"Rare Earth" :  Why Complex Life is Uncommon in the Universe

by Peter D. Ward and Donald C. Brownlee, Copernicus Books Springer-Verlag Publishers, Jan 2000. (amazon.com)

An argument for a paradigm shift in our understanding of the Origins of Life in the Universe.

Peter D. Ward, Ph.D., is a professor of paleontology at the University of  Washington and Donald C. Brownlee, also of the University of Washington, is a professor of Astronomy, a member of the National Academy of Sciences and chief scientist of NASA's $166 million Stardust mission designed to capture interplanetary
and interstellar dust.

The current astrobiology paradigm is the view that alien civilizations do in fact exist  and are numerous enough likely to be scattered among all the stars, but isolated from one another by the emptiness of interstellar space. Just for Earth's own galaxy, the Milky Way, experts have estimated that there might be up to one million advanced societies. The notion that alien civilizations are ubiquitous arose about forty years ago.

Frank D. Drake, then a young astronomer at a federal radio observatory in West Virginia, in 1960 was the first to scan the skies for faint alien signals, and was quickly joined by like-minded experts, including astronomer Carl Sagan, of Cornell University. Drake laid out his ideas in 1961, in what came to be known as the Drake Equation. The equation made educated guesses for the rate at which stars form, the fraction of stars with planets, the number of those planets on which life arises and so on, including the average lifetime of technological civilizations. By his equation Drake estimated that the Milky Way had about 10,000 civilizations capable of interstellar communication.

Later, Carl Sagan revised the calculations and raised the estimate to a million alien worlds. Since the cosmos holds hundreds of millions of galaxies, by that analysis the total number of alien societies could be
astronomical.  One estimate put the number at roughly 10 trillion alien intelligent societies.

This belief in extraterrestrial intelligent life is so strong that several of my biology classes, when asked, "Do you believe in extraterrestrial life", all raised your hands affirmatively. The paradigm is such a part of our popular culture, that  countless books, movies and television shows -- not to mention hosts of Klingons, Wookies and Romulans -- but a long scientific hunt that uses huge dish antennas to scan the sky for faint radio signals from intelligent aliens. A few zillion radio band are collected by a huge antennae dish at Mount Arecibo, P.R., (the biggest and best single antenna, at 1,000 feet in diameter, for gathering faint signals) in our search for intelligently made radio signals amid the vast celestial static.

Human culture is so captivated by the current alien paradigm that 1.6 million of us in over 200 countries have recently joined an effort that harnesses home and office computers to the job of sifting through the radio signals for signs of intelligent life among the stars. The SETI Institute project of the University of California at Berkeley uses idle computers linked to the Internet to plow through collected radio signal data.

SETI@home, via the Web, distributes free software that enables a home computer to crunch the Arecibo data in what its creators call the world's largest ad hoc supercomputer. The software works as a screen saver, analyzing data only when a home computer is idle. Once the data has been analyzed, often over several days, it is returned to Berkeley via the Web, for another batch of data. 

Since May 1999, when the project first got underway, home volunteers have donated 165,000 years of computing time to analyzing radio emissions from our Universe.

If your buy into the current pop culture, you may download the needed freeware from  www.setiathome.ssl.berkeley.edu. To date 1.6 million copies of the freeware have been downloaded says Dan Werthimer, chief scientist of the SETI project. Several corporate sponsors have bought into the project including: the Planetary Society, the University of California, Sun Microsystems, Fuji Film, Paramount Pictures, Intel, the SETI Institute and many private private donors.

With the publication of their book in January (2000), Ward and Brownlee are throwing rocks at the existing paradigm.  They present data to say the conventional wisdom is wrong. SETI and the alien search, they add, is likely to fail.

Drawing on new findings in astronomy, geology and paleontology, the two argue that humans might be alone, at least in the stellar neighborhood, and perhaps in the entire cosmos. They base their argument on a number of facts, which taken together they argue provides evidence that we are alone.
Their evidence includes the following points:

1. The right chemical elements are too rare in abundance. Modern science suggests that Earth's composition and stability are extraordinarily rare. 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, 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.

    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."  

2. Most everywhere else in the Universe, the radiation levels are too high. Added to that is the intense radiation and explosions of galactic interiors. The star-filled sky conveys a false impression of immutability. Recent 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," Ward said.

   Hospitable planets too few in number:  Earth’s planetary properties are rare in the Universe. Ward and Brownlee discuss the 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.

