If the Earth is Rare, We May Not Hear from ET

by Jean Tate on February 8, 2010

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Earth - Moon System

Image Credit: NASA

If civilization-forming intelligent life is rare in our Milky Way galaxy, chances are we won’t hear from ET before the Sun goes red giant, in about five billion years’ time; however, if we do hear from ET before then, we’ll have lots of nice chats before the Earth is sterilized.

That’s the conclusion from a recent study of Ward and Brownlee’s Rare Earth hypothesis by Duncan Forgan and Ken Rice, in which they made a toy galaxy, simulating the real one we live in, and ran it 30 times. In their toy galaxy, intelligent life formed on Earth-like planets only, just as it does in the Rare Earth hypothesis.

While the Forgan and Rice simulations are still limited and somewhat unrealistic, they give a better handle on SETI’s chances for success than either the Drake equation or Fermi’s “Where are they?”

“The Drake equation itself does suffer from some key weaknesses: it relies strongly on mean estimations of variables such as the star formation rate; it is unable to incorporate the effects of the physico-chemical history of the galaxy, or the time-dependence of its terms,” Forgan says, “Indeed, it is criticized for its polarizing effect on “contact optimists” and “contact pessimists”, who ascribe very different values to the parameters, and return values of the number of galactic civilizations who can communicate with Earth between a hundred-thousandth and a million (!)”

Building on the work of Vukotic and Cirkovic, Forgan developed a Monte Carlo-based simulation of our galaxy; as inputs, he used the best estimates of actual astrophysical parameters such as the star formation rate, initial mass function, a star’s time spent on the main sequence, likelihood of death from the skies, etc. For several key inputs however, “the model goes beyond relatively well-constrained parameters, and becomes hypothesis,” Forgan explains, “In essence, the method generates a Galaxy of a billion stars, each with their own stellar properties (mass, luminosity, location in the Galaxy, etc.) randomly selected from observed statistical distributions. Planetary systems are then generated for these stars in a similar manner, and life is allowed to evolve in these planets according to some hypothesis of origin. The end result is a mock Galaxy which is statistically representative of the Milky Way. To quantify random sampling errors, this process is repeated many times: this allows an estimation of the sample mean and sample standard deviation of the output variables obtained.”

Forgan simulated the Rare Earth hypothesis by allowing animal life – the only kind of life from which intelligent civilizations can arise – to form only if homeworld’s mass is between a half and two Earths, if homesun’s mass is between a half and 1.5 times our Sun’s, homeworld has at least one moon (for tides and axial stability), and if homesun has at least one planet of mass at least ten times that of Earth, in an outer orbit (to cut down on death from the skies due to asteroids and comets).

The good news for SETI is that a galaxy like ours should host hundreds of intelligent civilizations (though, somewhat surprisingly, there is no galactic goldilocks zone); the bad news is that during the time such a civilization could communicate with an ET – between when it becomes technologically advanced enough and when it is wiped out by homesun going red giant – there are, in most simulations, no other such civilizations (or if there are, they are too far away) … we, or ET, would be alone.

But it’s not all bad news; if we are not alone, then once contact is established, we will have many phone calls with ET.

To be sure, this is but a work-in-progress. “Numerical modeling of this type is generally a shadow of the entity it attempts to model, in this case the Milky Way and its constituent stars, planets and other objects,” Forgan and Rice say; several improvements are already being worked on.

Sources: “A numerical testbed for hypotheses of extraterrestrial life and intelligence” (Forgan D., 2009, International Journal of Astrobiology, 8, 121), and “Numerical Testing of The Rare Earth Hypothesis using Monte Carlo Realisation Techniques” (arXiv:1001:1680); this too will be published in IJA, likely in April.

Jean Tate February 10, 2010 at 12:34 PM

Hon. Salacious B. Crumb,

Your post was not consciously deleted, but was probably posted in Universe Today on the old server, around the time Fraser moved us to the new one, and got stuck there (Fraser mentioned that server moves always result in problems of one kind or another).

Sorry about that.

Thanks for your clarification.

Lawrence B. Crowell February 10, 2010 at 4:16 PM

@ Mr. Man: I think it is more likely that the people behind her would not permit it. She is not mentally competent. When she gives a speech that is anything beyond glittering generalities and mocking terms she degenerates into speaking incoherent sentence fragments. If by some disaster she became President, she would largely be a puppet or a manipulated figurehead.


Hon. Salacious B. Crumb February 10, 2010 at 4:39 PM

@ Jean

I though that it might be something like that as I could see any problems with what I’ve said. these thing happen I suppose.
It is interesting that the Drake equation has SFR as the first item (R*) [R star], which was originally given by Drake as “the average rate of star formation in the galaxy”
Most have interpreted this as “the mean star formation rate over the age of the galaxy”

In the end, this important part of the equation is really based on our knowledge of star formation (and recently planetary formation) and the processes taking place. As we have yet to even understand why, for example, the outcomes of nebulae produce say many small stars with a few massive ones or just a whole lot of medium solar-type stars.

