When we talk about the likelihood of aliens existing, we tend to think about how many millions and millions of planets there are and figure, well, shit, the odds are at least one of them nearby has green people on it we can talk to. But what we tend to totally discount is time.
And Sam Rockwell. He's so underrated.
Think about it; Earth was round for four billion years before humans appeared. Maybe there was a thriving alien civilization nearby, and maybe they mastered space travel and gave us a visit. And maybe all they found here were a bunch of big, dumb lizards. So it's not just that two intelligent, space-faring civilizations would have to arise physically close to one another, but they would have to overlap chronologically, too.
And without making this too depressing, the odds are overwhelming one or the other is going to go extinct before that happens. In keeping with their long-standing tradition of ruining everything for everyone, scientists came up with something called "The Doomsday argument," which is a statistical equation to determine the number of future members of the human race given a rough estimate of the total number of people born so far. Essentially, it is a formula that predicts the maximum number of human beings that can ever possibly be born. And according to the formula, humans will probably be dust long before we master interstellar travel, or by the time any aliens bother to roll up in their flying saucers and show their green faces.
Or before Steven Tyler has to sing a song about asteroids.
The equation basically says that human beings have a 95 percent chance of becoming extinct within the next 9,000 years, which in the grand cosmic scheme of things isn't a whole lot of time. If we want to meet some aliens, we need to hurry up and develop interstellar technology, because they sure aren't in any kind of hurry to contact us.
The Earth is relatively young compared to the rest of the universe, which has theoretically been spinning around in operatic blackness for billions of years. Statistically speaking, any number of the trillions of celestial bodies out there would have produced intelligent life millennia ago -- if they haven't mastered the science of awesome spaceships by now, they probably never will. And if our remaining 9,000-year estimate turns out to be correct, our window to do the same is equivalent to a tiny coiled butt hair on the epic universal timeline of existence.
You are exactly this insignificant.
Even if we do manage to achieve interstellar travel, the Doomsday argument has a "many worlds" application to account for the possibility of multiple advanced civilizations, meaning that any alien life with the presence of mind to build interplanetary flying machines is just as likely to have an equally finite lifespan. So there is a very real possibility that our historic first visit to another galaxy might yield nothing but a bunch of big-headed skeletons and about a zillion gallons of wasted rocket fuel.
The Rare Earth hypothesis, put forth by two scientists named Peter Ward and Donald E. Brownlee, suggests that since the development of life as it is on Earth was the result of a laundry list of geological and astrophysical events so cosmically random yet so crucially specific down to the smallest detail, it is ball-smashingly unlikely for a comparable civilization to have come into being anywhere else in the universe. That is, while some kind of bacteria or algae or cosmic mushroom may exist underneath some rocks on some far distant planet, the chances of there being another race of intelligent and industrious living things are about the same as you winning the lottery every single day for the rest of your life and then dying on the morning of your 200th birthday after getting struck directly in the face by Doc Brown's time train.
While breeding a Shiny Ponyta.
First of all, the position of a solar system is vital -- if it's too close to the center of the galaxy, everything will get melted by supernova radiation, but if it's too far along the edge of the galaxy it won't be able to support life. Then, the star at the center of the solar system can't be too old, too bright, or too big, otherwise complex life won't develop (complex life is very fussy). Finally, the planet on which said life develops has to be in a perfect orbit. In Earth's case, if the orbit was 5 percent smaller or 15 percent larger we would all freeze or burn to death, respectively. The size and location of our moon keeps the planet on a stable axis, preventing rapid and cataclysmic climate changes -- if we didn't have exactly one moon of the exact shape and size orbiting at its exact distance, we would all be superdead (and likely would never have existed to begin with).
The sequence of geologic eras even plays a crucial part -- if the Mesozoic had occurred after the Cenozoic, for example, the exact conditions needed for human life to develop might never have been met, upsetting the evolutionary order and resulting in a race of dinosaur humans.
Broadcasting "2 Girls,1 Cup" into space also hasn't helped.
Even the other planets in the solar system can have an effect. For example, Jupiter plays a huge role in keeping us all alive because it acts like a giant defensive lineman, blocking us from cosmic debris and world-ending asteroids like a celestial Olin Kreutz. There are innumerable other variables, all of which played a part in intelligent life appearing on just one planet out of an entire galaxy. The odds of every one of those things falling into place in the exact configuration necessary to duplicate both the existence and success of human beings are virtually nonexistent. Therefore, the fact that we haven't made contact with any alien civilizations is probably because there isn't anything out there to contact.