Response to the August 12 post about Fermi’s Paradox was heavy, an indication that the physicist’s famous question — Where are they? — will not go away. A number of readers asked for background on my statement (drawn from Milan Ćirković’s paper) that the average age of Earth-like planets in the Milky Way is now thought to be 6.4 billion years, an indication that there should be planets that have had a two billion year head start on Earth in terms of evolving intelligent life.
The number 6.4 billion is a broad estimate, to be sure, but it has been the subject of intense investigation by Charles H. Lineweaver and colleagues. Lineweaver’s key paper “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect” ran in Icarus Vol. 151, No. 2 (2001), pp. 307-313 (available here in PDF form), and was followed by a paper in collaboration with Yeshe Fenner and Brad Gibson titled “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.” The latter ran in Science Vol. 303 (2004), pp. 59-62, and is also available as a PDF online.
Both papers, and particularly the earlier study, are worthy of more time than I could give them in the August 12 post. Look for a fuller discussion of each on Centauri Dreams in coming weeks.