ARE WE ALONE?
Dr. Frank Drake wrote a famous equation in the 1960s to help determine how many civilizations are in our galaxy, the Milky Way, in the here and now. This equation contained a very simple set of seven elements to determine this magical number. Some of these elements we know, most of them we do not. The only absolute certainty that we do know of at this time on the probability of life in the universe is the very fact that we, humans, are here. We are the only known proof that life, advanced life, and civilization-capable life exists. Rene Descartes once wrote: "I think therefore I am."
To check it out the Drake Equation simply search on the Internet or at your local library about Dr. Frank Drake. What I will do here is pull two important concepts from the equation and delve into those briefly. This should give you an excellent idea on where current theories are in determining life in the universe. First, let's assume for simplicity sake that life will evolve remotely similar to ours, requiring similar resources with the same restrictions our own life has, such as the need for water.
THE STAR OF A SYSTEM IS THE KEY
The first thing to determine is if there are any other solar systems capable of supporting life bearing planets in the universe. This is one major step shy from determining whether life and intelligent life is possible elsewhere. Our sun is the center of our solar system. It breathes life upon the planets through its cosmic energy. The earth itself was formed out of the same material that our sun is made of. In fact, we wouldn't be here if it wasn't for other stars exploding and dieing millions of years ago to spread their valuable material upon our corner of the universe. Analyzing stars is the first step in determining whether life in a system is possible.
Stars come in all sizes, temperatures, colors, and life spans. Each of these factors is highly deterministic upon on the others. That is, with one factor being a certain figure, for instance the size of the star, the temperature, brightness and lifespan are also determined. There is also a certain limit on the size of the star to keep all these other things in balance. Stars cannot form too small or too large. Every system in existence has some sort of upper and lower limit, so just think of it that way.
On the smallest end of the scale are stars labeled M-Type. If you've heard of M-Type planets like those found in Star Trek, that reference is different :). M-Type stars are cool, red, small stars. These stars are smaller because they do not need a large mass to keep their temperature and nuclear reactions in check. Also, just like the periodic table of elements where hydrogen is the most abundant, as well as the smallest and simplest atom, small stars are much more numerous in the universe. It is simply easier to make smaller, less complicated things than larger ones. The other end of the spectrum is very hot, larger, white to blue spectrum stars. Our sun is a nice average G-Type star.
We can discount the very hot stars right away because they do not live long as they burn their fuel in one glorious burst. Think of rowing a boat. You wouldn't get very far by putting all your energy into massive paddles. The net effect wears you out much too quickly. Rowing much slower and with smaller paddles will get you somewhere but it will take a very long time. It takes time for life to evolve, so the longer a star lasts the better. Since our sun is a G-Type star, it obviously has enough time to evolve life. The only problem with those very cool red M-Type is that the orbit of a planet has to be very close to get energy from the star and keep liquid water on the planet's surface. This is called the “Habitable Zone”.
THE HABITABLE ZONE
We've already proven planets exist around other stars, with over two hundred extra solar planets discovered so far. While binary star systems are problematic, there is still the chance for planets to form in them. What is important is where those planets are within a star system. How close or far away are they from the star, and in what kind of orbit are they in? Too close and water boils off the planet. It is simply is too hot like Mercury and Venus are. Too far away and you have a deep freeze, water is not sustainable in liquid form (except for rare occasions like some moons around large gas giant planets like Jupiter). Our planet is obviously in the right distance to our own sun, and Venus is almost within that distance. Mars is within that distance as well but is unfortunately too small to hold an appropriate atmosphere.
Study our moon's position one night. You will notice that the same side faces us perpetually. This is due to "Tidal Locking". When an orbiting body that is smaller exists close to a larger parent body, physics locks its rotation into place over time. Because of the closeness of planets in an M-Type star system they would tidally lock to their sun in a very short time. One face would forever face the sun, burning, while the other side of the planet would freeze. Whether or not this type of system could sustain life is not yet known.
It would be nice, however, since roughly eighty percent of the stars in the galaxy are these types of cool stars. It might be possible with enough carbon dioxide to smooth over plant-wide temperatures. Obviously there is more to the habitable zone that simply the planet’s distance in direct proportion to the star itself. I’d like to think of this habitable zone as having “soft boundaries”, depending on the planet’s makeup. Venus might just have been habitable if it wasn’t for its mysterious runaway greenhouse effect.
