The Green Bank Telescope.  Credit: NRAO

Less Than 1% of Exoplanet Systems Have Intelligent Life, Researchers Say

Article Updated: 23 Dec , 2015


Recent findings say that Earth-like exoplanets could be all around us in our cosmic neighborhood. But how many would be home to intelligent life?

A new study estimates that fewer than 1% of transiting exoplanet systems host civilizations technologically advanced enough to send out radio transmissions that could be detected by our current SETI searches.

That equates to less than one in a million stars in the Milky Way Galaxy that would have intelligent life we could possibly communicate with. But even with those odds, there could be millions of advanced ET’s in the galaxy that we could phone, researchers say.

A group of astronomers, including Jill Tarter from the SETI Institute and scientists at the University of California, Berkeley used the Green Bank Telescope in West Virginia to look for intelligent radio signals from planets around 86 of stars where the Kepler mission has found transiting exoplanets. These specific targets were chosen because they had exoplanets in the habitable zone around the star and there were either five or more exoplanets in the system, or there was super-Earths with relatively long orbits.

The search came up empty in detecting any signals.

“We didn’t find ET, but we were able to use this statistical sample to, for the first time, put rather explicit limits on the presence of intelligent civilizations transmitting in the radio band where we searched,” said Andrew Siemion from UC Berkeley.

The team looked for signals in the 1-2 GHz range which is the region we use here on Earth for our cell phones and television transmissions. Narrowing it down, the team looked for signals that cover no more than 5Hz of the spectrum since there is no known natural mechanism for producing such narrow band signals.

“Emission no more than a few Hz in spectral width is, as far as we know, an unmistakable indicator of engineering by an intelligent civilization,” the team said in their paper.

The telescope spent 12 hours collecting five minutes of radio emissions from each star. Most of the stars were more than 1,000 light-years away, so only signals intentionally aimed in our direction would have been detected. The scientists say that in the future, more sensitive radio telescopes, such as the Square Kilometer Array, should be able to detect much weaker radiation, perhaps even unintentional leakage radiation, from civilizations like our own.

The researchers said these results allows them to put limits on the likelihood of Kardashev Type II civilizations. The Karashev scale is a method of measuring a civilization’s level of technological advancement, based on the amount of energy a civilization is able to utilize. The team said that finding no signals implies that the number of these civilizations that are “noisy” in the 1-2GHz range must less than one in a million per sun-like star.

The team plans more observations with the Green Bank Telescope, focusing on multi-planet systems in which two of the planets occasionally align relative to Earth, potentially allowing them to eavesdrop on communications between the planets.

“This work illustrates the power of leveraging our latest understanding of exoplanets in SETI searches,” said UC Berkeley physicist Dan Werthimer, who heads the world’s longest running SETI project at the Arecibo Telescope in Puerto Rico. “We no longer have to guess about whether we are targeting Earth-like environments, we know it with certainty.”

Read the team’s paper.

Sources: UC Berkeley, MIT Technology Review

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35 Responses

  1. Andrey says:

    With the age of our galaxy – 13.2 B years, about 100 years of our civilization having radio and maybe another 100 years before we will switch to another method of communication, is less than a blink of an eye. Chances of another civilization using radio signals detectable by our radio telescopes are very very slim, approaching to zero. I believe this is the reason why we have not detected anything. I do believe that civilizations do exist, but if we will detect them, we will be very, very lucky!

    • Beck45 says:

      As I imply in my other comment, the chances of your guess being correct about the way life forms–let alone civilizations–generally develop, is also very very slim. 🙂

      • Especially given the fact that much of what passes for communication on THIS planet would not qualify as evidence of extant intelligent life.

      • Ernie Dunbar says:

        The ability to transmit narrow-band radio signals of the type they’re looking for is evidence enough, for enough intelligence.

        Our ability to transmit television broadcasts (regardless of content) is an amazing technological achievement in and of itself.

      • You clearly have not seen Honey-Boo-Boo or anything else on “The Learning Channel”.

        One viewing would drive away any extraterrestrial explorers.

