Exoplanet news part 3: There may be hundreds of *billions* of planets in our galaxy!

By Phil Plait | January 13, 2012 7:01 am

[I’m trying to catch up with all the news that’s been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Here’s Part 1, and here’s Part 2.]

A very interesting set of observations has resulted in a conclusion that is somehow, paradoxically, both expected and startling: there are hundreds of billions of planets in our galaxy alone!

It’s expected because all the research being done for the past few years has been zeroing in on how many stars have planets, and it’s looking more and more like they’re very common. I’ll get into that in a sec. But it’s also startling, because HOLY COW THERE MAY BE HUNDREDS OF BILLIONS OF PLANETS IN OUR GALAXY ALONE!

Ahem. OK. So what’s going on here?

The new result comes from what’s called microlensing. The gravity of a star or planet can bend the light coming from an even more distant star, briefly magnifying it. The way the star light gets brighter over time can reveal the mass of the object doing the magnifying — the "lens", as it were. If a star passes in front of another star, you get a rise and then fall in the brightness, but if a planet is orbiting that nearer star, you get a second, smaller bump as well.

This kind of event takes an extraordinarily precise alignment, so they’re extremely rare. To compensate, you need to look at a lot of stars. So astronomers did: a survey using two telescopes covered several million stars every night, looking for the tell-tale bump(s). Over the course of six years, they found three — yes, only three — planets orbiting other stars acting like wee distant lenses. But that number is actually pretty good: when combined with previous surveys, and also taking into account how many lenses they didn’t see (which is important, statistically), they can extrapolate with some confidence about the numbers and types of exoplanets out there.

Their most basic result, and the one causing the stir, is that they find that there are likely hundreds of billions of planets orbiting other stars in our galaxy alone. Given that there are a few hundred billion stars in the Milky Way, this means on average there are about one or two planets per star in our galaxy! Now, let me be clear: this is an average. I’ve seen reports saying every star in our galaxy has a planet, and that’s not necessarily the case. You could have one star, say, with ten planets, and then nine with none and get the same results here.

The results get even more interesting when you break them down by planet type. They get that 17% (+/- 6/9%) of stars host Jupiter-mass planets, and 52% (+/- 22/29%) of stars host a planet with a mass near Neptune, and a whopping 62% (+/- 35/37%) have planets with a mass between 5 – 10 times that of Earth! These numbers have a somewhat high uncertainty to them (as you can see by the +/-) but even taking that into account, it shows that a lot of stars have planets, and it’s likely that most do. In other words, stars with planets are more likely the rule rather than the exception!

That, my friends, is very cool indeed. Other astronomer agree, too.

A couple of things to note: their method was not sensitive to planets the mass of Earth. However, given other results that lower mass planets are also abundant, and their own results mirroring that, it would appear that Earth-massed planets are extremely common. It may be too early to make that statement firmly given these particular numbers, but other results — like the three tiny planets found around a red dwarf — do make that very likely.

Also, this finding is for planets orbiting other stars. An earlier study from last year talked about rogue planets, unbound from stars, wandering the galaxy. The number of those detected implied there were billions of those, comparable to the number of stars int he Milky Way as well. That makes sense; those planets would’ve formed around stars and then been ejected by the gravity of bigger planets, which would be left behind. So the two numbers should be close.

And finally, a thought: in 1990 — when I was just starting to pursue my degree — we didn’t know of a single planet in the Universe orbiting another star. Not one. Then a couple of years later we found a few, then a few more, then more… and now look. Not only are we finding them, we’re using multiple methods to do so, and the upshot is that it might be hard to find a place that doesn’t have planets.

The Universe itself hasn’t changed in that time by very much, but wow, has our perception of it changed a lot. But then, science evolves. That’s what makes it strong: it seeks out more knowledge, more understanding, and better ways of knowing things. We sometimes have to abandon old thoughts, outdated notions, and comfortable misconceptions. But look what we get from it! An amazing and profound change in the way we perceive everything around us, including a galaxy overflowing with planets.

That sounds like a pretty good deal to me.

