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Where IS Everyone? (and Why Hasn't ET Phoned Home Yet?)

Bruce G. Marcot


"If in fact we are alone, it means that we're not only the heirs to the cosmos, but its guardians, which is a portentous thought.... Either alternative is amazing: whether we're alone or not alone." 
        - Arthur C. Clarke, Wired magazine, 1993

"If it is true that our species is alone in the universe, then I'd have to say that the universe aimed rather low and settled for very little." 
        - George Carlin
  

It's a mystery worthy of exquisite attention.

Some astronomers cautiously suggest that the number of planetary bodies in our galaxy alone may be ... well, astronomical.

And that at least some of these planets may be in the "life zone," that is, orbiting their suns at distances and in orbital configurations such that water, if present, could occur and persist in liquid form.

This is the assumed starting condition for the formation of life-as-we-know-it here on Earth, although recent findings also report bacterial life and other "extremophiles" on Earth occurring inside solid rock, in sulfer springs, in "black smoker" geothermal vents on the ocean bottom, and other unlikely places.

Even if a relatively small percentage of such planets sustain life, and if such life can evolve to consciousness, to intelligence, and to technological civilizations, there should still be untold numbers of such races throughout the galaxy.
 

So Where is Everyone?

Why have we not yet achieved contact?

One effort underway to search for extra-technology signals is the SETI@home project -- an amazing use of distributed processing to evaluate radio signals piggy-backed from the Arecibo radio telescope in Puerto Rico.  After an incredible amount of searching, this project has YET to confirm a single radio signal from an extraterrestrial intelligence.

(I personally have been running SETI@home software for some time now, downloading data packets and scanning for signals.  As of May 2004, I had analyzed some 158 data units for a total 5,902 hours of CPU analysis time.  So far, I haven't found any candidate extraterrestrial signals.)
 

Why the Skies are Empty

Some authors have suggested a variety of scenarios to explain why we have not achieved contact.  Here is my own list of possible explanations:


Other explanations are possible.  But none of these instills much delight.  We are either lonely, Narcissistic, ants, or engage in outmoded methods of communication, or are waiting for someone to contact us, or maybe several of these together.  Or, they are already here or have been here, but somehow remain invisible.

I will leave it to social psychologists to explore our depths of Narcissism or our perception as ants or as mere receivers of signals.  I also will leave it to the engineers to determine if our communication is based on some outmoded "Morse Code" signal and what to do about it.  I also will leave it to the cryptozoologists or the alien-abduction psychologists or the ancient-astronaut authors to explain how ET can be here, or have been here, and yet remain invisible or undetectable.

However, I will explore here the We Are Lonely Hypothesis.  How many of "us" -- intelligent, technological, and communicative races -- call them "ITC races" -- are there?
 

Probably, or Probably Not?

Let's take a probabilistic approach to determining how lonely we might be in this galaxy, that is, how many ITC races there may be.

I'll use a simple freeware computer program called E.T. Counter (from http://www.sf-soft.de/etcount.html).  This calculates, in 4 general steps, the joint probability (running product) of a series of factors affecting estimates of the number of ITC races in the galaxy.  This is a nice update to the standard Drake Equation, which is a simple formula introduced by Frank Drake in 1961 to estimate the number of ITC races.

The 4 Calculation Steps of E.T. Counter:

Step 1 -- Stars in Our Galaxy, Part 1:
Number of stars in our galaxy
Proportion of stars that:
    - have planets
    - belong to the second generation of population I type stars
    - are part of a multiple star system

Step 2 -- Stars in Our Galaxy, Part 2:
Proportions (sum = 1) of star types, of:
    - dwarf stars
    - sun-like stars (0.33 - 1.4 sun masses)
    - giant stars
Probability that a sun-like star has a useful ecosphere in case:
    - it is not part of a multiple system
    - it belongs to a dwarf star
    - it belongs to another sun-like star
    - it belongs to a giant star

Interim calculation at this point:
Number of sun-like stars belonging to the 2nd generation of population I, with at least one planet and an ecosphere

