How Rare is *Intelligent* Life in the Galaxy?

The Current State of the Drake Equation and the Chances for Intelligent Life Beyond Earth

Photo by Brooke Lark on Unsplash

Elon Musk’s SpaceX just sent a Tesla Roadster into space in February. It will travel almost 250 million miles into space before shifting into an orbit around the Sun. And yet such an extreme distance is nothing compared to the cosmos as a whole.

The scales of time and distance when it comes to astronomy boggle the mind.

Our nearest neighboring star, Proxima Centauri, is 4.243 light-years away. If you send a laser beam of light or radio or TV signals toward that star, the fastest-moving things physics allows — at 186,000 miles per second — , it will take over 4 years to reach the destination, and, potentially, to be detectable by any alien life that might live there.

Photo by Bryan Goff on Unsplash

The universe is unfathomably huge. It is at least so large that our most powerful space telescope can see galaxies almost 30 billion light-years away (

The current number for the radius of the observable universe is about 46.5 billion light-years. ( Lineweaver, Charles; Tamara M. Davis (2005). “Misconceptions about the Big Bang”. Scientific American.)

There may be as many as 2 trillion galaxies in this area, each containing an average of 100 thousand million stars.

(Credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team)

The Fermi Paradox

In 1950, physicists Enrico Fermi and Michael H. Hart followed this logic:

  • Since there are billions of stars in the Milky Way like our own Sun,
  • And many of those are billions of years older than our Sun,
  • And since there is a good chance some of these stars have might have Earth-like planets,
  • And if, like Earth, some have evolved intelligent life,
  • Then some of this life has developed space travel, and possibly interstellar space travel,
  • So even at the currently “slow”pace of space travel our current technology allows, one of the civilizations could probably traverse the Milky Way is a few million years’ time.

Given all of this, the Fermi Paradox asks why haven’t we seen or heard from anyone else? If there is alien life out there, intelligent alien life, why haven’t they come calling?

It wasn’t a brand new question, even then. Russian rocket scientist Konstantin Tsiolkovsky posited a similar question nearly 20 years prior to Fermi’s version.

The Drake Equation

In 1961, the astronomer Frank Drake, with some help from scientists Carl Sagan and John Lilly, devised what would be known as the “Drake Equation”. This string of variables represents the possible number of civilizations in the Milky Way galaxy (that we may be able to communicate with).

N = R* • fp • ne • fl • fi • fc • L

Per author Elizabeth Howell on

N = The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.

The original estimates that Drake and his colleagues used were as follows:

  • R∗ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
  • fp = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
  • ne = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)
  • fl = 1 (100% of these planets will develop life)
  • fi = 1 (100% of which will develop intelligent life)
  • fc = 0.1 to 0.2 (10–20% of which will be able to communicate)
  • L = 1,000 to 100,000,000 years (which will last somewhere between 1000 and 100,000,000 years)

Using the minimum values results in an N of 20. Using the maximum values would give us an N of 50,000,000. Obviously, the range of possibility here is almost too large to be considered worthwhile. There may be so few civilizations with sufficiently advanced technology that the chances of contact over such a vast area as our galaxy approaches zero, or there could be so many that me might expect contact to occur sometime in the next few decades.

Based on this wide variance, Drake says that we might reliably only state that the value of N is somewhere between 1,000 and 100,000,000, or roughly that N = L.

Let me restate that: There might be 1,000 advanced civilizations out there, or 100,000,000 of them.

Photo by NASA on Unsplash

Current estimates based on recent data from NASA and other sources are:

  • R∗ = 1 yr−1.5 to 3 (1.5 to 3 stars formed per year)
  • fp = ~1 (based on new data from gravitational microlensing that seems to show that most stars have at least one planet)
  • ne = ~4 (based on newer data that suggests a higher percentage of planets existing in habitable zones around stars)
  • fl = 1 (likely not as high as 1 based on newer data, but still a “guesstimate”)
  • fi = 1 (likely not as high as 1 based on newer data, but still a “guesstimate”)
  • fc = 0.1 to 0.2 (Still the same)
  • L = 420 years

That last one, L, is a rather large reduction from the previous value range of 1,000–100,000,000 years. The value of 420 years was proposed by Michael Shermer, a science historian, based on the average length of time 60 major civilizations lasted on Earth ( Shermer, M. (August 2002). “Why ET Hasn’t Called”. Scientific American: 21.).

