In 1956 when I was just beginning my journey out of atheism, our understanding of what it takes to produce a planet that can support life was pretty basic. We knew you needed a star with a certain kind of radiant energy emission, a planet with a certain size, and with a certain chemical make-up. Since the theory in vogue at the time was the nebular theory (which said that all stars and planets came from common clouds of matter floating through space), it was assumed that there were literally thousands of planets like Earth all over the galaxy and throughout interstellar space. I. S. Shklovskii and Carl Sagan wrote a book titled Intelligent Life in the Universe (Dell Publishing, New York, 1966) which in fact made that prediction.
In the last fifty years we have gained more and more understandings about what space is really like, and how many situations there are in other parts of space that would be fatal to life if those conditions existed here. The more of these variables we find, the more unlikely Earth's situation turns out to be. We have mentioned many of these things in our news and notes section of this journal over the years, but in this article we would like to be more specific and give a more comprehensive listing of what these variables are and how they impact our understanding of how special Earth is.
Collisions. In the late 1950s Eugene Shoemaker showed that there are certain kinds of rock that only form under the pressures that a meteor can deliver. The debate over what caused Meteor Crater was finally over, and astronomers had to recognize that astronomical catastrophes were a serious risk to life on Earth. In the early 1990s Louis Alvarez showed that from the time of the dinosaurs there was all over the world a clay layer containing large amounts of iridium which could only come from an asteroid colliding with Earth. Not too much later a 120 mile wide depression was found at the edge of Mexico's Yucatan Peninsula at a place called Chicxulub which contained the minerals of the asteroid and the rocks that Shoemaker found that indicate an asteroid collision with Earth. Now craters with similar conditions have been found in the Ordovician, Devonian, and Permian periods--all of which are older than the Cretaceous. As we have looked up close at other planets and moons like Mars, Mercury, Callisto (one of Jupiter's moons) and our own moon it is obvious that they have been pulverized by repeated strikes of material from space. Our atmosphere burns up the smaller of these objects, but it is obvious that getting hit by space junk is a real risk to life on Earth.
Another potential for collisions is from comets. We now know that there is a huge reservoir of comets in a huge cloud around the solar system. If a star came close to our system, it would knock these comets loose, with perhaps hundreds of them being moved toward the Sun. This hypothetical star has been called Nemesis and astronomers have suggested that all kinds of things could happen to Earth if something like this were to take place. Not only could you have a collision with the comet, but even coming close to a comet might trigger dust and microbes that life could not withstand. So far we have not seen any stars that would qualify as Nemesis, and our computer models show that the existence of the jovian planets (Neptune, Uranus, Saturn and Jupiter) forms a wall that protects Earth from such a collision, but it remains an example of how many things can happen to Earth just from the loose material floating around interstellar space.
Stars. We all know that Earth orbits a star we call the Sun. When I was in high school, we were told that the Sun was an average star similar to millions of other stars in our galaxy and throughout the cosmos. In the past fifty years we have found that the Sun is not typical at all. The Hubble telescope has found that an overwhelming number of stars in space are dwarf stars--red dwarfs, K dwarfs, and white dwarfs--all stars that could not support a life-bearing planet. We have also learned that most stars are not single stars like our Sun, but are binary or trinary stars with two or more stars orbiting one another. Planets in such a system would be too unstable to allow life forms of any kind to exist on them.
We have also learned in the past fifty years that many stars do not die peaceful deaths. Stars having a mass seven or eight times greater than the Sun die in a huge explosion called a supernova. If we were within thirty light years of a supernova, a lethal dose of cosmic rays would be directed toward our planet. The first thing that would happen would be that our stratospheric ozone would be stripped away, allowing the ultraviolet radiation to destroy the life forms on earth and convert our atmosphere into nitrogen dioxide. In addition to this, super blasts of gamma rays would wash across the planet destroying any life form that might have survived. On February 28, 1997, such radiation was picked up from an explosion of star GRB 970228 which happened half way across the galaxy. The name given to this huge gamma blast is "hyper nova." The fact that nothing like this has happened in our neighborhood is a testimony to the fact that Earth is very unique in its isolation from such events.
Another star issue is the black hole issue. Astronomers now have a variety of different photographs of black holes, and massive amounts of indirect evidence that shows that these huge collapsed stars are common in space. A black hole does not have to hit Earth to destroy it--just coming close would produce gravitational forces that would reduce Earth to a pile of rubble. We seem to be located in a very rare part of space in which the presence of black holes is not an issue.
Galaxy Issues. We exist in a galaxy called the Milky Way. Our galaxy is one of billions of galaxies, and most of them are not like us. Eighty percent of all galaxies are elliptical galaxies, which possess virtually no interstellar material, so there is nothing to make a planet out of nor is there anything to make life out of. Some galaxies explode on a regular basis, making a stable solar system impossible. Not only is our galaxy unique, but even our location in the galaxy is special. Stars located along the equator of the galaxy cannot have stable solar systems because the gravity from the axis of the galaxy would pull any planetary system apart. Near the center of the galaxy there are massive radiation areas, probably caused by the black hole at the core of the galaxy that holds the galaxy together. No planetary system could exist in that region, because the radiation constantly would sterilize any organic compound that might exist.
Not only are the type of galaxy and our location in the galaxy relevant, but the interactions of the galaxies are also critical. We now see galaxies in space that are so close to each other that their gravitational fields work on each other disturbing stars and planets that might exist within the system. Such overlapping fields could throw a planet right out of its solar system leaving it homeless and in total darkness.
Some forty years after Shklovskii and Sagan wrote their book, numerous authors have pointed out that we are probably alone. Their point is that our knowledge since the 1950s has grown so much and we now understand so much more about how many factors have to be right to have a functional solar system that you can no longer postulate that life is common and abundant in the universe.
Skeptics will counter this argument by saying that maybe life is out there that is radically different from our kind of life. They would propose fire people, rock people, or some other fanciful form of life. This is a religious proposal in which the normal definition of life is ignored and something for which there is no evidence is suggested. We would suggest that as more and more variables become known as necessary for life to exist, it becomes less and less likely that life can be explained as a product of chance. The statement "The heavens declare the glory of God and the firmament showeth His handiwork" (Psalm 19:1) is more true today than it has ever been. Earth is not just unlikely; it is impossible by rote mechanical chance. We truly can "know there is a God through the things He has made" (Romans 1:19-22). --Reference: Natural History, May 2005, page 25.
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Does God Exist?, MayJun06.