Image credit: ESO/M. Kornmesser

Related posts:

How many habitable planets are there in the galaxy?
New study: 1/3 of Sun-like stars might have terrestrial planets in their habitable zones
The galaxy may swarm with billions of wandering planets

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

Comments (52)

  1. Andreas H

    Whenever I am confronted with such news I feel enormous joy and wonder, followed by a strong feeling of sadness. Realizing how full of amazing objects our galaxy is and at the same time knowing that there seems to be no conceivable way to get there (even with an unmanned probe) in my lifetime is utterly depressing.

    While our methods of observing our surroundings seem to get better all the time our ability to actually move around and interact with the objects seems stuck without any real hope of progress in the near future, maybe ever. Sometimes I feel like a child being shown the most amazing toys just to find out they are behind a glass wall and I can never actually get to play with them.

    Thinking about the Fermi Paradox the same nightmarish thought keeps coming back. What if there are other civilizations out there, what if the universe is full of life but no one has ever been able to overcome the limits of relativity? What if Einstein was right and no matter how much we know, how good our observations get we will never be able to move objects or just communicate fast enough to explore our universe? What if we are trapped in our own little corner of space, teased by the most amazing objects never able to reach them?

    Such a depressing thought…

  2. JMW

    …[science] seeks out more knowledge, more understanding, and better ways of knowing things.

    To seek out new knowledge, new understanding, to boldly learn where no one has learned before.

  3. Stan9fromouterspace
  4. Peter Davey

    With regard to Andreas H’s worries, Arthur C Clarke, in his book, “Profiles of the Future”, devotes the first couple of chapter to negative predictions – how flight was impossible, how we never could know anything about stellar bodies except the fact of their existence, how artificial satellites were impossible, how expeditions to the Moon were impossible, etc, etc.

    The difficult we do easily, the impossible takes a little longer.

    (Current) science may not be on our side, but history seems to be.

  5. Chief

    I’m glad the Drake equation is looking better and better.

    I have a thought on the distance between the worlds and the need to communicate between same. What is to say that if a civilization does send out information to a distant world that that civilization would be there thousands or millions of years later to know that it worked. Thus the best they can hope for is to say that “we are here” and just listen for someone’s else’s transmission of the same message. Now we have been listening for near 50 years now and have not heard anything. The past thinking was that we didn’t know where a likely candidate for a world to have the technology to send a message but if there are indeed that many worlds. Then the odds that a signal coming from any point in the galaxy increases. The issue then is when the civilization has become technologically advanced. I would really find it a stretch that we are the first in several million years to have reached it first. (figuring on a signal traveling several hundred to millions of years to reach us). Of course the signal could have been broadcast back in the dark ages and has long passed us by now.

  6. Daniel J. Andrews

    According to Brian Cox’s book, Why Does E=mc^2?, it is possible for a human, within their lifetime, to travel to another galaxy even travelling at less than the speed of light due to the relativistic effects. Only something like 50 years would pass for the traveller, but several million years would pass here on earth. So we’d never know if the travellers would arrive or not–we’d just have to go faith that we seeded the galaxy and haven’t gone extinct (although considering our track record so far, maybe replace “seeded” with “contaminated” or “polluted”).

    Just tried looking for that book reference to get my numbers right, but didn’t find it (there’s no search function on the software I’m using at the moment).

  7. We know how much matter there is in the universe, right? But we have no idea how much is planets versus how much is stars?

  8. Not to be a wet blanked, but my takeaway from this study wasn’t so much that the galaxy is brimming with planets, (we had a good clue that this should be the case already), but that the astronomers tried a very neat physics trick that will help us better target our orbital planet-hunting telescopes in the future, but which was not all that useful in predicting the composition of alien solar systems.

    In fact, your own post quoting their uncertainty margins show that according to the findings, between 25% and 97% of the observed stars may have an Earth-like planet. That’s not just a margin of error, that’s a yawning chasm of uncertainty unless I really misunderstood something important here…

  9. Wzrd1

    Thanks for clarifying the reports, Phil. One of the reports that were incorrect, claiming EVERY star had planets was on the BBC.
    My first thought was impossible! Close binary and trinary star systems would perturb any matter enough to prevent planetary formation or eject any planets that managed to form.

