Carnival of Space 625

The Carnival of Space 625 is up at Cosmoquest.

Richard Lawn of SETI Home is revisiting the Drake Equation.

Nearly everyone interested in SETI has heard of the Drake Equation, but views of its utility vary widely. The equation is a useful way to estimate the number of technological civilizations in our galaxy and the chances of detecting an extraterrestrial message. Others view it as a wasted effort, given the huge range of conjectures involved in its components. There is a middle ground to use it to update and assess the reliability of relevant data that we have and the ways to improve upon the uncertainties. It can certainly be an effective tool for stimulating curiosity on this subject. Frank Drake presented the Equation in 1961 at what may have been the first formal SETI conference with the intent to stimulate discussion and evaluate proposed research, not to arrive at a true estimate of the number of intelligent ETs whose signals we could detect by multiplying its components.

The Drake Equation is:

N = R* x fp x ne x fl x fi x fc x L where:

* N is the number of intelligent civilizations in the Milky Way Galaxy, who, for this exercise, emit radio, light or other transmissions that are detectable from afar.
* R* is the rate of star formation per year for the galaxy.
* fp is the fraction of those stars with planets.
* ne is the average number of planets capable of supporting life (think of e for “earth-like” or “ecologically fit”).
* fl is the fraction of those that actually develop life.
* fi is the fraction of those where life becomes intelligent.
* fc is the fraction of emitting detectable signals into space.
* L is the lifetime of a communicating civilization.

N is usually considered the number of communicating civilizations in just our Milky Way galaxy. Keep in mind that there are hundreds of billions of other galaxies in the visible universe. The factor R* is estimated to be about 1 by some astronomers, based on the current rate of star formation in our entire galaxy. But the rate of star formation was once higher, and dividing the number of stars in the galaxy (200-400 billion) by its estimated age (roughly 10 billion years) would yield an R* closer to 10 per year, the number I will choose to plug in.

It’s probably safe to plug in the estimate of fp = 0.5. (number of stars with planets is 50% or higher)

Richard’s personal tally for all of the Drake factors is 0.0625. L is the lifetime of communication civilization. By our definition of a technological (transmitting) civilization, we have been around for about 100 years. How sanguine are you about the future of homo sapiens? He personally can’t come up with a more precise conjecture than between 500 and 5 million years.

Richard’s current guess for N (number of intelligent civilizations) ranges from 31 to 310,000.

3 thoughts on “Carnival of Space 625”

  1. One of the major issues with the Drake Equation is that interstellar colonization and migration is not taken into account. In theory, the majority of communicating extraterrestrial civilizations (which is what SETI is looking for since aliens with stone age technology can’t communicate) could have branched off from a single civilization that was spawned on a completely different star system. Also, how would variable L figure into a group of civilizations that branched out from a common ancestor civilization. If the average life span of communicating extraterrestrial civilization was, let’s say, 10,000 years, but for most of that time was spent sending out colony ships, and those colonies sent out colony ships of their own and, by the time the original civilization fell, it has spawned 5 or 6 generations of civilizations?

  2. I think the point is that we don’t have the science to get a result for this equation.
    At this point the result is not a useful thing.

    But the structure of the equation itself is useful. It tells us where it might be a useful parameter to investigate. Until recently we had no idea about
    fp (fraction of stars with planets) and
    ne (number of life supporting planets per star (ie. liquid water to a first approximation (and we only have a first approximation))).

    But recent advances in astronomy have pushed fp from ??? to (.5-0.9) which is both a lot higher than earlier guesses, and a good enough guess that (for the purposes of the Drake eq.) we don’t need to look any harder at that and should concentrate on other factors.

    Likewise, probes to the other planets have pushed the number of planets with water from ????? to “maybe 4 to 6 per star” (Our solar system having, we are fairly sure, liquid water on Earth (OK, we are REALLY sure about that one) Mars (very likely) Mercury (surprise!) Eris, Ceres, Io, Europa and possibly even Pluto and Charon.

    Once again, far higher than earlier guesses and good enough to move on to other factors.

    fl is likely to be the next number to be nailed down. Either there is alien life on Mars. Io, Europa etc. OR fl is much lower than xenobiologists currently believe.

    The remaining factors are likely to be solved in reverse. Either we encounter alien civilizations in which case we give fl, fc and L high values. Or we don’t and they’re low

  3. How dependable is the result of an equation when the state of many of the variables are completely unknown and likely to remain so for the foreseeable future.

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