    • Eric Rooney says:

      Agreed. We haven’t even cataloged all the life on Earth. When I grew up we didn’t know there were planets outside our solar system. Even if there are civilizations looking for us, they would face the same obstacles of interstellar distances as us.

      • Correct. Remember that given the extreme and vast colossal distances, even the light-speed is slow to transmit/receive data, so I think that this barrier will never be beaten, because there’s no way to shorten the distance (according to Einstein, nothing can travel faster than light). I think that there could be life out there, but we will never discover it. Distances involved are simply too vast.

  2. John Stock says:

    As a species we started broadcasting EM signals about 120 years ago, give or take? We are also starting to remove those broadcasts in favour of fibre optics or directed beams. So it’s very likely that a civilisation only has around a 150-200 year window where they wastefully broadcast signals. So the litmus test described above could be massively inaccurate.

    • What exactly is very likely here? In the whole galaxy we have only one expample of a civilisation who is capapbe of interstellar communication by radio technology. Why should our civilisation be typical? There’s absolutely nothing to corroborate this, but I agree with your closing statement.

  3. George says:

    5 Hz wide signals would not contain much useful data, no matter what civilization produces them. Between 1 and 2 GHz, most of the transmissions from Earth are very low power (TV broadcasting is NOT in this range). Further, at those frequencies, wideband emissions are more likely to be useful and logically produced, since a society that can create signals at these frequencies is likely to have relatively advanced communications systems. It seems that, by looking for narrowband signals at these wavelengths, we could be missing a lot. I do applaud the effort, but drawing any conclusions from a small data set with unlikely parameters is extremely premature.

  4. How much is less than 1%? One millionth, one billionth. one trillionth? This says absolutely nothing about the number of civilisations that are like us.

    • Lorin Ionita says:

      They don’t have be like us. They just have to be technologically advanced (post industrial). The level of advancement is irrelevant here.

    • Ernie Dunbar says:

      Yes, because we’ve only surveyed about 100 stars with known earth-like planets, and found nothing.

      Therefore, the odds are less than 1%. If we had found *something* in those 100 stars, then we would know for absolute certain that it was greater than 1%.

      However, the problem is that we just don’t know of enough earth-sized planets in earth-like orbits to really do a proper survey just yet. So we’ll just have to go with what we’ve got.

  5. I just KNOW they’ll finally find life about 15 minutes after I die of old age.

  6. Beck45 says:

    On exobiology I really don’t think we know enough about how life develops to make any useful statements. Imagine having all modern knowledge of Earth’s physical non-living systems and trying to demonstrate what life would emerge. Good luck! Now add all our knowledge of biology. Even WITH that, we can’t say exactly what happened.

    *Materials science* is still a field of study. Even with the small number of elements on the periodic table, we still can’t even figure out all the possibilities of inanimate materials. One would think the top supercomputers churning away 24/7 would have discovered every possible material that can exist. Apparently not. So when we click the “Include living systems” checkbox on their software…it’ll take a few hundred years of computation I guess. 🙁

    So what do we know about what types of life forms are likely, possible, existent outside of those on Earth? Yeah, nothing, basically–obviously, since we have zero evidence so far. 🙂

  7. campbell s w says:

    Why are we always looking in a mirror to find other intelligent life?

  8. lcrowell says:

    I suspect there may only be a few thousand planets in this galaxy with the level of complex life we have on Earth. There may be far more planets with elementary life, such as prokaryotes, or something analogous. I base this judgment on some analysis of stable orbits at 1AU that could exist in known stellar systems of planets. Then given that we have had electromagnetic technology for only about a century this is a window of time that is only about 3×10^{-8} of the duration of this planet. This last part is a bit of a frequentist argument, but it is all that I have to so with. As such we may find that there is no ETI in our entire galaxy.


    • Jared Davis says:

      the ‘goldielocks zone’ is not always 1 au from its star its spectral type and luminosity may make that zone closer or farther away to assume that the ‘goldielocks zone’ is always 1 au will influence your findings.

