"We are not alone- is certainly a statement to be reckoned with. Up until only the last two or three decades, the idea that there was life on other planets in our solar system than that of our own wasn't all that tolerated. Although we as humans have never truly accepted the idea that it is only on the earth that life has occurred, we can't help but be skeptical. From earliest times when we peopled the heavens with gods, goddesses and supernatural beings, through our space exploration activities of the 20th Century, we are driven to decide the question: "Is there life on other planets?-.
After reading the handout I believe there is enough evidence present to show that life on other planets is statistically probable. Science itself posts the greatest evidence when discussing the likelihood of "higher life- organisms and habitats. Harlow Shapely gave a rather just explanation and conservative guess as to why he believes there is a possibility of other planets in the solar system. He goes on to explain that life can't possibly exist on stars because of their extreme temperatures. He hypothesized that every star in a thousand has a planetary system and that perhaps one in a thousand systems has a planet with just the right temperature tolerances for life. Then one out of a thousand of these planets have sufficient quantities of air and water to transform organic molecules into protoplasm. Overall, using these figures, its estimated that even if only one star in a trillion meets all these requirements for life, the total number of stars in the universe is about hundreds of billions so the number of planets that actually have the suitable conditions to sustain life turns out to be a hundred million. Of course the Drake/Sagan formula gives it a more scientific approach and shows that there's quite a number of planets in direct competition with our own. As far as causal reasoning mistakes are concerned there don't seem to be too many.