The Drake equation is the estimate for the number of technological civilizations that may exist in our galaxy. Astronomer Frank Drake came up with a list of specific factors that everyone agrees are essential to the development of intelligent life. The Wiki entry is pretty good and worth reading if you have interest in these things. If you want something a little more casual, space.com has a nice article on it. The Drake equation is pretty much all the alien hunters have at the moment, given that we have zero evidence of life beyond this planet.
The reason for that is a mystery. In fact, it has a cool name as well. It is called the Fermi Paradox. There are billions of stars in the galaxy. The math says there should be millions with planets similar to earth and capable of life. That’s the paradox. The math says there should be lots of earth like planets teaming with life that has evolved for a lot longer than life on earth. Yet, as far as we know, we are alone in the universe, but we keep looking.
This story the other day is interesting.
Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultracool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star. The new results will be published in the journal Nature on 2 May 2016.
A team of astronomers led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope  to observe the star 2MASS J23062928-0502285, now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth . Detailed analysis showed that three planets with similar sizes to the Earth were present.
A light year is roughly 5.9 trillion miles so these planets are roughly 240 trillion miles from earth. To put that into some perspective, let’s pretend there is intelligent life on one of these planets. They decide to let us know they are there by using a light signal of some sort to send Morse Code. By the time we received the signal and decoded it, most of the people who sent it would be dead. By the time they got our reply, they would all be dead and most the people on our end would be dead.
Traveling to these planets would be impossible for humans. The fastest space vehicle we have is the upcoming Solar Orbiter that NASA plans to launch in 2018. It will travel at 450,000 miles an hour. If that were configured to haul humans, it would arrive in the vicinity of these planets around the year 62,899. Our astronauts would not even be dust at that point. Even assuming we can build a vehicle to reach something close to light speed, we’re still looking at having geezers showing up to the alien planet.
The other side of this is that the alien planet could have a species that has solved these technological problems. They have the ability to reach speeds in excess of light and the ability to survive in deep space for extended periods. The challenges of interstellar space travel are many orders of magnitude more difficult than anything we understand. That would most likely mean they are vastly more advanced than humans in every way.
The size of the technological gap between us and them would be something similar to modern humans and australopithecines . Our technology is amazing to us, just as sharp sticks were amazing to Australopithecus. To the people able to conquer interstellar travel, our technology would be the equivalent to the sharp stick. They will do things we cannot imagine doing, much less understand doing.
One of those things, most likely, will be the ability to conceal themselves from us. Interstellar travel will require manipulation of matter on a grand scale. Long before they figured out the Warp drive, they will have figured out how to hide from our level of technology. We’re getting pretty good at hiding from radar and the visible spectrum. Our alien visitors will certainly have expanded this ability into most of the electromagnetic spectrum.
Of course, the ability to control matter at the scales required of Warp drives means they would have the ability to control living matter as well. We like to think we’re complicated and by the standards of earth, we are complicated. By alien standards, we are single cell creatures in a water bath. They would be able to manipulate us just as easily as we control ants and roaches. For all we know, earth is just a really big terrarium anyway.
Then you have the evolutionary issues. Humans today are nothing like the humans of 200,000 years ago. Imagine what we will be like in 200,000 years. Intelligent life in a million years could very well be microscopic organisms living in silica. What if Hawking is more right than he knows and the future of intelligent life is at the smallest of small scale? That means our alien visitors could very well be a dust storm or the single facet of a snowflake.
The point here is that we are alone in the universe, as far we know and as far as we will ever know. By the time we can know otherwise, we will not be us. By the time the aliens can show up and set us straight, they will be so far advanced compared to us we will not be able to detect them anyway. For all any of us can know, we’re just a science experiment for some distant race of life. Earth is a terrarium sitting on a kid’s desk. Regardless, we are alone in this universe and we always will be alone.