Interactive Drake Equation

BBC Future has provided us with a colorful application for twiddling with the variables in the Drake Equation. The Equation is a thought experiment meant to give a very rough idea as to the number of inhabited planets and technological civilizations existing within the Milky Way galaxy. This version is divided into four sections, allowing you to see how many potentially habitable planets exist in the galaxy, how many are inhabited, etc.

(This is only an image. Go to BBC Future for the application.)

  • UrsaMinor

    Sadly, there is still simply not enough real-world data for this to be anything more than an amusing game. At best, it reveals your prejudices about the way you’dlike the universe to be.

    In our current state of knowledge, we can probably bookend the number of planets in the galaxy physically amenable to our sort of carbon-based life within fairly broad but solid limits, but we still know zero about the probability of life actually arising on any of them. Every statement about abiogenesis is hypothesis or handwaving until we have a sample size larger than one.

  • http://theotherweirdo.wordpress.com The Other Weirdo

    I’ve read about this equation years ago and basically it’s a wild guess based on wishful thinking, especially the parts we have no hard data about. And besides, why has the number of habitable planets per solar system been pegged at 2.5 when our own solar system, the only one we really know anything about, has just the 1?

    • UrsaMinor

      Data from physics and astronomy provide these estimates. I don’t find the number to be unreasonable. We have a good idea of the limits of solar energy that permit planetary conditions compatible with liquid water (between roughly 0.53 and 1.3 times the amount that Earth receives from Sol; there’s quibbling about the upper and lower boundaries, but the broad range is known), and every day we are getting better statistics on the way that planets are arranged around their host stars. Combining the two pieces of information allows you to arrive at the average number of planets in the habitable zone of an average star. Incidentally, the number for our own solar system is currently 2, not 1. Mars lies in the habitable zone, albeit towards the outer edge.

      Of course there’s more to it than just stellar insolation. The size and composition of the planet are major factors as well. Luna receives exactly as much solar energy as Earth, but it’s a barren rock because it’s too small to generate or hold onto an atmosphere. Mars’s surface isn’t currently habitable as far as we know (although there is a real possibility of subsurface environments suitable for microbial life), because it has a very low mass, low escape velocity, and low outgassing rates. Consequently, most of the atmosphere has either escaped into space, combined with silicates to form rocks, or frozen out on the ground because of the waning greenhouse effect caused by a combination of the removal of atmospheric mass by the first two processes and an outgassing rate that can’t keep up with the losses. If it were as massive as Earth with an active carbonate-silicate cycle due to sufficiently high levels of radioactive decay in the core to generate enough outgassing, it would probably have a thick atmosphere dominated by carbon dioxide, a substantial greenhouse effect to warm the surface, and liquid water oceans. Complex multicellular life would be possible. But the game of planetary billiards during the solar system’s formation left it with insufficient mass.

      Sol is also a dynamic object. If you factor time into the equation, Venus was once in the habitable zone before Sol’s luminosity increased enough to trigger a runaway greenhouse effect, and Mars may have initially been outside the HZ in the early days of the solar system. In this sense, you would be somewhat justified in saying that our solar system has 2.5 potentially habitable planets.


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