The Drake Equation has been around since 1961, https://en.wikipedia.org/wiki/Drake_equationand there have been some later extrapolations as well. The probability of there being life somewhat similar to Earth life somewhere else in our Milky Way galaxy seems to be strongly dependent on the equation factor fl = the fraction of planets that could support life that actually develop life at some point.This seems to me the most speculative of all the terms in the Drake equation. For example, the following article discusses the possibility that the Earth's unusual moon may have played an essential role in life occurring on our planet. http://www.scientificamerican.com/article/moon-life-tides/ I have unsuccessfully tried to find any source that discusses the probability of an exoplanet "that could support life" having an unusual moon like the Earth's. One way of describing the unusual nature of the Earth's moon is that it is the only moon in our solar system that has a larger angular momentum than its planet's rotation.Quite a few computer simulation models have been developed to explain the moon's origin. Can someone cite a reference that discusses the statistics from such models so that some ballpark estimate might be made for the probability that an exoplanet might have a moon like ours?