# Is the Drake Equation an accurate way of finding the probability of life on planets?

I was interested in finding out if there was life on any planets or moons. I saw ReNiSh A R's question, and I saw the answer about there being life on Europa, but it didn't accurately answer what I wanted to find out.

The Drake Equation is on: http://en.wikipedia.org/wiki/Drake_equation.

I have also heard that there could possibly be life in the upper atmosphere of Jupiter, which could have floating beasts, and also on Titan, which does have surface liquids, but I am pretty sure most of it is in solid form.

If the Drake Equation is equal to a low number, then why is it pretty common if there is a possibility of life on 2 moons and 1 planet? Does that make the Drake Equation a non-reliable source?

I'm also looking for an accurate answer, so can you please give it to me in detail!

• Let's see we can address you instead of giving just an answer: what is the Drake equation used for? – Py-ser Oct 28 '14 at 4:45

The Drake equation is little more than a randomly assembled guess of relevant factors for intelligent life, and any use of it requires a number of hard to justify guesses for the various probabilities.

It is otherwise an attempt to give a foundation for making Fermi estimations on the number of civilizations, and the associated Fermi paradox. A Fermi estimation is an order of magnitude estimate. It doesn't try to get a particularly accurate estimate, it only tries to tell you how many digits you can expect the answer to have. And the Fermi paradox is simply stated as "There is an absurd number of stars. So absurd that finding extraterrestrial life should be easy. But in fact it is hard. Why?"

The resolution to the paradox is the second sentence: it assumes we have reasonable basis to suppose life is easy enough for the sheer number of stars to give us lots of observable life. We do not. The Drake equation is in one sense a way of singling out as many factors as possible so we have a concrete goal to figure those factors out, and so the entire problem.

But mostly it is used by pop science and sensationalists to rile us up with lines like "there should be thousands of civilizations more advanced than ours in our own galaxy!" and "there could be life in Jupiter's atmosphere." These things have no solid science behind them, and are just attention grabbers dressed in pseudoscience.

Because the Drake equation has so many variables that have to be supplied out of whole cloth, there is no way that one can say it is accurate. As we learn more about the cosmos, we are getting a better range of values for some of those variables though. We know fairly accurately (read: within an order of magnitude) how many stars there are in the Milky Way. We are getting more refined estimates of what percentage of them have planets, and it is higher than anyone would have guessed previously (the highest estimate I saw before we started using doppler effects to find planets was 10%, but now we see ~22% of surveyed stars have planets). We also know that stars which have planets almost always have more than one. We also seem to find life pretty much everywhere we look on earth, but have still found none beyond our planet. Still, I think most educated guesses nowadays put this percentage in the 90-100% range.

I think the better way to phrase this question is, is the Drake Equation a reasonable way of estimating the probability of finding intelligent life? Since 1961 when it was first proposed, have we found any reason to call into question any of the assumptions Dr. Drake made? I think it is reasonable. I think the greatest room for modification is in the definition of "technology that releases detectable signs of their existence into space" and what signals we search for. But the variable itself is broad enough to encompass whatever we might come up with.

One further variable might be added though: Percentage of likelihood that we would recognize an alien signal for what it is.

EDIT: We could potentially find signs of life without receiving technologically created signals. During stellar transits we have the opportunity to examine the spectroscopy of their atmosphere and check for methane and other organic gasses that are unlikely to have formed without life, but I think we'll have to find life on Europa (where we know there is methane) before that test will be commonly accepted.

The Drake Equation is a formula for gestimating the number of intelligent extraterrestrial civilizations we might be able to detect, which will most likely be a small subset of planets in the universe that host lifeforms.

Let's see if we can break this down a bit. From the Wikipedia article:

The Drake equation is:

$$N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L$$

where:

$$N$$ = the number of civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);

and

$$R_{\ast}$$ = the average rate of star formation in our galaxy

$$f_p$$ = the fraction of those stars that have planets

$$n_e$$ = the average number of planets that can potentially support life per star that has planets

$$f_{\ell}$$ = the fraction of planets that could support life that actually develop life at some point

$$f_i$$ = the fraction of planets with life that actually go on to develop intelligent life (civilizations)

$$f_c$$ = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

$$L$$ = the length of time for which such civilizations release detectable signals into space

When Dr. Frank Drake first proposed this formula for estimating potential SETI signals in 1961, very few, if any, of these factors had enough reliable data upon which to form a good estimate. In the intervening decades, astronomers have done much research on the process of star formation. Also in recent decades, a great number of extrasolar planets have been discovered; the massive amount of data returned from the Kepler space telescope is especially helpful when estimating $$f_p$$. In addition, the developing theory of the Goldilocks zone, the orbital band in which an exoplanet can have water in liquid form, gives us a perspective on the third factor. Thus we are beginning to get some idea of the magnitude of the first three factors.

The last four parameters, however, are entirely speculative. As we only know of one planet which supports life, Earth, and know only one intelligent civilization in the universe, our own, we have insufficient data upon which to form any reasonable conclusions. The current estimates for each factor are summarized in the Wikipedia article.

I think it's important to note that finding life and finding intelligent life are 2 different things.

We might expect there to be unintelligent life on a significant number of planets, but the number of factors to create intelligent life are exponentially less plausible than just creating life.

We must also factor in the very real possibility that any life intelligent enough to discover E=MC2 will quite probably destroy themselves.

The cosmic window of any intelligent life could be very small, and the chances of any 2 planets with intelligent life at the same time could be remote.

Although homo-sapiens have been intelligent for a few thousand years, we've only really discovered how to send our message into space in the last 50 or so years. There is a good chance that our species will die out in the next couple of hundred years (global warming, nuclear threat, super-bacteria, etc). 250 years is not even close to a blink of an eye in cosmic terms.