The NY Times has an article today about a book out last month, Rare
Earth by paleontologist Peter D. Ward and astronomer Donald C. Brownlee.
The book's thesis is that advanced multicellular life is likely to be
very rare in the cosmos.
The article's URL is
and if it asks you for an account, use "cpunks" and "cpunks" for username
and password (or register your own account).
Here is the part of the article where they describe the specific
arguments. Most of these we have seen before, but perhaps not all
brought together in one place. I wonder if the book actually does any
quantitative estimates using the Drake Equation or something similar.
Dr. Ward said he was drawn to the topic because of his studies of mass
extinctions. Increasingly, top culprits are seen as speeding rocks
from outer space that hit Earth in huge explosions, with one 65 million
years ago killing off many plants and animals, including the dinosaurs.
New studies, Dr. Ward said, suggest that things could be worse. For
instance, the rate of terrestrial impacts could be as much as 10,000
times higher but for the presence of Jupiter, the solar system's
largest planet, which absorbs many killer rocks and flings others
into deep space.
"We're right on the edge of the abyss," Dr. Ward said, in terms of
higher bombardment rates that have probably precluded the development
of advanced life.
Recent finds of giant Jupiterlike planets outside the solar system
offer no solace. Most of their orbits, he said, are wildly eccentric,
which would abet destructive chaos among smaller planets rather than
"All the Jupiters seen today are bad Jupiters," Dr. Ward said of the
31 finds to date. "Ours is the only good one we know of. And it's
got to be good, or you're thrown out into dark space or into your sun."
Dr. Marcy, the planet finder, said such analyses were adding to his
doubts about the existence of extraterrestrials.
Dr. Ward said that even if some distant Jupiters were found to be in
stable, circular orbits, other factors might overwhelm their protective
effect and demolish any life. For instance, closer to the center
of the galaxy where star populations are far denser, the frequent
passage of one star past another could trigger cascades of comets,
trillions of which are thought to orbit the icy fringes of most stars.
"If you're in the interior of the galaxy," Dr. Ward said, "you're
always getting bombarded."
Added to that fury, he said, is the intense radiation and explosions
of galactic interiors. The star-filled sky conveys a false impression
of immutability. New studies show that the cosmos, especially galactic
centers, are hotbeds of violence swept by killing waves of X-rays,
gamma rays and ionizing radiation.
"So I don't think there's any life in the centers at all," Dr. Ward
Dr. Brownlee, the astronomer and co-author, said the odds for complex
life were similarly bad at galactic edges.
The analysis of starlight from the fringes shows they are relatively
poor in elements like iron, magnesium and silicon, partly because of
less recycling of stellar materials over the eons and partly because
of the rarity in such regions of supernovas, the stellar blasts that
help make heavy elements in enormously hot explosions.
These elements, Dr. Brownlee said, and even heavier ones that are
radioactive and also made in supernovas, appear to be prerequisites
to the formation of terrestrial-type planets that have sufficient
gravity to retain seas and atmospheres and that have plate tectonics,
which is powered largely by the heat of radioactive decay.
According to the book, the slow movement and recycling of planetary
crust into a planet's hot interior are key ingredients for the
evolution of complex life. Plate tectonics, the authors say,
promotes biodiversity by producing mountain chains and other kinds
of environmental complexity, lessens the odds of extinctions, helps
keep planetary temperatures even through the recycling of carbon and
makes dry land on which advanced civilizations can flourish.
"We're critically dependent on mass," said Dr. Brownlee. "Being bigger
or smaller might rule out plate tectonics."
Whole galaxies are metal-poor and therefore probably devoid of animal
life, Dr. Brownlee added. Only spiral galaxies like the Milky Way and
its nearby neighbor in Andromeda appear rich in metals, and even then,
only in their inner regions. In contrast, elliptical and irregular
galaxies, he said, are barren.
"Lower metal abundance means you can't make a planet as big as the
Earth," Dr. Brownlee said. "It seems like something a lot of people
don't want to hear."
The scientists discuss other planetary characteristics that are
probably rare in the universe but are increasingly seen as critical
for making Earth so favorable to complex life. Among them are these:
*An orbit that keeps a planet at exactly the right distance from its
star to ensure that water remains liquid, not vapor or ice.
*A large moon at just the right distance to minimize changes in a
planet's tilt, ensuring climate stability.
*Enough carbon to aid the development of life but not so much to
allow for runaway greenhouse conditions, as occur on superheated Venus.
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