  10. GebradenKip

    Aren’t numbers like 52% (+/- 22/29%) ridiculous? That means that we are 95% sure (two-sigma) that the value is somewhere between 0 and 96%.

  11. Satan Claws


  12. LSO Bob

    Thanks for that great series, Phil.
    Will this explain some of the dark matter and dark energy that we are trying to understand?
    If we consider humans have had intelligence for at least four thousand years, then sending out signals to announce we are here on a planet we call Earth for the last fifty years represents a small fraction of that time frame. We still have a lot to learn.
    The odds of life on other planets is increasing but as we see these new planets around stars many light years away, when we find the right one, we cannot know at what stage of their evolution we will see the planet. With the majority of the universe unknown to us, we need to keep taking what seem to be small steps to understand what it is and how it works. I think we are doing a great job of learning about the universe and need to continue solving the problem one piece at
    a time.

  13. aleksandar

    Lightspeed limit is irrelevant for implications of Fermi’s Paradox. Even at 1% or 0.1% of C your Von Neumann probes should be in every system in the galaxy after some 100 million years.

    Then. Any hypothetical super civilization (Kardashev 3) would be likely observable across entire local group or even further. Even if extremely rare their lifespan and visibility across intergalactic distances would make them most likely ones to be detected.

    And we see nothing, nowhere.

  14. Bruce of Canuckistan

    @1 Andreas

    “.. knowing that there seems to be no conceivable way to get there (even with an unmanned probe) in my lifetime is utterly depressing…”

    Or looked at another way, our best bet for seeing an interstellar probe in our lifetimes isn’t investment in the space program, it’s investment in biotechnology.

  15. Sam H

    @13 aleksandar: You might find this of interest then :)http://www.centauri-dreams.org/?p=14157. And how do you know our system hasn’t been visited in it’s 5 billion-something-year lifetime, especially when any probes might well be nano-sized (as small size means light weight, therefore higher speed)?? Sure we can’t assume that any are there without evidence, but we’ve barely searched – hell we have a hard enough time just sending space probes to natural bodies under austerity measures!!

    And I’m not sure about Kardashev III civilizations in general – to attain the energy of an entire galaxy would take eons, and it may not even make sense anyway. Intelligence is probably rare in the universe – at most I think there’s only a few hundred to a few thousand civilizations in our galaxy at any given time, not the millions envisioned by Sagan and others. The fact that any of these will be at least several thousand ly away explains the Fermi Paradox relative to SETI – and 50 years is a blink of an eye even on human historical timescales!! KII civilizations I think are more likely, and while Dyson Spheres or stellar beacons are visible over interstellar distances, again we haven’t searched much for these. And there are likely more efficient means of energy production – black holes anyone?? :)

    Point is – the universe is queerer than we can suppose. The same thus applies to alien civilizations. And we’ve only searched for 50 years in the radio spectrum, and less for other types of possibly alien activity – When we search for 1000 years or so, THEN we can realistically say they are or are not there.

  16. Ganzy

    From a science-fiction perspective.

    What if we as a species made some major breakthrough in physics in the not to distant future which allowed us to achieve intergalactic travel through some funky spacetime distortion effect, much like the timelord utilizes himself.

    Could it be possible for our extremely technologically advanced descendants 300 years into the future from now, if, say, they had mastered the ability to travel back through time, visit us in the here and now? If only to satisfy the same curiosity we would have if we could travel back in time, say 2000 years on earth.

    Didn’t Hawking say something along the lines of: If the species had mastered time travel, then why I haven’t we been visited by our descendants from the future yet?

    If our future descendants do master time travel, and did drop by to check up on us
    from time to time. One reason they would not make contact, is because by doing so, they may change the course of our evolution and ultimately theirs.

    They don’t mind us clocking a fleeting glimpse of their equipment to pique our curiosity, but a meet and greet would be out of the question for fear of changing our mutual timeline.

    This story line must have been played out in some science fiction novel somewhere?
    Any sci-fi-fans recommend a good read along these lines?