Plugging in these new assumptions, we get N = 252 as our minimum and N = 1,008 for our maximum.

Simply by trimming down the huge range gained by using the original values (1000–100,000,000), we now have what seems to be a much more realistic range which states that there may be between 252 and 1008 civilizations in the Milky Way galaxy emitting detectable electromagnetic signals.

More realistic- but not nearly as exciting or hopeful as that massive 100,000,000 possibility.

Of course, L might also be a much larger number, thus taking us back to a wide range. Astrobiologist David Grinspoon in his book Lonely Planets wonders if L should be replaced with fIC · T. Here, fIC equals the fraction of civilizations that emit electromagnetic signals (and thus can communicate) that reach a point in their development where they essentially become “immortal”, while T is the length of time it takes a civilization to reach that point.

It took life on Earth (very) roughly 4 billion years to develop anatomically modern humans, and it took humans 200,000 years to reach the stage we’re currently enjoying, sending all sorts of signals radiating out into the cosmos along with tons and tons of beautiful space debris like Pioneer and Voyager.

So the equation could instead be:

N = R* • fp • ne • fl • fi • fc • L • (fIC • T)

Using our current estimates for the variables, and using .1 for fIC (essentially the same value as fc) and 200,000 for T, we get an answer of N = 12,000 for our minimum number of civilizations in the Milky Way. For a maximum, we get 96,000.

From the original estimation of between 1,000 and 100,000,000 million possible civilizations, we have narrowed it quite a bit. Compared to estimating based on Michael Shermer’s L value, with a range of 252 to 1,008, we have expanded a great deal while keeping the top end somewhat restrained.

So, could we say there is a good chance there might be advanced alien life in the Milky Way we might some day communicate with? Are there tens of thousands of such civilizations with the capabilities required?

I think the only thing we can conclude at this point is that we may be improving in our ability to estimate these possibilities, but at the scales we are dealing with we will need more technological advances ourselves to be able to make better conjectures.

New Research, New Problems

Some of the latest research has introduced a new limiting factor to the equation, however — that of the element phosphorus. Phosphorus is the “P” in the ATP molecule that carries energy for our cells. On Earth, it’s a requirement for life, which means that it is absolutely needed to produce intelligent life forms as we know and understand them.

The problem is that phosphorus is actually a very rare element throughout the universe. Phosphorus is created in supernovae, and then has to find its way to a planet aboard meteorites. According to the researchers, there doesn’t seem to be enough phosphorus to go around. Not enough, anyway, that every planet in a habitable zone would stand to find itself in possession of some.

Thus, our variable ne (the number of planets per solar system that might have an environment suitable for life) might be far lower than in most Drake Equation estimates. It doesn’t much matter if a planet is simply in a habitable zone around its star if it doesn’t have the requisite elements for life on its surface. Rather than multiple planets around a given star having the potential for life, there may be at most 1, and likely far less…perhaps a fraction as low as .1 % of such worlds in habitable zones might contain the needed phosphorus. True, it is believed that a supernova occurs in the Milky Way roughly every 50 years, and over the expanse of universal time that has resulted in many millions of such events and all the element P those events have produced, but space is so vast and mostly empty that the odds of some phosphorus landing in every solar system, on the habitable worlds — having to get past the gravity of the star itself and any gas giants — would be vanishingly small.

And there are yet other variables that would affect the outcome of the equation. One of those is the very position of any given solar system in the Galaxy. The closer planets are to the bright, denser center of the Milky Way, the more those worlds are bombarded by cosmic radiation such as gamma rays. Unless those planets have extremely powerful magnetic fields and thick atmospheres, it would be less likely advanced life could evolve.

Though I, and probably most enthusiasts of this speculation would prefer to be as optimistic as possible, what is possible must be restrained by reality. Perhaps Michael Shermer’s result of between 252 and 1,008 potential worlds with intelligent life is one of the closest. Taking into consideration the phosphorus problem, that might easily drop to 1 to 10 worlds.

And Earth may be the 1.

Whether there is 1 world in the Milky Way with intelligent life, or 10, or 250, or 10,000… The universe is so large that all such planets, and the life on them, is unique and special.

Life itself is the rarest element of all.

Thank you for reading and sharing!