Step 3 -- Habitable Planets:
Probability that:
    - a planet exists within the ecosphere of such a star
    - the planet has a life-enabling mass and element mixture
    - it does not have an extremely elliptical orbit, has an axis of rotation with an appropriate angle of inclination, an appropriate rotation speed, and a suitable atmosphere
    - life actually evolves on such a habitable planet

Interim calculation at this point:
Number of currently inhabited planets in our galaxy

Step 4 -- Advanced Civilizations:
Number of years:
    - following formation of the planet at which life first forms
    - following formation of life at which civilization appears
    - the planet is inhabited by advanced civilizations
Average distance between neighboring stars of population I, 2nd generation, in our galaxy
Average size of galaxies relative to ours
Number of galaxies in the universe

Final calculations:
Number of currently existing advanced civilizations in our galaxy
Number of currently existing advanced civilizations in the universe


The final calculations don't account for the "C" (communicative) part of my characterization of ITC (intelligent, technological, and communicative) races -- that is, even if there are technologically advanced civilizations in our galaxy, would they be interested in communicating with us?  For the SETI listening project, this means that another civilization would need to transmit, not just receive, signals that we can at least identify as originating from intelligent beings, even if we cannot decode the message per se.  (See the We Are Ants Hypothesis and the Waiting for the Other Shoe to Drop Hypothesis, above.)  For the sake of simplicity, we can ignore these factors for the moment.

OK, let's also ignore the calculations for the entire universe (what's the error rate of that estimate?) and just focus on the calculation for our own galaxy, the Milky Way.  In so doing, let's tinker with several input variables, in the spirit of a sensitivity test of model performance.

We'll be testing the plausibility of the We Are Lonely Hypothesis.  How many ITC races are there in the Milky Way?
 

Is That an Echo or Are You Happy to See Us?

Here is the first set of calculations.  The parameter values listed below with an asterisk * denote the ones that we will vary in the next calculation set; all other parameters (without asterisks) will be held constant, presuming that their default values are correct enough within at least an order of magnitude.
 

Scenario 1:  Default Settings
In this scenario, each factor seems to be a conservative value, but the final product is an estimate of well over half a million ITC races in our galaxy alone!

Step 1 -- Stars in Our Galaxy, Part 1:
Number of stars in our galaxy  =  200,000,000,000
Proportion of stars that:
    - have planets  =  0.75 *
    - belong to the second generation of population I type stars  =  0.10
    - are part of a multiple star system  =  0.60

Step 2 -- Stars in Our Galaxy, Part 2:
Proportions (sum = 1) of star types, of:
    - dwarf stars  =  0.55
    - sun-like stars (0.33 - 1.4 sun masses)  =  0.40
    - giant stars  = 0.05
Probability that a sun-like star has a useful ecosphere in case:
    - it is not part of a multiple system  =  1.0 *
    - it belongs to a dwarf star  =  0.67 *
    - it belongs to another sun-like star  =  0.4 *
    - it belongs to a giant star  =  0.0

Interim calculation at this point:
Number of sun-like stars belonging to the 2nd generation of population I, with at least one planet and an ecosphere  =  4,302,600,140

Step 3 -- Habitable Planets:
Probability that:
    - a planet exists within the ecosphere of such a star  =  0.5 *
    - the planet has a life-enabling mass and element mixture  =  0.5 *
    - it does not have an extremely elliptical orbit, has an axis of rotation with an appropriate angle of inclination, an appropriate rotation speed, and a suitable atmosphere  =  0.5 *
    - life actually evolves on such a habitable planet  =  0.5 *

Interim calculation at this point:
Number of currently inhabited planets in our galaxy  =  242,021,258

Step 4 -- Advanced Civilizations:
Number of years:
    - following formation of the planet at which life first forms  =  1,000,000,000 *
    - following formation of life at which civilization appears  =  4,500,000,000
    - the planet is inhabited by advanced civilizations  =  25,000,000 *
Average distance between neighboring stars of population I, 2nd generation, in our galaxy  =  7.6 light-years

Final calculations:
Number of currently existing advanced civilizations in our galaxy  =  605,000


Scenario 2:  Marcot's Estimates
In this scenario, I have decreased some of the default setting probabilities in Steps 3 and 4 (items changed are denoted below with an asterisk *).  The result is that .... we are entirely alone as an ITC race in the galaxy!

