The holiday time of year, for some reason, causes me to reflect on the Big Picture. By that I mean, the Really Big Picture subjects. In the fall of 2018, I had a blog titled, “Are We Alone in the Universe?” This blog was mostly about the Drake Equation and speculation about the chance for other intelligent beings existing in our galaxy or at all.
The Drake Equation was a response to the Fermi Paradox. Astronomers knew there were trillions of stars in the universe, probably many trillions of planets and that, with those odds, surely intelligent life had evolved elsewhere. Speculation was that there were thousands of civilizations amongst the stars. With enough time, these advanced civilizations could surely have explored all the Milky Way, our home galaxy.
Walking to lunch one day in 1950, Enrico Fermi and several other physicists were discussing this very topic. Fermi thought about it awhile and then asked, “Where is everybody?” Where indeed.
From Fermi’s question has arisen the field of astrobiology—the study of life in the universe and specifically outside Earth. Carl Sagan and others promoted the idea of thousands or millions of worlds inhabited by intelligent beings. And still we’ve heard nothing. No visits, no radio signals, nothing.
Another lunchtime conversation occurred at the University of Washington faculty club in Seattle. The two authors realized that they both believed that life is less common in the universe than is commonly believed and decided to write a book about it. That book was published in 2000. It is titled Rare Earth and subtitled, Why Complex Life is Uncommon in the Universe. The authors are Peter Ward, Professor of Geological Sciences, and Donald Browlee, Professor of Astronomy, both at the University of Washington in Seattle.
The authors have updated the Drake Equation for other factors. The equation for the number of planets that would host intelligent life now has eleven variables. The book walks through the various factors and how they led to intelligent life on Earth. There may be other paths to creating intelligent life, but we have a sample of one to study. I’ll briefly mention a few of these factors below.
Habitable Zones
Many have heard of Habitable Zones in a solar system. The notion is that if a planet is too close to its star, it will be too hot for life. Too far away and it will be too cold. The habitable zone is where the temperatures are such that liquid water can exist for billions of years; enough time for evolution to works its magic.
Our Sun is also bigger than 95% of stars. Smaller stars could support life, but that implies the planet must be much closer to its star. That risks ‘tidal locking,’ which means the planet doesn’t rotate. This is like our Moon, with one side always facing Earth.
Just as there are habitable zones in a solar system, there are habitable zones in each galaxy. Most areas of a galaxy are too full of gamma or x-rays, radiation that will quickly sterilize a whole planet.
Rare Earth Factors
It is not enough that a planet is the right distance from a star to maintain a temperature that accommodates liquid water. The planet must also have a circular orbit around its star. An elliptical orbit causes too much range in temperatures.
The planet also needs the right tilt. Most planets wobble a lot on their spin axis, which is devasting to whatever life is evolving. The tilt of Earth is stabilized by the Moon. We happen to have a very large moon for the size of our planet. This alone is rare.
How to Build Animals
While life can exist in extreme circumstances, and in the case of Earth, may have originated in extreme environments, the complexity of bacterium is much different from any multi-cellular animal. The number of genes in a bacterium is in the thousands. In a large animal it is in the millions.
It took 1.5 billion years for complex cells to evolve. About one billion years ago, the pace of evolution of complex animals accelerated. Animals that were big enough to be seen unaided first showed up about 600 million years ago.
Then, 550 million years ago, sizable, and diverse life burst into the oceans. This is known as the Cambrian Explosion. Over that short period of time, all animal phyla either evolved or appeared in the fossil record. The Cambrian Explosion involved huge numbers of new species with novel body plans.
By the end of the Cambrian, all animal phyla had been formed; none have occurred since. Did something trigger the Cambrian Explosion? One theory is that oxygen reached a critical threshold that made diversified life possible.
Mass Extinctions
Mass Extinctions occur when much of the planet’s biota are killed off by environmental factors. An asteroid strike killed the dinosaurs. Mass Extinctions have happened many times in the history of Earth. Animals are fragile and easily killed, while microbes are less vulnerable.
The frequency of these events varies but can be thought of in a manner similar to weather events, for example, 1,000-year floods. Some mass extinctions sterilize the entire planet, eliminating all life. Most eliminate much of life, maybe even most, but seem to pave the way for other varied forms to flourish. For example, the end of the dinosaurs allowed the age of mammals, including humans. But too many mass extinctions and all life gets wiped out.
The Surprising Importance of Plate Tectonics
Earth is unique in our solar system as the only planet with liquid water on its surface. Liquid water is necessary for life as we know it. This, in turn, requires a temperature range that allows water to remain liquid.
Plate tectonics is the global thermostat; it recycles carbon dioxide to keep the amount in our atmosphere relatively constant. This is the single most important factor in maintaining liquid water for four billion years. Plate tectonics also creates continents, which is important for supporting higher life forms. Tectonics also makes possible the Earth’s magnetic fields; one of our most potent defense systems.
Jupiter
Jupiter, a huge gas planet, has cleaned up the solar system of floating debris, via its enormous gravitational pull. This has (mostly) spared the Earth from mass extinction events caused by collisions with asteroids and small planets. Jupiter acts to guard Earth but it is also far enough away that it won’t draw Earth in.
The Rare Earth Hypothesis
The Rare Earth Hypothesis is the unproven supposition that although microscopic, sludge-like organisms might be relatively common in planetary systems, the evolution and long-term survival of larger, more complex, and even intelligent organisms are exceedingly rare.
We have the tools to search for alien life. We have sent probes to nearby planets. We can study the atmosphere of planets with telescopes and detect signs of life. And we can try and detect radio signals sent by other civilizations. This has been going on, on a limited basis since 1990 with SETI—Search for Extra Terrestrial Intelligence. One of the problems with this approach is we don’t know the life span of civilizations with radio technology. And could any other civilization respond in a timely manner?
How Rare is Earth?
The Rare Earth Equation produces daunting odds against other intelligent beings in our galaxy and maybe anywhere. And if any of the eleven factors approach zero, so too does the result. If there is other intelligent life in our galaxy, the distances are so great that it is likely that we could never communicate with them. For all intents and purposes, we are alone.
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