Many of us have been led, over the years, to believe something that’s not necessarily true. When we wonder about the absence of alien life or the enormity of the universe, we are told that Earth is a world like many others. It is not of tremendous size nor is it located in any special sort of galaxy. The fact that we are the only intelligent life in the universe is such a mystery to us because there is nothing special about our planet. It is, as scientific literature and programs and speeches will tell us time and time again, unordinary in any way.

 

Dr. Frank Drake’s equation tells us that, after taking into consideration seven distinct factors, there should be at least 1,000 civilizations in our galaxy alone. Carl Sagan estimated this number to be 1 million in 1974. Yet it was Sagan himself who taught us that Earth is not a typical place in the universe. There is no such thing as a “typical” planet, moon, or star. Every celestial body is precious; most of our universe is defined as dark and empty.

Our sun, too, is officially described as an average star when in actuality around 90% of stars are less massive. This means that their habitable zones are far closer to the stars, placing those planets in a position vulnerable to radiation and tidal locking. During tidal locking the same side of the planet will always face the host star, leaving it susceptible to a scorching, sterile surface on one side while the other remains an unwelcoming tundra.

The more one peers into the history of our evolution, the more one realizes it took an incredible tangle of factors to bring us to where we are today. We were not inevitable. We were the result of an elaborate scheme, and of great fortune on a rocky planet that happened to be in the right place at the right time. The Rare Earth hypothesis attempts to be an answer to the question of alien life. It reveals to us just how delicate our history has been. Great odds have confronted life before. We scraped by, crawled, slithered and fought our way to the rich complexity of Earth’s flora and fauna. And there may not be a chance for life to form again in the universe’s future.

The metal content of a galaxy is associated with its size. Galaxies with poor metal content wouldn’t encourage the formation of life, yet we were gifted with a metal-rich Solar System and sun. A unique star system, in a possibly very unique galaxy. The sun is seen here as a yellow orb orbiting in the plane of the Milky Way. Image by ESA.

The Rare Earth hypothesis does not tell us that life is rare in the universe. In fact it admits that simple, microbial life may be very abundant on distant planets or even on moons like Europa which we suspect has a liquid ocean beneath its ice. Microbial life cropped up on Earth almost as soon as the environment allowed for its survival. Because life arose as soon as it was possible for it to do so, there’s no reason to believe this wouldn’t be the case on other planets and moons. Our fossil record suggests primitive life spurs easily from non-living material. These microorganisms should be common in the galaxy. After all, early Earth was replete with infernal temperatures, pressures, and a stifling lack of oxygen. Yet these are the conditions under which life began — the same conditions shaping so many more worlds around us.

So how does this hypothesis explain our solitude? Why is only the Earth in song while an ominous hush falls over the rest of the cosmos?

The many factors, as outlined in “Rare Earth”, that have contributed to the formation of Earth’s complex life.

This is because while simple, single-celled life may be common throughout the universe, animal life is not. It is one thing for life to begin under intense conditions deep in a hostile sea so many billions of years ago, it is another thing for these organisms to evolve into multicellular creatures with eyes and wings and teeth. And not only is the transition from single-celled organisms to animal life a mysterious one filled with accidental mutations and mysterious leaps, but even if alien microbes did attain animal status it would be difficult to maintain it. Animals were a result of many failures and a few successes on the part of microorganisms. There is no guarantee that where there is bacteria there will necessarily evolve anything more complex.

Even life here was almost extinguished five times in the planet’s mass extinction events. In all these situations life has nearly lost its unsteady grip, with so many species rising and falling just as quickly as they appeared. We are here, in large part, due to sheer luck. One can imagine how many other alien lifeforms haven’t been as fortunate.

Everything is a balance allowing us to exist. We do not orbit a typical star and we are not at a typical point in time in our universe’s lifespan. We exist just at the right moment when enough stars have exploded to allow for the heavy elements which created our planet and its organisms. One of the elements birthed by supernovas — uranium — is thought to be essential for the Earth’s plate tectonics. The radioactive decay creates the planet’s internal heat which in turn influences the planet’s atmosphere, oceans, and vegetation. There was not enough of this element in the universe’s first 2 billion years to create environments like Earth’s. In the future less and less uranium will be created in stars, meaning that Earth-like planets at other points in time won’t have the necessary elements for plate tectonics. Plate tectonics keeps our temperature steady. And if there’s anything life needs to flourish, it’s stability.

Further stability is provided by our sun. Earth has enjoyed consistent energy output from our star for billions of years. Scientists believe it would be difficult for life to form around stars with varying energy output or in systems with more than one star. Kepler-16b is an exoplanet orbiting two stars. This is the hypothetical view from the planet’s moon. Image by NASA.

There are two ways to test the Rare Earth hypothesis. Both involve the continued search for life beyond our home. Finding either living microbes or their fossils on other worlds would help confirm that single-celled life appears easily and perhaps even frequently on warm worlds with liquid oceans. The second test is the search for advanced alien civilizations. Large telescopes in space would help us part the heavy and mysterious curtain of uncertainty.

In the book Rare Earth by evolutionary biologist Peter Ward and astrobiologist Donald E. Brownlee, the authors emphasize just how much more significant the loss of every animal species is in the light of their hypothesis. If we are at a special moment in time — and at a special place in the great breathy cosmos — where life can exist when all the odds are otherwise against it, then every species of animal on this planet is that much more precious. Far from any of us being common creatures in the lush of common landscapes, we are each of us an anomaly. What seems everyday to us is a treasure in the cosmic web. Our planet’s oceans glimmer like jewels. Its colors are the colors of riches we won’t find anywhere else. This does mean that we are alone — alone in our extraordinary existence.