  9. Aqua4U says:

    Any day now.. in fact, every time I open this webpage.. I expect to hear that evidence for the existence of ETI has been found. My personal belief is THAT evidence won’t come from radio or light wave signal sources. Instead it will come from explorations and discoveries within our own genome. We humans might BE that long awaited and anticipated transmission! Our very existence the ‘proof’…

    Step for a moment outside the sphere of our blossoming yet rudimentary physical technologies and set aside the light speed limitations we presume. Imagine extra dimensional or parallel universe physics where light speed limitations are not a restrictive factor. Imagine realities beyond our physical limitations. Having done that… what possibilities exist? What infinities to explore?

    I think the real question might be… When will we evolve far enough along in this time line to begin that exploration? Why not now? Indeed, it appears that our prophets, mystics and seers have at times broken through the confines of ‘normal reality’ and consciousness and have seen into a far more infinite universe?

    Reminds of the fairly recent realization that there are distant galaxies. BOOM! Our universe suddenly got exponentially larger! Thank you Mr. Hubble!

  10. ITSRUF says:

    Really… How can this be known? Was a computer model involved?

  11. Olaf2 says:

    I knew it. 99% of the aliens that come here with their UFO are not intelligent at all 🙂

  12. Me says:

    1st of all there is the goldilocks zone. 2nd is having a moon. This subject has a billion elements of guessing. Is there H2O? An earth like planet is so rare plus it having a moon like our beloved earth has is another rarity. Soooooo many factors, so many guesses, I need a few tokes from a sweet doobie to calm me down!… God Bless all ;-)… .

    • Torbjörn Larsson says:

      Creationists shouldn’t comment on science sites, it is hilarious and makes deconverts from religion, see Dawkins’s Convert’s Corner.

      This comment has lost its creationist script, presumably lost in the implied drugs. The religious use of the daft Rare Earth idea revolves around its haphazard requirement list having _a large, close_ moon (so rotation axis stabilization). That is now known to be erroneous as all the other components on the yet again too unconstrained bayesian estimate (see Fermi’s question in my other comment).

      The early moon modelers made a mistake of an order of magnitude. Even without large moons planets are now known to be stable over geological periods, even if inherently chaotic. Plenty of time for life to evolve, and then plenty of time for language capable life to evolve, in the periods between large tilt changes.

      Habitable zones are not “goldilocks zones” except in the sense of being in between extremes. The primary habitable zone is so wide that most stars have 2-3 planets in them, Earth with just 1 habitable planet (Mars being too small to be surface habitable today) is a rather abysmal turnout.

      [And according to Kasting et al recent revision, barely so as we are just 1 % of its width from the inner edge. It was Earth that is a rare marginally placed, marginally large runt, not the common more habitable “Earths” expected.]

      We now know that eta, the number of systems with Earth analogs, is ~ 5 %. That is not “rare”, considering the 10^10 stars in the Milky Way alone.

      Added to that is the moon habitable zones, the tidal heating zones around gas giants, that in our system alone has yielded 5 (known) – 15 (suspected) potentially habitable subsurface oceans. Those are prime estates, by the way, some of them easily potentially larger biospheres than our own dried husk of a planet.

      • Me says:


        I remember you from other postings over the past few years. You really need to smoke a fatty!….lol. Your answers are black & white w/no concept of a novelty. I was NOT referring to gas giants or any other myth like planet with a blink in front of its star or a wabble. It is all a guessing game! So for we humans have not even scratched the surface yet. 2 probes are out there and the 1st is not close to the ort cloud yet. Although, I do admit, it is a great feat of our “infant human 1st steps out there”. I was/am referring to earth like planets. Take away our moon. What will happen Tor? MAYHEM, thats what! Although the article is about exo-planets and what you say does have merit I agree with. Your 1 way thinking and non-agreeable ways makes you look ablit too robot’ish. I know you are an ATHIEST. That is obvious. Hey, to each his/her own belief. I do NOT want to be you at death. The vast universe and the vast many dimensions with in dimensions is the creators mind that is so vast it is uncomprehensible. God Bless, …ME

      • Me says:

