The Overwhelming Probability of Life Elsewhere in the Universe

One of the greatest mysteries of the universe is whether or not there is life -- in particular, advanced forms such as ours -- elsewhere in the universe. The discovery of another planet with an advanced civilization on it would be the greatest discovery ever made on Earth. It would, in fact, be difficult to imagine what its consequences would be. This may not happen in the next few years but there's no doubt that it will eventually happen. (It will be difficult for us to communicate with them because of the tremendous distances involved.) The reason we are so sure of this comes from the data that the spacecraft Kepler  has collected and is still collecting. It's object is to find Earth-sized planets around some of the nearby stars and determine as accurately as possible how many of the billions of stars in our galaxy may have Earth-like planets. And it has succeeded beyond our wildest dreams. As of January, 2012, it has discovered 2326 candidates, with 207 of them are similar to Earth in size.(There's no doubt that this is a monumental discovery.)
     For life to arise on a planet we not only need a planet similar in size to Earth (mostly because of problems with gravity) but we also need the planet to be in the ecosphere or life-zone of the star. In addition, the star should be similar to our Sun. Of the 207 Earth-like planets discovered, 48 appear to be in the life-zone of their star.
     It's important that the planet be in the life-zone because water would be in a liquid state most of the time in this zone -- and this is critical. There's no doubt that life needs water to survive. but would life form naturally if water was there? If there was an atmosphere composed of the proper chemicals it would (methane and ammonia would be needed, but they are very common). Scientists showed many years ago that with the proper atmosphere, proper temperatures and liquid water, a very elementary form of life would evolve naturally. From there it would no doubt evolve to a higher form of life. It would take a long time -- millions of years -- but there's no doubt that it would occur. After all, the universe has been around for billions of years.
     With only 48 good candidates, you might think that the probability of it occurring in them is pretty low -- and indeed it is. But with the data that the Kepler team has, they have been able to show statistically that 5.4 % of all stars in our galaxy have Earth-sized planets. Furthermore, they showed that 17% of all stars have many planets orbiting them. And with over 200 billion stars in our galaxy that gives a lot of Earth-sized planets -- about 10 billion. For most people, 10 billion is hard to visualize so I'll give you a simple picture. It's about equal to all the grains of sand in all the beaches on Earth. I think that should convince you that it's a lot.
     If only a tiny fraction of these planets -- say, one in hundreds of thousands -- had a planet similar to Earth -- we'd still have millions of planets out there like us.
     And this isn't the end of it. So far I've only talked about our galaxy -- the Milky Way. It is only one of billions of galaxies in the overall universe. There are, in fact, galaxies as far as we can see, and we know we're not seeing the end of the universe. We may only be seeing a small fraction of it. So, needless to say, it's a big place with an incredible number of planets similar to Earth. On the basis of this, what's the chance we are the only advanced civilization in the universe? I would say it is zero, or very close to it. As hard as it may be to believe, there has to be millions (and possibly many more) of civilizations in the out there similar to us.