Only a self-centered fool would believe that we are alone in the Universe. We live in the Milky Way Galaxy, which is only one of millions of galaxies in the observable Universe. The Milky Way contains about 200 billion stars according to Astronomy magazine. Our Sun is nothing more than a star. At the galactic center of our galaxy is a super-massive black hole, devoid of life. So let's say that the 200 billion stars in the Milky Way may or may not have life on planets orbiting around them. To calculate life, I will use the one-percent galactic-life rule.
One percent of the 200 billion stars stated above will show that our galaxy has 2 billion stars with planets capable of supporting life. At one planet per star, that would be 2 billion planets.
One percent of 2 billion planets will show that our galaxy contains 20 million planets that do support life. That could be anything from bacteria to beings living in huts.
One percent of that total will show that we have 200,000 planets with intelligent life. These life forms would be on a higher level living in towns and practicing agriculture.
One percent of that will show that we have 2,000 planets with advanced-intelligent life. These beings would live in cities and towns with a diverse society. Inventions would come at a fast pace enabling its population to constantly improve itself.
Finally, one percent of that will show we have 20 planets with life capable of space travel. These life forms have successfully entered space. Some maybe only competent of orbiting their own worlds. Others maybe skilled at exploring far-off stars in the Milky Way and beyond. Maybe even visiting Earth.
This simple equation, the one-percent rule, shows unequivocally that the Milky Way has at least 20 planets with life capable of space travel. That is a small number when it is compared to the millions of other galaxies that we can see with our telescopes. With trillions of planets orbiting billions of stars in millions of galaxies, there must be life else where in the Universe. To think that Earth has the only life out of millions of galaxies is nothing less than delusional.
Reference: July 2011 issue of Astronomy magazine, page 23.