Step 1 -- Stars in Our Galaxy, Part 1:
Number of stars in our galaxy  =  200,000,000,000
Proportion of stars that:
    - have planets  =  0.10 * [I decreased this from the default estimate of 0.75, based on recent planetary inventories suggesting that stars in star clusters seldom have planets, even though we are finding a number of extrasolar planetary bodies]
    - belong to the second generation of population I type stars  =  0.10
    - are part of a multiple star system  =  0.60

Step 2 -- Stars in Our Galaxy, Part 2:
Proportions (sum = 1) of star types, of:
    - dwarf stars  =  0.55
    - sun-like stars (0.33 - 1.4 sun masses)  =  0.40
    - giant stars  = 0.05
Probability that a sun-like star has a useful ecosphere in case:
    - it is not part of a multiple system  =  1.0 * [I decreased this value and the next two based on my guess from recent findings on extrasolar planetary searches]
    - it belongs to a dwarf star  =  0.67 *
    - it belongs to another sun-like star  =  0.4 *
    - it belongs to a giant star  =  0.0

Interim calculation at this point:
Number of sun-like stars belonging to the 2nd generation of population I, with at least one planet and an ecosphere  =  347,200,011

Step 3 -- Habitable Planets:
[I decreased the following 4 parameters based again on my readings of recent extrasolar planetary discoveries, but they are very much guesses.  Few if any extrasolar plants discovered so far seem to be in habitable orbits (but search techniques to date do not yet lend to finding Earth-like planets), and I don't think that evolution of life is all that likely where it is habitable.  See Gonzalez et al. 2001, and other readings at the end of this essay.]
Probability that:
    - a planet exists within the ecosphere of such a star  =  0.05 *
    - the planet has a life-enabling mass and element mixture  =  0.005 *  [interestingly, if I increase this value by a factor of ten, to 0.05, it does not affect the outcome; there is still only 1 ITC race predicted for our galaxy]
    - it does not have an extremely elliptical orbit, has an axis of rotation with an appropriate angle of inclination, an appropriate rotation speed, and a suitable atmosphere  =  0.2 *
    - life actually evolves on such a habitable planet  =  0.001 *

Interim calculation at this point:
Number of currently inhabited planets in our galaxy  =  15
(This is where my estimates depart from the dafault values that generate an estimate here of  242,021,258 currently inhabited planets in our galaxy!  Even if I boost some of the above factors by an order of magnitude, the estimate here remains very low, especially considering the size of our galaxy.)

Step 4 -- Advanced Civilizations:
Number of years:
    - following formation of the planet at which life first forms  =  1,500,000,000 *  [I added an extra half-million years onto the default estimate as a fudge factor]
    - following formation of life at which civilization appears  =  4,500,000,000
    - the planet is inhabited by advanced civilizations  =  2,500 *  [I think I'm even being generous with this estimate, given the propensity of our civilizatiion toward mutual destruction and territorial imperatives]
Average distance between neighboring stars of population I, 2nd generation, in our galaxy  =  7.6 light-years

Final calculations:
Number of currently existing advanced civilizations in our galaxy  =  1 *


We're Number One

So that's pretty much all it takes to reduce the estimate of ITC races in our galaxy to just ONE -- us.

Even with more optimistic values of some of the parameters in Step 4, the number of ITC races seldom exceeds just a few.  Even changing the total time the planet is inhabited by advanced civilizations from 2,500 years to a billion years results in just 2 ITC races.

So a lot seems to depend on the factors in Steps 1, 2, and 3 -- and these are factors that, given in the near future, we ought to be able to actually estimate from ground- and space-based observational tools.

Now, change the parameter of total time that a planet is inhabited by advanced civilizations, in the first (optimistic) scenario above, from its default value of 25,000,000 years (which seems wildly optimistic to me) to 2,500 years (still a long time for a human civilization to maintain itself, given our own human history).