It is also interesting that this is the only stellar parameter in the equation. It has always been ill-defined and ambiguous in its interpretation. Thinking about it, Duncan Forgan and Ken Rice have really done the same – interpreting how stars are formed throughout the whole spiral galaxy.

As a open comment, it is surprising that the Drake equation dos not have a universe evolution component. I.e Where life can only exist within a range of time – say when the amount of metals (those elements heavier than Helium) were available for planet formation or when the available nebula gas in a galaxy is depleted sufficiently to make star formation a rarity.
In this view, for example, life is unlikely to have been created in the first billion years or 100 billion years from now.

Hon. Salacious B. Crumb February 10, 2010 at 4:42 PM

Mr. Man Says:
February 10th, 2010 at 11:56 am
Palin is not the sharpest tool in the shed, but I sincerly doubt she would launch salvos of Nukes just to bring Jesus back.Mr. Man

Question: Should this be added as another variable in the Drake equation. I.e. f_sub_p ??

Lawrence B. Crowell February 10, 2010 at 5:34 PM

I think the Drake equation has a factor involving the duration of any intelligent life. The problem as I see it is that our intellectual abilities are a sort of byproduct of our complex social behavior and language. We did not evolve to build machines, probe the atom or look deep into the unvierse. Yet, we have that ability and we have gotten far with it.

At the core our behavior is from our hominid evolution, which is really just a branch of apes in general. So here we are as somewhat brainy ground apes largely rampaging exponentially out of control. We also have nuclear bombs, and are tearing down the planet at an astounding rate. It would be if you left loaded guns near a chimpanzee troupe and watched the show —best done at some distance I should think.

So I suspect most ETI confront this problem, and many go extinct I would imagine. Intelligent life does not evolve to learn quantum mechanics, but does so as a byproduct. This would in many cases lead to conflicts between their base behavior and their intelligence. StarTrek advanced a fictional species of aliens, the Vulcans, who managed to get a grip on their behavior, individually and collectively. That is the route for any long term survival of an intelligent species of life. We are long away from this goal, and our exponential trends make time frames for addressing this issue rather short.


Dark Gnat February 10, 2010 at 7:28 PM


Your right that war will certainly play a part, along with pestilence, famine and death.

It’s strange that we will bring the things we fear the most upon ourselves, especially when we have the ability to avoid them.

I consider myself very conservative. I believe in small government and that the right to bare arms is critical for democracy, but people like Sarah Palin frighten me. Both Democrats and Republicans have become shallow reactions to each other, and are neglecting the people they represent and their constitution.

When people become desperate, they become gullible – that’s how dictators come to power. Certain politicians also tend to play the “Providence” card as if they are destined to lead people to greatness.

They prefer to keep the populace uneducated and distracted while they quietly remove rights and liberties under the guise of “security”, “public safety” “crime reduction” or whatever else is convenient. All governments are corruptable, and larger governments tend to lead to larger corruptions.

Watching “The Road Warrior” the other day, I thought to myself that the film was probably an accurate depiction of the future – much more accurate than Star Trek, for example.

Uncle Fred February 10, 2010 at 7:33 PM

This forum has (understandably) become a place to vent our doom and gloom frustrations.

LC, I was hoping to hear your answers to the questions I posed earlier. I feel few have addressed these questions directly in any recent article. Here is what I gathered since last night:

1/2. On the statistical likelihood of finding a Earth-like planet within 50-100 light years (your optimistic variables):

- Guesses are to be avoided, but many claim one could be found within 1-5 years. Does this seem reasonable? Kepler perhaps? Problem I find with Kepler is that it is only looking at a small chunk of the sky. Yet since Kepler is looking further afield then 100 lights years, it may find something further out.

3. If we do find it, what can we discern from it using current, or near-term technologies/techniques?

- From my understanding, and I am no scientist, but we could deduce its mass, internal composition, orbital characteristics, median temperature, and traces of it’s atmospheric composition. We could infer (guesstimate) on other basic areas such as rotation and tectonic activity.

Am I correct? Have I missed something? Let me know.



