Are we on a Goldilocks planet?

Perfect Planet, Clever Species: How Unique Are We? By William C. Burger, Prometheus Books, Amherst, New York, 2002. ISBN 1-59102-016-6. 345 pp. Hardcover, $29

"In 1978, Isaac Asimov [in Extraterrestrial Civilizations] generously estimated that there ought to be 530,000 technological civilizations in our galaxy, separated by an average of around 630 light-years" (273). Not so, says William Burger (Curator emeritus, Field Museum of National History, Chicago).

And in Eight Little Piggies, Stephen Jay Gould argued for the certainty of life on every Earth-like planet in the universe when he wrote, "The oldest rocks that could contain preserved organic remains are 3.5 to 3.6 billion years old ... and ... do feature fossils of single-celled creatures similar to modern bacteria." He saw that as evidence that "life, arising as soon as it could, was chemically destined to be, and nor the chancy result of accumulated improbabilities." But it was only life at the lowest level that Gould deemed inevitable, since the evolution of multicellular animals did not occur for a further three billion years.

So life will almost certainly evolve on an Earthlike planet. And there, according to Burger, lies the problem. To qualify as "Earthlike," a planet must not only fall within a reasonable approximation of Earth's composition, mass, gravity, and spin. It must also be located in what astronomers have dubbed the "Goldilocks orbit." "To sustain complex lifeforms anywhere in the universe, it seems likely that a planet has to be at a distance from its parent star that provides a temperature range in which water is maintained in its liquid state.... Planet Earth sits comfortably within this Goldilocks zone" (32). Also, for a star to have a life-supporting planet, it must not be part of a binary system, it must be located inside the comparatively radiation-free area between the galaxy center and its halo and sufficiently far from galactic dust zones, and it must contain more heavy elements than eighty percent of otherwise sun-like mainstream stars.

A further uniqueness that makes Earth life-supporting, found nowhere else in the solar system and therefore conceivably rare in the galaxy, is its active crust: "If the Earth's surface were smooth, the overall depth of water covering our planet would be about 8,800 feet (2,700 m). Simply stated: without plate tectonics we'd all be fish" (50). And fish, as inhabitants of an environment with no dry land, could not develop the technology that makes intelligence a survival factor.

Add to all of that the stabilizing effect of an enormous moon, created by statistically improbable circumstances, that at the time of its formation increased Earth's spin rate, thereby blowing away a suffocating atmosphere such as still exists on Venus, as well as increasing Earth's magnetosphere to a level where it prevents the solar wind from stripping away the ozone layer. Later, by tidal effects, the presence of the moon reduced the spin rate to a level that reduces temperature variations to life-supporting limits, while simultaneously minimizing axial tilt. Those combined circumstances are sufficiently rare that Burger suggests there may be no other star system that meets all necessary specifications anywhere in this galaxy. He writes that "although the string of new data confirms that planets are common, at this stage it seems that planetary systems configured like our own are rare indeed" (29).

Many of the other factors Burger cites as necessary accidents in Earth's evolution as a life-bearing planet, such as constant bombardment from water-laden comets at precisely the right historical moment, cannot be rare in galactic terms. But for all such beneficial accidents to happen to the same planet was indeed a low-probability coincidence. He writes, "Because of biological activity based on the photosynthetic splitting of water, Earth's atmosphere became uniquely rich in oxygen. Thus, a combination of lucky breaks and unusual trajectories confronts us with the very real possibility that our blue and white planet may be unique, not only in its own solar system, but among the many thousands of other star systems as well.... We're not just talking Goldilocks's orbit here; we've got the Goldilocks planet. Our gravity, spin, crust, atmosphere, and water are all 'just right'" (55-56).

And precisely because conditions are "just right," certain mammals were able to evolve into apes, apes into the species Homo, and Homo into Homo sapiens. However, I was surprised to find Burger equating Homo heidelbergensis with an early form of Homo sapiens, directly descended from Homo erectus, which he identifies as a human ancestor. According to Klein and Edgar (The Dawn of Human Culture), the hominid line split into Homo erectus and Homo heidelbergensis, with the latter leading to Homo sapiens and the extinct coexisting species, Homo neanderthalensis, and the former to extinction. Their conclusion that all modern humans are descended from ancestors who lived exclusively in Africa until 50,000 years ago, far later than H. erectus, is supported by strong evidence. Burger's rebuttal: "Those who insist on the 'African Eve' scenario must postulate that the eastern Asian facial and cranial features arose twice: once in the distant past and a second time after the original features were swept away by modern sapiens genes. The chance of a double origin for such regionally unique features seems utterly unlikely."