What's the effect?  It reduces the number of ITC races from 605,000 to only 61, with an average distance between neighboring civilizations (if evenly spread throughout our galaxy) of 5,241 light years.  This is an ENORMOUS distance, over which radio conversation would be utterly impossible.  It would take over 10,000 years for them to say "hello there," for us to hear it, for us to say "hello back," and for them to hear that.  And another 5,241 years for us to hear them say "we hear you too."

If a 2-way conversation of "hello there ... hello back ... we hear you too" were to be desired within the lifespan of a human (let us be generous and say 100 years), this means they would have to be only 33 light years away.  For this to be possible under Scenario 1 above, the average total time that planets are inhabited by advanced civilizations would need to be well over the average age of the planets they inhabit, an impossibility unless such civilizations somehow outlive their own planets of origin.  This means they would have the capacity to inhabit interstellar space or to travel to and colonize other planetary systems once their own planets ended.

And for such a 2-way conversation to take place over the typical length of one person's (radio-monitoring) career, let's say 40 years, the other civilization would need to be within 13.3 light years distance of Earth.  Is this even plausible?  The surprising answer is ... yes.

Yes, "but..."

According to the Gliese 2.0 stellar catalogue, there are 82 stars within 22.28 light years (7 parsecs) of our sun (not including sol itself).  However ... over half of these are likely multiple-body systems (binary, trinary stars) not conducive to having planetary systems with long-term, equable, inhabitable ecospheres.  And most of the rest are very different than or own sun, a type G main sequence star.  For example, of all 24 stars within 11.7 light years (about 3.6 parsecs), 55% are multiple-body systems and only ONE is a G-type star (Alpha Centuri A, at 4.3 light-years distance).  So, nearly all of our nearest stellar neighbors may be mostly the wrong type (luminosity, age, size, and temperature) of star to promote stable planetary systems and the evolution of life and ITC civilizations.
 

Only the Lonely?

My conclusion?

THEY may be very well be OUT THERE ... but ITC races may be so thinly spread at any one time within our galaxy that only by sheer, remote chance would any be close enough for us to receive their signals to learn of their existence.

And the odds of them being close enough to engage in 2-way radio communication (with radio waves traveling at light-speed) over a person's lifetime are immensely low.

My hope is that ongoing astronomical observations of extrasolar planetary systems will greatly help refine some of the parameters, especially those in Steps 1-3, and maybe adjust them upwards from my values.

Or maybe the Lonely Hypothesis is not to be discounted, after all...
 



 

For further reading:

Gonzalez, G., D. Brownlee, and P. D. Ward.  2001.  Refuges for life in a hostile universe.  Scientific American 285(4):60-67.

Hart, M. H., and B. Zuckerman.  1995.  Extraterestrials: where are they?  Cambridge University Press.

Taylor, S. R.  1998.  Destiny or chance: our solar system and its place in the cosmos.  Cambridge University Press.

 


Updates:

New evidence suggests that Earth-like planets may be more common than previously considered.

New evidence that most stars are singles, not multiples, so planetary systems may be more common than previously thought.

Calculating odds of other habitable Earths.

Where is Everybody?  A revisit of the Drake equation.

  


   
Addendum:  Some further ideas on WHY there has been no clear contact with other civilizations.

Assuming we're not alone, the WHY can be answered by several hypotheses, not necessarily mutually exclusive:

1/ They're out there, and it's a cosmic conspiracy to not show themselves to lesser-advanced civilizations.

2/ They're out there, and it's a cosmic conspiracy to not show themselves to US in particular.

3/ They self-destruct (the nuclear option) just when they reach communication stage.

4/ They exist, and operate in, an entirely different and (to us) undetectable form of existence (dark matter?), dimensional extent, frequency of communication, spatial scale, or time scale.

5/ We just happen to be in a corner of the cosmos and/or cosmic time in between the appearances of other civilizations. 

6/ We have not yet learned to recognize evidence of their existence, which might be more obvious if only we knew how.

7/ Reliance on "our technology" is largely or uniquely a human affair, or is that of a too-youthful civilization to worth bothering with; that is, they communicate and interact using other means (psychical, hyper-dimensional, quantum physical). 

8/ ... what are your ideas ... ?
  

  


 

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