This time there were very few questions which most of the class got wrong, so that’s good. Tests are good for identifying things that I am not teaching well. I learned from this test that I need to go back over the file drawer problem. That problem does not affect the conclusions of published studies reporting positive results. It affects the conclusions of meta analyses which survey the literature to look for general patterns. If negative results are not being reported, it is hard to properly assess whether the positive results are real — or just flukes.

I also asked about Dean Larson’s discussion of the recent estimate of the number of earth-sized planets in the Universe. In his slides, he gives the figure of 17 billion in our Galaxy. He and I discussed this in class – if there are 100 billion galaxies, then that means 17 billion x 100 billion earth-size planets in the Universe.  

One of my post-docs said it was unfair to ask about numbers, but in this case I think it really matters. There are evidently a staggering number of earth-size planets in the Milky Way, and a staggering number of galaxies. The chance that life exists elsewhere is going up and up the more we learn. It’s a very fast evolving area of science with profound implications for the way we think ourselves in the universe. Just how special are we?

In fact it’s such a fast moving area of science, the Dean’s figure (from March of this year) is now apparently outdated. The latest estimates, released yesterday, are of 40 billion habitable earth-sized planets in the Milky Way (= 40 billion x 100 billion in the Universe). So the best estimates of the chance that we are alone became even smaller – during the time this class has been running.