RARE EARTH

Reviewed 3/14/2004

Rare Earth, by Ward & Brownlee

RARE EARTH
Why Complex Life Is Uncommon in the Universe
Peter D. Ward
Donald Brownlee
New York: Copernicus, 2000

Rating:

4.0

High

ISBN 0-387-98701-0 333p. HC/GSI $27.50

Building a Habitable World

Science is only now discovering how many factors must come out just right in order to create a world that is hospitable to the development of large animals and ultimately intelligent life forms such as ourselves.

Table of Requirements for Habitability

The world must: If not:
Not be in an elliptical galaxy Stars there are too old to contain the amounts of heavy elements required to form rocky planets the size of Earth. Also, most solar-mass stars have become red giants, roasting any inner planets they might have.
Not be in a globular star cluster Stars there are too old to contain the amounts of heavy elements required to form rocky planets the size of Earth. Also, most solar-mass stars have become red giants, roasting any inner planets they might have, and the stars are so close that planetary orbits are unlikely to be stable for long enough.
Be located within a spiral galaxy's habitable zone Too close to galaxy center, planet sterilized by supernovae, gamma-ray bursters, or central black hole; too far out, insufficient heavy elements.
Be in a single-sun solar system. Multiple stars in a system perturb the orbits of any planets, negating long-term stability.
Orbit a suitably long-lived, stable star. Large stars burn too quickly to give complex life time to evolve; stars with highly variable output change the surface temperatures of their rocky planets too much.
Orbit a star of a mass not much less than that of our sun. Planets of small stars would have to orbit close to be warm enough for life; then they would be tidally locked to the star, and life probably could not develop under that condition.
Be located within solar system habitable zone. Planet's surface is too hot or too cold for life.
Have a giant planet in the system to act as an "asteroid sink". No gas giant means a longer period of bombardment by "space rubble".
Not have the giant planet too close, or in a highly elliptical orbit. Gas giants near sun ("hot Jupiters") tend to toss rocky planets out of the system. Those with elliptical orbits are similarly disruptive.1
Have radioisotopes to keep core molten, drive magnetic field, make volcanoes & plate tectonics, enhance biodiversity. No protection from star's particle radiation, no continents, no continental drift, no carbonate-silicate cycle to prevent runaway greenhouse effect
Have a relatively large moon to stabilize its obliquity.2 Planet wobbles, leading to excessive surface temperature variations over time.
Incorporate sufficient heavy chemical elements (carbon, silicon, oxygen, nitrogen, phosphorus, and various metals) along with hydrogen in the form of water. Without an abundance of such elements (found in generation 2 and later stars), rocky planets would be too small, and the essentials of life too scarce.
Not have so much water that large continents cannot form. The carbonate-silicate cycle will not operate, or will be much less efficient.3 The likely result is a runaway greenhouse effect.
Have enough mass to induce sufficient gravity to hold atmosphere and water. These vital substances escape too soon.
Undergo period(s) of global freezing, with recovery to warm temperatures. Hypothetically, animal life will not develop without such "snowball" events.
1 It is significant that all the extrasolar planets detected to date are either close to their suns ("hot Jupiters") or in elliptical orbits with high values of eccentricity. Note too that gas-giant planets cannot form close to a sun; they must form farther out and drift sunward — a process likely fatal to any rocky inner planets.
2 The authors also claim (page 227) that the moon's tides, and its effect in slowing its planet's rotation, are essential for life. The book does not support this claim.
3 Both large continents and extensive shallow seas are necessary for the carbonate-silicate cycle. The authors peg the range for the area ratio (continent area:ocean area) as one-third to two-thirds, but acknowledge that these numbers are not well-supported. I can't resist quoting Mike Nelson of the old TV show Sea Hunt: "You know, three-fifths of the Earth is covered by water — and how little most of us know about that underwater world."
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