You might be surprised to learn that there is also a galactic habitability zone. We revolve around our galaxy every few million years. This also determines certain conditions of life sustainability, chiefly the amount of radiation our system receives and how "metallic" our system is (which determines if rocky bodies like our earth can form at all). Just think what we need, what we can't have, and where in the universe, in the galaxy, in the solar system, we would best be to have these elements. If all these conditions are met you have a chance, perhaps, of a life bearing planet that will spawn a civilization.
THE FERMI PARADOX - TOO MANY NECESSARY STEPS
There a few other factors to consider. Actually, there are many, so many in fact that the Fermi Paradox was created in order to describe the immense "luck" we have in our current situation in the universe, as well as the mystery of not finding anyone else out there. I will only briefly summarize some of these other factors, so please feel free to look them up to learn more about them.
First, even if a planet is the right size, distance from the parent star, it must also be in a circular orbit to maintain temperatures evenly. Planets must also rotate fast enough to produce a rotating molten core, which produces a magnetic field to protect against a stars radiation. Enough water must also initially exist on the planet to produce temperature moderating effects. Believe it or not, periodic meteor impacts are also possibly essential to life, as they "reset" the planet’s progress for another chance at developing different, perhaps better, life. Wiping out the dinosaurs gave us mammals a chance to evolve for instance, and eventually humans. I thank the dinosaurs for their sacrifice. Even the moon helps to stabilize our orbit and temperature, as well as produce life-bearing oceanic tides.
LIFE TAKES ROOT, NOW WHAT
Let's say that life actually evolves on a planet. This takes time. Single cellular life on our planet existed for a couple of billion years before multi-cellular, complex life evolved. There are arguments that this sudden emergence of complex life spawned rather quickly, fueling speculation about what actually caused it. The chief theory is that single cellular life had to change the atmosphere into an oxygen rich mix for multi-cellular to take hold.
Fortunately, it seems that itself is self-sustaining, a "ghost within the machine" so to speak. The greatest part about this phase of development in a planetary system is that once complex life grabs a hold of a planet current probabilities suggest that it will stay there for a long time, with the occasional resets caused by asteroids and other random astronomical phenomena. Once life grabs a hold of a planet, given the abundance factor of life on a planet, intelligent life will result. After that, in short order it is almost a given that a civilization will form, if the intelligent life form has at least a moderate physical capability of making tools.
WE MIGHT AS WELL BE ALONE
It is up to us, and other intelligent species and their civilizations, to take it from there. At no known point in the history of our planet has a species had the ability to control its destiny. Space may be our salvation for longevity. It will likely start with the mining of precious metals on mars and the asteroid belt. Then trade may extend to the outer solar system amongst the many gas giant moons. After that, who knows? Star Wars might become a reality, except for one last issue, distances beyond the solar system.
As is quite obvious, it takes light one year to travel one light-year. The galaxy itself is roughly 100,000 light years across. So, if a civilization was on one side and another civilization was on the other, it would take 100,000 years for one of the civilizations to detect the other's signal, and another 100,000 to respond, if they were looking to begin with. Will any civilization last that long to detect such a signal, and if they do will that sending civilization still exist, especially for another 100,000 years to hear the “hello”? Even if we reduce this to a miniscule 100 years, the space-time reality is challenging.
The one thing that should trouble our species is the apparent fact that no alien species have visited us yet. There are those that would say things like “they just don’t want to be detected”, or “the government knows…”, but that really is hog-wash in most likely scenarios. Any aliens that would go to the trouble of coming here wouldn’t be playing cloak-and-dagger games with potentially the only civilization they will ever meet. Perhaps they have visited us, it was just a long time ago, or they are on their way but will arrive a few thousand years too late...
OUR SPACE-TIME IS WHAT IS IMPORTANT
Can this conundrum be rather complex to figure out? Yes, mainly because most of the pieces of the puzzle we have to theorize. That is why we have a long way to go in understanding the basics of how life evolved, what is required to get it started, and the low probability that intelligent life forms will meet each other in the vastness of space. I wanted to explain all this to help you understand what our current thinking is about life in the universe, or at least in our own neighborhood.
Is there going to be a part of the universe that has dozens of advanced aliens running around at the same moment in history? At this point it is just as possible that we will discover alternate universes and meet aliens that way, or find a way around the light speed barrier. One thing to keep in mind with the "low" probabilities and so many events required to get where we are today, is that many of them may actually be very common. Large moons around inner planets, stable orbits and Jupiter's roaming in the distance may all be more of "just how it evolves" than the exception we see today.
I will let you be the judge of what is possible. Star Wars here we come! I just wish I could get that Fermi Paradox out of my mind...