Lawrence B. Crowell February 11, 2010 at 4:18 AM

I did my analyses by computing the Lyapunov exponent for a three body system, such as Sun, Jupiter and Earth (ignoring other planets), for the chaotic drift of the Earth-like planet. I then ran some Bayesian calculations of the likelihood of an Earth like planet at a radius r ~ 1AU orbit etc around a F_l, G, K_h (h and l mean high and low) class of star. If the Jovian planet is too close to the star it rattles the orbit of any 1AU radially distant planet too much for the planet to remain their in a stable configuration. I came up with close to a thousand such planets may exist in our galaxy, using as a Bayesian prior estimate based on our solar system and another one found which is a “close match.” Now a part of this estimate, an priori assumption is also used based on the existence of a band in the galaxy with stars that have statistically an optimal metallicity (elements higher in atomic number than helium) that is about 10% of the galaxy. This is a sort of habitability zone in our galaxy. So let us estimate in a ballpark fashion how close these stars would be. For a 100,000 light year diameter galaxy that is a disk area of about 3e^{10}ly^2, where the area which we think. Now using my estimate of 1000 stars bearing a planet at least comparable to Earth, then the area which includes these stars is ~ 3e^8 ly^2 divide by 1000 and take a square root and we get an average distance between these stars as 550 light years. For amore optimistic 10,000 Earth-like planets this is an averaged distance of 173 light years. Yesterday I came up with the 100 light years by ignoring the factor of pi in the area estimate I ran in my head. So at 173 light years averaged distance between stars with a stable 1AU orbiting planet we should be able to observe one or more of these. The Kepler spacecraft, or subsequent generations of similar spacebased observing systems, may indeed find such a star. In my book I discuss this in the context of the physics of sending a spacecraft to another star. If we get lucky and find such a planet within a few tens of light years we might be able to send a probe to this star system to take a close look.

In observing extrasolar system the orbital characteristics (6 parameters of an orbit in Newtonian mechanics) can be estimated, the mass and density of the planet can be further estimated. In an optical interferometer a closer look can be made, and the chemistry of the atmosphere and surface measured in a coarse grained fashion. The surface geology (tectonic plates etc) is harder to infer. From such data signatures of possible life might be found, which might be strongly suggestive of a complex biologically active planet. There are of course other possibly competing factors, such as whether the rotational axis or angular momentum of the planet is stable or not. Earth is pretty highly stabilized by the presence of the moon, which by tidal interactions in a sort of “pin-orbit coupling” keeps the rotational axis of the Earth fairly stable. On the other hand it is possible that a planet not so well stabilized might evolve life capable of adapting to such changes.

As for the “doom factor,” this does enter into this discussion with respect to the longevity of ETI and ourselves. To be honest I don’t think things look terribly good for us over the next few centuries. I will also say that I think that there is a lot of hubris in connection with our ideas about what we might be able to control and have power over in the future, such as controlling planetary orbits, colonizing interstellar space etc. These ideas in some measure seem to reflect our bias that the universe ultimately exists for us, or that we are at the center of things. This bias seems to be expressed in a science, or science fiction, type of format.


Dark Gnat February 11, 2010 at 5:44 AM

I don’t mean to sound like a downer, but the reality of the situation makes our chances of survival slim…at least civilized survival.

Humanity might survive and once again regain “civilization” but it is my hope that when that happens they will be aware of what brought us down.

It’s possible that there are many ETI’s out there but unless they have found a way around the limits of the universe, they may figure that there isn’t much point in contacting us anyway. After all, what could we do other than say “Here we are.”

If they are 700 light years away, we might be able to beam signals to them, but by the time we get a responce, at least 1400 years will have passed on Earth. Empires could have risen and fallen in that time. This of course assumes that the signal is still detectable by the time it gets there. Obviously I hope they are closer.

I predict that if we are visited by ETI’s, then they would likely be very far ahead of us technology wise, and possibly thought capacity. Looking at how destructive we are, they may decide that we are simply roaches to be exterminated.

Or they will be nice and give us hovercars, teleporters, and replicators. But what would we have to give in return?

Paul Eaton-Jones February 11, 2010 at 6:44 AM

If there is a is a galactic civilisation that trades, exchanges ambassadors and goes to war it is more likely to be found inward of our position out here in the suburbs. Why would any starfaring nation come out here where the distances between star systems are so vast. The idea that they might send out ‘genertional’ ships is ridiculous. A civilisation with ‘slow’ star ships might do that but if you’re part of an interstellar community you’re not going to come out here unless you have FTL technology. Also why would you send signals out to the ‘burbs if communication is going to take thousands of years each way. That is the more likely reason we haven’t heard from them – not because they’re not there but because it’s not worth while chatting to people 15,000 light years away. They’re probably there but not interested. I think that would come as more of a shock to humans [intelligent life not intersested in us] than the fact we’re totally alone.

Paul Eaton-Jones February 11, 2010 at 6:56 AM

Again we get bogged down with things we can’t yet prove i.e. numers of stars that can allow the formation of planets with atmosphere, liquids etc. The number of planets that can support life once it’s appeared and lives long enough to bring forth intelligent life capable of reaching the staers etc etc. All the discussions are based SOLELY on the fact that life evolved here, on earth, on one planet. Reducing the number of stars that may just allow life to evolve in its system down from 200/400 billion to a billion, a million, a thousand or just one is pointless. Informed/intelligent guess work? Based on what? Nothing. It seems all calculations are based around water, carbon, a large satellite, plate tectonics,equitable temperatures we have here and anything different and it won’t occur or be extremely difficult! Sheer hubris. Until we find unequivocal evidence from beyond earth speculation based on unknowable numbers and conditions and making [firm] declarations is verging on the ludicrous.

