N ~ 1: Alone in the Milky Way
The Search for Extraterrestrial Intelligence (SETI), as well as popular sci- fi culture, often premise that the number N of advanced civilizations in our Galaxy is large, of order ~103-104 or more. A large value for N justifies SETI efforts targeting even nearby stars. A large N, however, is at odds with observation, hence the Fermi Paradox.
This study reevaluates N on the basis of latest knowledge of the different factors affecting its value, using available data only and avoiding a priori biases. To evaluate N, we use the Drake Equation in the form: N = R* fp ne fl fi fc L, where R* is the average rate of star formation in the Galaxy, fp the fraction of stars with a planetary system, ne the average number of environments suitable for life within planetary systems, fl the fraction of planets suitable for life on which life actually emerges, fi the fraction of planets with life on which it becomes intelligent (creates new technology), fc the fraction of planets with intelligent life on which it becomes an advanced civilization (capable of, although not necessarily engaging in, interstellar communications), and L the average longevity of advanced civilizations. Each term is evaluated based on analysis of current astronomical, geological, and historical data. This study results in the adoption of likely values for R*, fp, ne, fl, fi, fc, and L of 20, 0.5, 1, 0.5, 2×10-4, 0.1 and 104, respectively, which yields N ~ 1, i.e., a small number. Large uncertainties remain, of up to 1 order of magnitude for R*, fp, and ne, and possibly several orders of magnitude for fl, fi, fc, and L.
N ~ 1 would imply that intelligent societies in our Galaxy, and by extension in any galaxy, are likely few at any given time. In the Milky Way, we might be it. While life itself is likely common across the Galaxy, especially in primitive form, including possibly within our own solar system, intelligent societies may be exceedingly rare. The Fermi Paradox is resolved – indeed dissolves – if N is small, consistent with observation to date. SETI efforts should focus on extragalactic searches for greater chances of detection.
