The Universe
One star in billions,
One galaxy in billions,
A speck, but our all.
Light years away
life blooms and dies.
Inconsequential.
Once upon a time, the Earth was thought to be at the center of the universe. That was a long time ago, several hundred years ago. We're much wiser now. We know that the Earth is one of nine or maybe ten planets that rotate around the sun, and that our sun is just one of an estimated 400 billion stars in the Milky Way galaxy. We are less than a speck on the edge of a spiral arm of this unfathomable galaxy.
The immensity of the Milky Way is mind-boggling. Its diameter is estimated to be about 100,000 light years, and a light year (the distance light travels in a year) is about six trillion miles. The human mind cannot even conceive of such distances; nor can we conceptualize four hundred billion stars. To us, our little planet seems gigantic!
Our sun together with its Solar System is orbiting the Galactic Center on a nearly circular orbit. We're moving at a speed of nearly one million miles per hour. It takes about 220 million years to complete one orbit. Consequently, the Solar System has orbited the Galactic Center a mere 20 times since its formation about 4.6 billion years ago.
The Milky Way is a member of a cluster of thirty-three galaxies. The largest of these is the Andromeda galaxy, about 3 million light years away. The French astronomer, Charles Messier, first described this cluster of galaxies in the late 18th century. These galaxies are identified by their Messier (M) numbers. The Andromeda galaxy, for example, is referred to as galaxy M31. We can be proud that our galaxy is the second largest member of this cluster.
During the early part of the 19th century, British astronomers identified thousands of distant galaxies, most occurring in clusters. As the numbers of known galaxies increased, and more refined studies became possible, the American astronomer, Edwin Hubble, conducted studies that led him to conclude that all of the galaxies are moving away from one another, and that the universe is therefore expanding. This concept led to the Big Bang theory about the origin of the universe. It is believed by many scientists that the universe is about 13 billion years old. However, the origin and the ultimate fate of the universe are very much in question.
We now know that there are probably well over a billion galaxies in the universe, each consisting of billions of stars. Light from distant galaxies takes so long to reach the Earth, that with our most sensitive telescopes, such as the Hubble telescope, we are observing light that was emitted over 10 billion years ago, just a few billion years after the proposed Big Bang. Even though we can see them, we don't know that these galaxies still exist!
Like our own star, there are probably several planets rotating about each star in the universe. For the sake of simplicity, let's say there is just one planet per star, on the average. Then assuming at least one billion stars per galaxy, and one billion galaxies (undoubtedly a gross underestimate), we must conclude that the universe contains at least 1 billion x 1 billion planets. This equals 109 x 109 = 1018 planets. Then let us assume that just one in a million (106) of these planets possesses the right gravitational pull, a moderate temperature, and the correct elemental composition to generate conditions favorable for supporting life. This would require the presence of water, atmospheric gases and solid matter in proportions that don't differ greatly from those found on Earth. If so, then the universe contains 1018/106 or 1012 planets that can support life (1012 is one thousand billion, or one million million planets). Now let's assume that since life either arose on Earth or was seeded from outer space, the same could have happened on many of these other planets. Just to be on the safe side, let's further assume that conditions appropriate for extended life support occur on only one in one hundred of these planets. That still leaves us with 1010 or 10 billion (10,000 million) planets on which we can expect to find life. Such deductive reasoning leads to the conviction that the Earth is not alone in possessing a biosphere. There may be great biological discoveries out there waiting to be made. I have no doubt that outer space will reveal many novel and fascinating life forms. However, I'm afraid that although I'm a scientist with a very inquisitive mind, I don't have the time or the energy to travel even a few light years to reach such a destination.
Although we may never come to prove the existence of life in outer space, I find these calculations and considerations immensely reassuring. One of my concerns deals with the appalling rate at which we humans are destroying the biosphere on Earth. With an estimated 20 million species of living organisms on Earth, we are currently causing species extinction at a rate of about 50,000 per year. This rate, maintained over a 200-year period, will cause extermination of at least half of the living species on Earth, very possibly including our own. However, global warming alone is expected to result in the extermination of one quarter of all species on Earth within the next 50 years. So the rate of species extinction is likely to increase dramatically within the next few decades.
Because of my attachment to this home of ours, the destructive tendency of Homo sapiens distresses me. I feel tremendous frustration with the stupidity of mankind. However, when put in the perspective of the universe, who cares? We are just one life-supporting planet in 10 billion. When we destroy our environment and return it to the microbes, we will not have perturbed the universe at all. In fact, the effects of our actions will not even be noticed.
Rowan, L. and Stone, R. (2000). Portrait of a galaxy. Science 287, 61-91.
Galaxies - http://www.windows.ucar.edu/tour/link=/the_universe/Galaxy.html
The Milky Way Galaxy - http://www.seds.org/messier/more/mw.html
Stephen Hawking's Universe - http://www.pbs.org/wnet/hawking/html/home.html