There are two schools of thought about whether earth is rare or common. One school puts together an equation with a small number of factors, multiplies by the number of eligible star s and comes to the conclusion that there are suitable planets everywhere. Another school of thought, called the rare earth hypothesis, puts in more factors before you conclude that earth really is incredibly rare. You could very well persuade yourself that earth is the only planet of its type in the galaxy. I've already persuaded myself of that, but other astronomers, i i don't think it's generally agreed to. What we're interested in is whether intelligent life can thrive all over the universe, or whether only an extremely limited areas like