We have discussed the importance of the size of a planet and also its distance from the sun. These two relationships govern the atmospheric composition and the temperature of the planet. They are critical for life on earth and any other planet.
It sounds like we are very very special and unique. Well, not really, I think. However, I also think we may never find out, so this may be just an interesting speculation for which no one can prove any position of belief they might have. But we can speculate on the possibilities and even the probabilities.
We know that many and probably just about all stars have orbiting planets. More and more are being found all the time. There are 1022 (that is 1 with 22 zeros after it, or 10,000,000,000,000,000,000,000) stars in our Milky-Way galaxy. That is a lot of stars and a lot of solar systems. Astronomers think that as many as 1/3 of them have a planet about earth’s size about the same distance from their sun. But lets assume that only one in a thousand have a planet that has conditions much like earth (same temperature and gravity and could have the same atmosphere) That lowers the number to 1019 Earth-like planets. But we are not finished. There are thought to be 1022 galaxies in the universe. That means the number of total “Earths” is now 1041 ! That is too long a number to write out. Now, how does life form? That is a scientific and for some a theological question. If given exactly the same conditions as Earth in which life formed, and if the chances of life forming were one in a million, then we would still have “life” on 1035 Earth-like planets with life of some kind. We are not alone. Make the numbers as pessimistic as you want and, unless you say ab initio that life can only exist on Earth no matter what, you must conclude life could exist elsewhere and that we could live there. We must also recognize that the Periodic Table of all the elements is the same everywhere in the universe. All planets everywhere are made basically of the same stuff. There are no new elements, different physics, or different chemistry anywhere in the universe.
What would life be like? Our DNA and all life from bacteria to plants to animals is a combination of the four bases that can be formed on a particular sugar backbone. That is simple organic chemistry. The devil (or his opposite) is in the details. But it is certainly possible that our, or some other chemical composition could form and create life. The odds for this to happen are overwhelmingly good. But it not at all clear that life anywhere else would resemble that on earth. It could be much more “intelligent” or very rudimentary, or something in between. SETI has been searching in vain for signs of intelligence for years. Personally, I doubt we will ever discover any and even if we do, the distances are so enormous that I doubt there would ever be any form of communication. Do I think there is life “out there”? Yes. Do I think we will ever benefit or be harmed by it? NO. I think we have our hands full here on Earth.