone, but assert that there must be one based on
the general decrease of "metals" as one goes outward from the Galactic
center. It is true that there is such a general decrease, but there
is also considerable scatter about the general trend. Thus, the outer
radius is probably far more "fuzzy" than they describe it.

On the topic of the Galaxy's habitable zone, the authors seem to imply
that the Sun has little or no interaction with the Galaxy's spiral
arms and that the inter-arm regions of a spiral galaxy have a lower
stellar density than inside the spiral arms. Neither is correct. It
is true that the Sun is not now located in a spiral arm (or if it is,
it is located in a "spur"). However, the Sun orbits the Galactic
center, taking about 250 million years to do so. The Galaxy's spiral
arms do *not* rotate with the stars. The Sun therefore probably
passes through at least one spiral arm every orbit. Over its lifetime
the Sun has made approximately 20 orbits, plenty of time to pass
through multiple spiral arms. Indeed the authors seem to be unaware
of a proposal that massive extinctions in the Earth's past are caused
by passage of the Sun through a spiral arm. (The proposed mechanism
is that the gravitational effects of the large dust and gas clouds in
a spiral arm could cause an increase in the number of comets falling
into the inner solar system and thereby striking the Earth.) Nor are
the inter-arm regions are devoid of stars, as the authors seem to
imply. Indeed, if one looks at a spiral galaxy in the red light
produced by the numerous low-mass stars (like the Sun), the spiral
pattern can nearly disappear. Spiral galaxies appear spiral largely
because of the enormous quantities of light produced by the hot,
relatively rare, and short-lived hot stars.

The authors also do a poor job of arguing that there may not be many
Earth-like planets. They note that there have been many planets
discovered recently, but all are larger than Jupiter. They note,
correctly, that there have been considerable selection effects against
the detection of smaller-mass planets (around main-sequence stars).
They end up concluding that terrestrial planets may therefore be rare.
(Not only that, elsewhere they suggest that Jovian planets might
rare!) Indeed, it is the prevailing opinion is that the stars
currently known to be orbited by a Jovian planet probably do not have
terrestrial planets. (The reason is that these Jovian planets are
thought not to form in the current location but to have migrated
closer to their star from their formation location which was similar
to Jupiter's current distance from the Sun. In the migration process
any terrestrial planets would have been ejected from the system or
scattered into the star. Of course nobody expected to find these "hot
Jupiters" in the first place so maybe many of those stars are also
orbited by terrestrial planets?)

What about those stars, the majority of those surveyed thus far, that
are not orbited by "hot Jupiters"? Are they orbited only by "cold
Jupiters" that did not migrate inward? or might they also have
terrestrial mass companions? The following line of reasoning is, to
this reviewer, equally or more plausible as the one suggested by the
authors. The first planets ever detected convincingly are around the
pulsar PSR B1257+12. They are terrestrial mass. Their existence
suggests that planets can form in a variety of environments.
Therefore we should expect planets to be nearly ubiquitous. Moreover,
given the range of masses in various systems---PSR B1257+12's planets
range in mass from lunar mass to greater than Earth mass, the Sun's
planets range in mass from Jupiter to Mercury and Pluto, even the
masses of the various "hot Jupiters" range over a factor of ten---we
would expect that other planetary systems might have a wide range of
masses, too. Thus, terrestrial planets might be plentiful.

(After the publication of _Rare Earth_, the discovery of Saturn-mass
planets was announced. From the census of planets around
main-sequence stars, there is the *hint* that lower-mass planets are
in fact more numerous than higher mass planets. I do not criticize
the authors for not knowing the future, but I do think these results
suggest they are wrong.)

These are a few examples of how the authors' points seem muddled or
poorly thought out. There are a host of other examples. They present
the "snowball Earth" hypothesis as near fact even though it is by no
means widely accepted by geologists. Even if we accept that the Earth
went through a snowball phase, was it essential to life's later
diversification? or did it slow down life's inevitable
diversification? and on how many other Earth-like planets would a
snowball phase occur? (Of course the answer to all of the above is,
we do not know.) They seem to overestimate both the fraction of stars
in open star clusters and the likelihood of a close encounter between
stars in an open cluster which could disrupt planetary orbits. They
seem to spend an inordinate amount of time discussing stars in
globular clusters, even though only a miniscule fraction of stars in
the Milky Way Galaxy are in globular clusters and there are a number
of reasons to suspect that planets might not be frequent in globular
clusters. They discuss the possible importance of lunar tides in the
ocean without ever noting that the solar tides are not that much
weaker (only a factor of three).

