Rare Earth*

It is helpful to read Rare Earth by Ward and Brownlee to understand why intelligent life may be extremely rare in the universe, even if simpler life appears and evolves rapidly when conditions are favorable.

As science gained a better understanding of the galaxies in which stars and planetary systems form, it became apparent that the reduction of probabilities for intelligent life to develop begins with the galaxies. There are many different kinds of galaxies. There are galaxies that are too dense and too violent, and others too light and low on the heavier elements necessary for the development of life. Even life-friendly galaxies have only narrow bands of habitable zones far from the lethal radiations of the center and equally far from the thin edges that are poor in the essential elements.

A similar rule applies to the planetary systems. The habitable zone of a planetary system is a narrow band not close to the host star where it is too hot, and not far from it where it is too cold. While this process has already eliminated most of the cosmos for nursing life, the process of elimination continues further.

Under most circumstances gravity will arrange matter on a planet evenly, without mountains. A planet without mountains would be flooded when water is added and would remain without dry land. Such a planet is not likely to evolve intelligent animal life. To have dry land, a planet must be mostly molten with only a relatively thin solid crust floating on the molten surface.  On such a surface, plate tectonics create mountains and continents that rise above the water. Clearly, if you want more than algae and fish on your planet, you must have plate tectonics.

After all that, your planet would still have frequent and severe ice ages preventing the development of higher animal life. This would be due to the wobbling of the rotational axis of the planet. To stabilize rotation you would need a large moon in an extraordinarily close orbit to your planet, which does not happen by normal formation, but by a rare accident.

Scientists have theorized that Earth collided with a large body during its early development that split our planet with a particular force. As a result, the severed material settled in close orbit to form our moon. Earth would have reabsorbed the material had it been impacted with a smaller force. A larger impact, on the other hand, would have sent the material into an orbit too far to have a stabilizing effect on our planet.

But what is the use of having a stable planet when large asteroids collide with it and sterilize it too often to allow intelligent life to evolve? To save Earth from such frequent catastrophic events, we have our “guard planet.” Jupiter acts as Earth’s personal heavyweight bouncer with a muscular gravitational field that sweeps clean the outer solar space of most asteroids. Jupiter guards and saves Earth from frequent and catastrophic collisions with asteroids, thus securing the very long peaceful period required for the development of intelligent life.

Many of these conditions are unusual by themselves. The probabilities progressively diminish for two or more of such events to come together. It is a statistical rarity to have so many cosmic coincidences converge. If there is intelligent life elsewhere in the cosmos, it must be extremely rare. Indeed, we have looked and listened for extraterrestrial intelligent life for decades, but so far have found none outside of ours here on Earth.

* Rare Earth – Why Complex Life Is Uncommon in the Universe  by Peter D. Ward and

Donald Brownlee   Copernicus/Springer - Verlag,  New York, NY.