Jean Tate February 11, 2010 at 6:57 AM

So far as planets with life, complex life (a.k.a. multicellular), animals, and intelligent animals are concerned, the science is severely constrained by having a sample of one.

I’m not sure what the current status is for the number of times multicellular life evolved – more or less independently – from simple life, but there certainly were several resets in the evolution of animals (there are at least five mass extinctions, of which at least two caused a pretty major change in the fauna of Earth).

There’s a lot more data on the rise and fall of civilizations, and in terms of the kind of simulation Forgan and Rice ran, it sure is fun to think about what the planetary conditions for intelligent animals to develop civilizations which can engage in SETI are.

For example, Jared Diamond’s book Collapse suggests that human societies are not especially good at managing their resource usage sustainably (at the society level, you might say they are suicidal).

Also, why didn’t science in the Middle East (during the European Dark Ages) or China (ditto) get to the point of discovering that the rules which govern celestial events are the same as those down here on Earth?

Lawrence B. Crowell February 11, 2010 at 8:49 AM

A lot of this depends upon whether life is fairly common or not. This is different from what I would call a bio-exuberant planet like Earth. I think Mars might have life, and there are ideas of Jovian and Saturnian moons having sub-iceshelf oceans with life. Yet in these cases it appears that these bodies are not bio-planets, but cases where life emerged at some point and has managed to continue by adapting to severe environments. Martian life, should it exist, is not likely to be much more than prokaryotic-like forms making a living in the soil or in little niches free from solar UV and where they can catch a bit of water now and then. Time will tell of course whether this is at all the case. Yet if life is a fairly normal product of chemistry or geochemistry, and if it is found to happen in a number of cases in our solar system, we can pretty safely assume that it is common throughout the universe. My estimate was only that there are about 1000 planets in some orbital and stellar configuration similar to Earth’s. If this estimate is close to the mark (say by an order of magnitude or so), then these planets might be expected to have life and temperature and energy flow through which might foster complex life in fecund ecological systems. Even without the moon a planet like Earth might still harbor complex life, though of an entirely different nature than we have here. By complex life I am referring to any multi-cellular (like) form, and the early Cambrian period with small shelled animals, spirogyra like plants, worms etc is an example of complex life. So there might exist millions of planetary bodies in the galaxy similar to Mars or the Jovian moons with unicellular life, but I suspect far fewer planets are bio-philia zones similar to Earth.

Finding a bio-planet in some other stellar system of planets is the great coup we might be working for. If we get some signal from an ETI that would approach some the dimensions of a religious revelation of some kind. I am frankly rather dubious about our prospects for ever getting an ETI call. Yet if we find a bio-planet or two out there we can infer the existence of ETI. The cosmological data indicates the universe is a flat space (k = 0 in FLRW) which is expanding outwards (a time-time curvature in general relativity) according to so called dark energy. In such a universe if we know life exist elsewhere in abundance on some planets then it would also throughout the universe. This a Copernican principle: The universe on average appears the same everywhere. So doubtless there then should exist other intelligent life forms.

We have to then bag our first bio-planet out there! Maybe Kepler will find one. Until then we can only at best try to estimate upper or lower bounds on occurrences of situations favorable to life and to make other estimates which might be better than complete shots in the dark.


William928 February 11, 2010 at 4:36 PM

An interesting article, and even more interesting comments. @Paul Eaton-Jones:
I don’t believe anyone is making any ironclad suppositions about life on other planets, simply offering food for thought, and in my opinion, interesting topics for discussion. It’s fairly clear that until we discover space travel approaching C, it’s highly unlikely we’ll ever meet any intelligent life. This doesn’t preclude speculating about what may be out there however.

Paul Eaton-Jones February 12, 2010 at 1:07 AM

Agreed that The Copernican Princilple states that the universe on average appears the same wherever you look but I don’t thinkit should be used to infer that life must/probably/possibly exists elsewhere just because it does here. I personally feel that the Milky Way is teeming with life at all levels of complexity and organisation. We should stop using humanity as a yardstick against which we measure the viability of ‘other’ ET civilisations. Just because our 5-6 thousand year history is littered with wars, self-induced famine, environmental degradation etc does not mean for one second mean that someone else will/has followed the same path. Again, hubris. Doing so moves the argument close to the pre-20th century “we’re the centre-of-the-universe-and-aren’t-we-special”. Just because we have the tendency to screw up don’t think ETs have to.

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