In many places the presentation also seems muddled. I am not sure if
this is the fault of the editor or the authors. However, if there is
an entire chapter on the "snowball Earth" hypothesis, why are we asked
to conduct a thought experiment in at least two other sections of the
book on the consequences of the Earth's oceans freezing over? Why not
just refer to the appropriate chapter? Similarly, if we are told that
Jupiter is more than 300 times the mass of the Earth (p. 235), do we
really need to be told less than three pages later that Jupiter's mass
is 318 Earth masses (p. 238)?

Finally, the tone of the book, in many places, struck me like a book
report. Perhaps it is because I manage to keep abreast of
developments in fields outside of astronomy by reading the first
sections of the journal Science. (Indeed, this is a good strategy for
other interested in recent developments in astronomy, planetary
science, geology, and biology. Subscribe to a journal like _Science_
or _Nature_ and read the short updates that fill the first few
sections of the journal.) Yet I somehow have the sense that having
read sections of _Science_ over the past few years, I too could have
written this book.

In summary, if you are already of the persuasion that animal life
and/or technical civilizations are rare in the Galaxy, reading this
book probably will provide you with warm fuzzies. Your opinion will
be seconded, but you probably won't gain much. If you are not of this
persuasion, you probably have better things to do with your time than
read this book. If you really feel you must, wait until your local
library obtains a copy, then stroll down there some afternoon when you
have nothing better to do.

(After I finished this review, I read the review by C. P. McKay in the
2000 April 28 issue of _Science_. McKay is one of the premier
astrobiologists. His review seems lukewarm. He describes the authors
as "[making] the case (if not always convincingly) that the situation
on our Earth is optimal for the devlopment of complex life." Later he
also writes that "we have only one example of life" and that the
"assessment of [the] probability" for the development of life "is
uncertain at best." He concludes that "theories of life and
evolution" should guide us but not constrain us"---they may be wrong.
In this spirit _Rare Earth_ provides a sobering [...] perspective in
just how difficult it might be for complex life [...] to arise.")

Lt. Lazio, HTML police   | e-mail: jlazio@patriot.net
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From: willner@cfa183.harvard.edu (Steve Willner) Newsgroups: sci.astro Subject: Re: Book Review: Rare Earth (correct version) Date: 9 Jun 2000 17:25:53 -0500

In article <m24s7q17ao.fsf@patriot.net>, jlazio@patriot.net (T. Joseph W. Lazio) writes: [most of nice review deleted] > Similarly, they provide no outer radius for their > Galactic habitable zone, but assert that there must be one based on > the general decrease of "metals" as one goes outward from the Galactic > center. It is true that there is such a general decrease, but there > is also considerable scatter about the general trend. Thus, the outer > radius is probably far more "fuzzy" than they describe it.

I think you could have been a bit stronger here. Conventional wisdom (e.g., Zaritsky et al, 1994 ApJ 420, 87, Table 3) is that the typical abundance gradients are of order -0.05 dex/kpc but with considerable uncertainties and differences among galaxies. (One galaxy even shows a positive gradient, but it's less than one sigma above zero.)

If we accept the typical gradient, the Sun at 8 kpc should have 40% of the metals at the Galactic center, and a star at 16 kpc should have 40% of Solar metallicity. But who says 40% of Solar isn't enough to form planets and life? And even if not, _most_ stars in the Milky Way are closer to the Galactic center than the Sun. (Stellar density in the disk is exponential with a scale length between 5 and 8 kpc.) So any metallicity "cutoff" is unlikely to have a big effect on the number of stars with planets or life unless you are prepared to believe that the Sun itself is well below the metallicity cutoff and only has planets and life as an unlikely fluke.

Based on some recent work and things I've heard, it is my _suspicion_ (not yet anything so strong as an _opinion_ or even _guess_) that the conventional wisdom of abundance gradients will need reexamination. I expect much progress once SIRTF is operational.

-- Steve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.)

************************************************************************ Amara Graps, PhD | Max-Planck-Institut fuer Kernphysik Heidelberg Cosmic Dust Group | Saupfercheckweg 1 +49-6221-516-543 | 69117 Heidelberg, GERMANY Amara.Graps@mpi-hd.mpg.de * http://www.mpi-hd.mpg.de/dustgroup/~graps ************************************************************************ "Never fight an inanimate object." - P. J. O'Rourke

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