I think it’s now increasingly likely the nearest stars to our Solar System could contain rocky crust planets. Find one where there is water vapor in the atmosphere and there is a good chance some form of life exists on that planet.
Water vapor is all over the place. Find elemental oxygen in the atmosphere. Elemental oxygen is the 2nd most reactive substance in the universe; without photosynthesis, it’s certain to have reacted with *something* on the planet.
Maybe. Maybe not.
We now know that complex organic molecules are common throughout the galaxy. Apparently, all it takes to create them is:
1. The necessary elements;
2. An energy source;
3. A turbulent medium (either liquid or gas) that will contain the elements and energy so they can react with each other; and
4. Millions of years.
The theory is that with more millions or billions of years the complex organic molecules will somehow evolve into living organisms. The problem is that no one has ever actually figured out how to convert complex organic molecules into even the simplest living organism.
It is possible that, given enough time and the right conditions, life will eventually evolve from organic molecules. It is also possible that the evolution of life on Earth required an infinite number of trillion to one occurrences that have never or extremely rarely all occurred anywhere else.
It is like the old infinite monkey theorem: Given enough time and an infinite number of monkeys banging on typewriters, one of them will eventually reproduce all of the works of Shakespeare. The universe of planets could be the infinite number of monkeys banging on typewriters and Earth could be the only one that resulted in life.
If we actually find even simple living organisms on Mars, Europa, Titan, Enceladus or anywhere else in the solar system that appears to have the necessary mix of conditions, then it is likely that life is common throughout the universe. If, however, it turns out that the rest of the solar system is sterile, then it is possible that the rest of the universe is just as sterile.