The Galaxy is Getting Crowded: What Does the Drake Equation Say About Kepler’s Discoveries?

NASA’s now-disabled Kepler Space Observatory

Unless you’ve been living under a rock the last month or so, you’ve no doubt heard the exiting news coming from NASA.  On November 4th, a team of scientists working with NASA’s Kepler data announced through a paper published in the journal Proceedings of the National Academy of Sciences, that there are 8.8 billion Earth-like planets in our galaxy.

That’s a whole lotta planets, with a whole lotta potential!

The new findings, which were the result of an in-depth study of data from NASA’s now-crippled orbiting telescope, the Kepler Space Observatory, tell us that in the Milky Way galaxy – our galaxy – which consists of some 400 billion stars of varying size and type, there are just shy of nine billion planets of comparable size to Earth.  All of which orbit stars of the same type as our sun, and reside in what’s now commonly called the Goldilock’s zone (not too close/hot and not too far/cold).

This is held, and rightly so, as an important and profound discovery.  Over the past several years, Kepler has been peering at an area of space containing approximately 42,000 stars.  Using data and images from that tiny slice of our galaxy, scientists looked for Earth-like planets orbiting sun-like stars and then extrapolated that data to accommodate the entire galaxy, resulting in the number 8.8 billion, with an error rate of less than 8 percentage points.

There are ten different methods for detecting extra-solar planets, most rely on effects that the orbiting planet has on its parent star, such as changes in the red-shift of the light emitted by the star, or in changes to the star’s brightness, thereby allowing scientists to deduce information about the planet in question.  Kepler uses, or rather used, the Transit Method for detecting extra-solar planets.  That is, it’s equipment measured the transits of planets orbiting distant stars across the plane of the star, which results in a regular but very slight dimming of the star’s relative brightness, which when detected, can be used to determine the position, orbital distance and size of the planet in question.

A graphic representation of the Transit Method of exoplanet detection

The Kepler project has been wildly successful, and the numbers above are really only the tip of the ice burg.  Scientists focused only on relatively small and dim sun-like stars, which are not at all common, whereas the data covered all types of stars.  Earlier estimates suggest that at least 15% of red dwarf stars in our galaxy may have Earth-sized planets orbiting in the Goldilock’s Zone.  Combining those numbers gives the incredible result of more than 40 billion right-size, right-place planets, and that’s just in our galaxy.  There are hundreds of billions of galaxies out there!

Of course, what does this all really mean?  So there are some billions of planets that, at least superficially resemble Earth, that doesn’t mean, by any stretch of the imagination, that there are that many planets that can support life (as we know it).  We know from our own solar system, and our study of Mars and the moon, that water, at least in our system, is or was at one time somewhat abundant.  And we know that water is crucial for the development and survival of life (again, as we know it), but we know nothing about the presence of water outside of our system.  It’s speculated that it probably does exist, in frozen form if not liquid, on at least some planets, however few, but that’s hardly a sure thing.

When it comes to life outside of Earth, we know very little.  Even though our knowledge is severely limited in that regard, it does seem somewhat naïve to dismiss the possibility that life exists elsewhere…the numbers are compelling.

As reported here, there are a few methods we know of for estimating the likelihood that extraterrestrial life exists.  In most cases, scientifically speaking, when one discusses extraterrestrial life, they mean, implicitly, microbial or simple life.  Others though are less selective in their thinking.  The subject begs to be discussed.  The most popular, if not the least reliable method for speculating on the likelihood of alien life, that is complex, intelligent alien life, is the Drake Equation.

Formulated in 1961 by the world renowned Astrophysicist Frank Drake, the Drake Equation provides a method for statistically considering how many intelligent civilizations might evolve within the Milky Way galaxy (N).  The equations is expressed as follows:

N= R* x Fp x Ne x Fl x Fi x Fc x L

To derive N, one must consider:

R* = the average rate of star formation per year in our galaxy

fp = the fraction of those stars that have planets

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

f = the fraction of the above that actually go on to develop life at some point

fi = the fraction of the above that actually go on to develop intelligent life

fc = 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

Early results, by Drake and independently by some of his colleagues, were quite low.  Drake originally cited N=10, and later calculations provided for wildly differing conclusions, ranging from -10 to 10,000.  The reason for this discrepancy is due entirely to the nature of the values involved.  All of the values, up until now, have been completely arbitrary and based on, at best, educated guesses.  Guesses that were extremely difficult to justify.  This fact prompted critics to cry foul, claiming that the equation was useless and that any result derived from it worse than speculation – due mainly to the fact that the public was/is likely to misunderstand the failings of the idea and thereby assign much more importance to the figures than was warranted.

Dr. Frank Drake

The complaint that nearly all of the variables in the equation are unknowns (with the exception of R*) has now been silenced, at least partly.  Thanks entirely to the work done by NASA’s Kepler Scientists, we now know (with acceptable margins for error) the true value of Fp, allowing us to calculate N with all the more accuracy.

When one works through the equation with this new information and using best known estimates for all other values, we find now that N=10,800.  This fits with Drake’s later calculations, as well as those of others in various fields who have endeavoured to legitimise the idea.  Of course, while the Kepler data serves to resolve what amounts to possibly the most important value in the equation, the results remain inconclusive.  Though one is required to remember that the Drake Equation was always only ever meant to provide an estimate of the prevalence of intelligent life in our galaxy, and to provide a starting point for further discussion and research.


Those with an affinity for astrophysics and/or statistics may find the following to be, well…awesome.  The brilliant fellows over at Onomaly Extraterrestrial Software have developed a smart phone app based on the Drake Equation.  With it, users can input their own values and watch as the application calculates the result graphically, showing elegant visualizations of the Milky Way based on whatever outcome the equation generates…and so much more.

If you have a smart phone, and the above interests you in any way…you have to get this app!

DrakeEQ is now available in the Apple App Store.

(This has NOT been a paid advertisement, I just find it to be worth sharing)

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