Let's reevaluate the Drake equation by analyzing each term separately. R, the rate of star formation in the Milky Way per year, is indeed currently about 1 — astronomers are quite sure of that. In fact, astronomers have recently determined that stars formed at a higher rate several billion years ago, when the stars that might now bear intelligent life were being born. So a value of R = 3 or 5 is more realistic.

How Many Planets? f_p

The second variable is f_p, the fraction of stars that have planetary systems. Discoveries that many or most young stars are surrounded by planet-forming disks, and detections of some 3,000 actual or likely planets orbiting Sunlike stars (as of 2012), confirm what astronomers had already suspected: planets are common.

So-called "protoplanetary disks" are routinely detected by infrared observations and are seen directly in, for instance, Hubble Space Telescope photographs of the Orion Nebula, one of the most prolific star-forming regions in our part of the Milky Way. Submillimeter-wave observations have shown much more tenuous dust disks around many older stars, including Drake's first target, Epsilon Eridani. Many of these disks are doughnut shaped, with central holes apparently swept clear by planets accreting the disk's gas and dust. Some such planets have even been directly imaged. But the vast majority of the evidence comes from the gravitational wobbles that orbiting planets induce in their stars, and transits of planet-sized silhouettes across star's faces.

So, what fraction of stars have planets? As of 2012, astronomers finally have a solid answer: nearly all of them. And small bodies like Earth are more abundant than the more easily detected giants. This age-old question has at last been settled. So, f_p is large and is certainly not a bottleneck in the Drake equation.

How Many Good Planets? n_e

The news is also good when we turn to the equation's next term, n_e. This is the average number of worlds in a typical solar system that have environments suitable for the origin of life (the "e" stands for "Earthlike"). In his 1992 book Is Anyone Out There?, Drake recalled that the participants in the Green Bank meeting concluded that the minimum value of n_e lay between one and five. In other words, every planetary system was expected to contain at least one minimally Earthlike place (defined as where liquid water is possible), and that there might easily be three, four or five hospitable worlds per system.

That optimistic view was based on the assumption that our own solar system is typical. Today Mars and Jupiter's moon Europa are being considered as possible sites of early biology, making three possible "Earths" (by the Drake-equation definition) in our solar system. And indeed, the extrasolar planetary systems being found as of 2012 indicate that our solar system's basic setup is not some kind of rare fluke. Rocky worlds with liquid water on their surfaces should be pretty common.

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