        Come on Tor! Where is your reply? And NO Tor, weed is harmless if done w/class. Your love putting other peoples papers/views/etc..ahead of your own. Do you actually have a life Tor? And again, us creationists have merit ole boy. Your so into this universe that your lost(or never ever have had)life reality. What about the “paranormal” Tor? Oh ya, don’t beleive

      • Me says:

        Come on Tor, your killin me!! Reply damn it!…lol. The “goldielox zone” is REAL! Infact we are 2/3rd’s the way out in the “gal’x Tor. That is the “goldielox zone”. I KNOW you know that you Sweedish brian child. It is a “Rare Earth” Tor, …it is so RAREjust like you are Tor. Your so rare its not fair! God Bless you Tor. That should hold you for awhile! PEACE!

  13. Torbjörn Larsson says:

    I think many or most commentators are agreed on that the generic question (Fermi’s question) is too unconstrained to be tested in this fashion. However, it is relevant research and the findings are consistent with, i.e. less than, the known eta (~ 5 % Earth analogs).

    The Kardashev scale borders on pseudoscience – the KIII hypothesis is physically impossible (no such coherent organizations possible due to the distances) – but the Dyson cloud idea of the KII is doable and possibly even relevant. (I expect it is as rare as language capable intelligence in relation, see below, that is (exceedingly rare)^2.)

    The reverse question, whether life leads to complexity at all, is starting to yield to an interesting data set. Recently the phylogeny and clock dates of oxygenating bacteria of relevance, cyanobacteria, was elucidated for the first time.

    It turns out that the clade, as many other prokaryotes, most likely evolved multicellularity before the oxygenation of the atmosphere but that its diversification and complexification was greatly enhanced by its own reengineering of the planet. [“Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event”, Schirrmeister et al, PNAS Early Edition, 2012]

    As for life, the early evolution (within 0.5 – 1 Ga from first cells) predicts from simplest possible statistical process models that both oxygenating photosynthesis and multicellularity are efficiently easy and common enough on old planets. (Here ~ 100 % on 8 Ga old habitable planets.)

    To evolve complex genomes takes a mitochondria like energy plant endosymbiosis. (Lane’s energy theory on eukaryotes.) This is yet again early, first eukaryotes are perhaps 1 Ga later.

    Evolving language capable intelligence however is quite another thing. It is expected by biologists to be exceedingly rare. Even if Snowball gives us a run for the money:

    “Snowball (hatched c. 1996) is a male Eleonora Cockatoo, noted as being the first non-human animal conclusively demonstrated to be capable of beat induction— perceiving music and synchronizing his body movements to the beat (i.e.,dancing).”

    • Me says:

      OK Tor, …here at M.I.T., most of us know there is a creator. It is the atheist here who also are closed minded like you are. God is real. I cannot prove it to you. . Remember Tor, it is the eyes and ears of the “SOUL” that can see and hear and not only the eyes and ears of the body. Your soul is blind and deaf. But never the less. God Bless you no matter what your soul allows to see and hear the “truth”. PEACE OUT!

  14. Eric Rooney says:

    Radius of Milky Way = 50000 ly
    3.14*50000^2 = 7850000000 ly^2

    assuming approx 100000 ETs for our galaxy

    7850000000 ly^2 / 100000 ETs

    = 78500 ly^2 / ET

    therefore distance between ETs should equal

    2(78500 ly^2)^.5 = 2(280 ly) = 560 ly

    Also note “Most of the stars were more than 1,000 light-years away”

    So I think we had better look farther/broader for unintentional communication and closer for intentional communication. Plus, if you think about it, even if there was a civilization at 500 ly away, they would have just learned that we discovered America and it will take another 500 years for us to learn what they think of that if they are inclined to contact. Fermi paradox solved I think. (For a further digression, according to the Fermi paradox, Native Americans would have been quite right in assuming that no intelligent life existed beyond their continent, that is, until the Europeans arrived)

  15. What makes this finding so useless to those of us who want to estimate thenumbers of ETs is that it only establishes what they think is an upper limit. But NO lower limit. Except one, of course..

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