3. The rain of killer meteorites is too intense for life ever to have evolved into advanced communities. Ward said he was drawn to the topic because of his studies of mass extinctions. The suggestion is that meteorites hit Earth in huge explosions, with one 65 million years ago killing off many plants and animals, including the dinosaurs.

   New studies suggest that  the rate of terrestrial impacts could be as much as 10,000 times higher, but for the presence of Jupiter, our solar system's largest planet, which absorbs many killer rocks and flings others into deep space.

   The Earth may be in a unique protected “Life Zone” where because of the fortuitous presence of Jupiter we are not hit as hard as we could have been by meteorites. "We're right on the edge of the abyss," Ward said, in terms of higher bombardment rates that would have probably precluded the development of advanced life.

    To date about 31 planets circling around other stars have been found so far. Recent discoveries of giant Jupiter-like planets outside our solar system offer no help. Most of their orbits seem to be wildly eccentric, which would abet destructive chaos among smaller planets rather than shielding them.

   Even if some distant Jupiter-like planets 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.

4.  Alien microbes may survive in many places as a kind of cosmic shower scum, they say, but not extraterrestrials civilized enough to be awash in technology.

5.  New findings suggest that the Drake Equation is riddled with hidden optimistic assumptions.

6.  Brownlee argues that "People say the Sun is a typical star," he remarked in an interview. "That's not true."

7.  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  Brownlee. "Being bigger or smaller might rule out plate tectonics."

Opinions expressed about “Rare Earth”.....

Their book, "Rare Earth" (Springer-Verlag) is producing whoops of criticism and praise, with some detractors saying that the authors have made their own  simplistic assumptions about the adaptability of life forms while others call it "brilliant" and "courageous."  [William J. Broad – NY Times]           

"We have finally said out loud what so many have thought for so long --  that complex life, at least, is rare," said  Peter D. Ward of the University of Washington, a paleontologist who specializes in mass extinctions and whose previous works include "The Call of Distant Mammoths" (Springer-Verlag, 1997). "And to us, complex life may be a flatworm."

Geoffrey W. Marcy of the University of California at Berkeley, a leading seeker of planets around other stars, 31 of which have been found so far,  hailed "Rare Earth" as likely to spark a revolution in thinking about extraterrestrial life. "It's brilliant," Marcy said in an interview. "It delineates many things I've been thinking about but does a much more credible job of listing and explaining the various issues." For instance, he said, it shows how the giant planets discovered so far outside the solar system bode ill for the development of complex life. "It's courageous," Marcy added. "It's rare in literature and science that a stance goes so far against the grain." 

In the book's conclusion, the authors say the Rare Earth hypothesis is testable, and they strongly encourage such work. Powerful new telescopes will shed light not only on gas giants, but on the abundance of smaller, terrestrial planets around distant stars, and will also show whether their orbits are stable and protected by larger planets from cosmic bombardment.

New telescopes also might find evidence of planets enshrouded in ozone and oxygen, which in sufficient concentrations imply the existence of life. 

The two authors of “Rare Earth” scientists also call for searches of Mars, the Jovian moons Europa and Ganymede, and Saturn's moon Titan for signs of alien microbes. That discovery would answer the question of whether life is an inherent property of matter, as most scientists believe.

Frank Drake, the originator of the extraterrestrial paradigm and currently president of the SETI Institute, a private group in Mountain View, CA, said the book's main failing was undue pessimism about life's tenacity. "The basic flaw in all those arguments," Drake said, "is that they don't allow for the opportunistic nature of life, its ability to accommodate or alter itself to cope with environmental change."

If the “RARE EARTH” hypothesis turns out to be true then it greatly increases the significance of loss, each time a plant or animal is driven to extinction and strengthens the responsibility for humans to be good stewards of the planet.

Also, Ward remarked in an interview, if the Milky Way is truly devoid of alien hordes, then it might be humanity's destiny over the eons to spread into the wilderness of stars, unopposed by ancient legions. "If we are as rare as we think we are," Ward said, "it raises the stakes, intellectually and morally."

Excerpted from an article in the New York Times, February 8, 2000 – Science Times Section D – by William J. Broad – page D1 and D4.

University of Miami Page  |  Biology's Page  |  Dr. Mallery's Page  |  On-Line Testing Center.
copyright c2003, Charles Mallery, Department of Biology, University of Miami, Coral Gables, FL 33124  
Last Update - 02 April, 2007