    With all those planets out there it wont be long now before we find the crib of the original Kryptonite cornflake cruncher and man of steel himself: Wzrd1.. woops.. I mean.. Sir Clark of Kent! 😀 😀

  17. Brian

    Does our planets make us look fat?

  18. Chris Winter

    As Aleksander suggests in #13, this discovery makes the Great Silence that much more puzzling.

  19. @Chief,

    I’ve always thought that our not finding a signal could be more of a result of our own limitations than of a lack of other civilizations out there.

    First of all, a signal would need to be timed just right. It would need to have been sent out long enough ago that we would receive it but not so long ago that (as you said) it would pass us during the Dark Ages.

    Secondly, we would need to be looking in the right direction. When SETI searches the skies for signals, they do so one portion at a time. So while they are searching Section 23, a “Hi There from Zorax 3” could be passing by in Section 42.

    Thirdly, we would need to be using the right technology. We’re searching for radio transmissions. Perhaps a species’ use of radio is limited (and thus difficult to detect). Maybe all of the aliens out there communicate via subspace transmissions or something else that we haven’t figured out yet.

    Finally, we would need to be able to recognize that it isn’t gibberish. Would you be able to tell that a phrase in Russian wasn’t gibberish? (If you know Russian, substitute some language you don’t know. Preferably one with a different alphabet than English.) What if the Russian message was encoded as a MP3? What if the encoding wasn’t MP3 but some private encoding schema that you’d never seen before? Now mix an alien language and alien encoding scheme in the mix. We might get a “Hi there” message from a planet and think it’s just noise because we don’t recognize how they encoded the message.

  20. Bruce of Canuckistan


    Agreed. Even for radio, I’ve read that the highest efficiency in terms of energy/bit is achieved with a spread spectrum signal encoded using pseudo-white-noise. So if you didn’t know the encoding, and especially if you were slightly out of the beam, a truly efficient signal would be nearly invisible.

    So the only way radio SETI would work is in the case of a beacon deliberately sent out for newborn civilizations, left on for a very long period of time. And why bother, if newborns are typically loud and dumb enough to announce themselves anyways?

  21. amphiox

    What if Einstein was right and no matter how much we know, how good our observations get we will never be able to move objects or just communicate fast enough to explore our universe? What if we are trapped in our own little corner of space, teased by the most amazing objects never able to reach them?
    Such a depressing thought…

    Then we will simply have to derive our joy from the knowledge that they are out there itself.

    Surely, if it is possible for many people to find meaning and satisfaction from believing in supernatural things they can never touch, never see, never feel, never even prove to exist, then it should be possible to be satisfied with knowing something as amazing and beautiful as this, even if we can’t ever go there.

  22. Michael

    I have to believe it is a safe assumption to say that there are many civilizations out there in the galaxy, and probably a few of them are capable of some level of interstellar space travel (broken into 2 groups – ‘one way trip’ and ‘can get back to where they came from’). Lets say we recognize both an alien message and its data packaging. There is no guarantee that we will be able to make use of the info. Since, unless we as a species manage to gain complete and total control over our planet (no earthquakes, no volcano eruptions, etc), we only have a very short amount of time to get off the planet and colonize. Otherwise, we go extinct, or darn close to it. I would guess maybe 2000 years at the outside. So, we need the closest exoplanet in the habitable zone and hope that it is not already occupied. And then we need the ability to go there en masse. All else is secondary, tantamount to party conversation, unless you don’t mind the human race being wiped out.

  23. amphiox

    Any hypothetical super civilization (Kardashev 3) would be likely observable across entire local group or even further. Even if extremely rare their lifespan and visibility across intergalactic distances would make them most likely ones to be detected.

    But would we recognize such a thing even if we did see it? Perhaps some active galaxies really are the handiwork of KIII’s. Maybe some GRBs really are alien weapons…. (or industrial accidents).

  24. sangos

    IMO radiowaves for alien communication is like trying to use a smoke signal in say Los Angelos to flag down the White House….its futile but thats the best we reached yet up the tech ladder! but keep doing it for if the other earths have more advanced life they will see us coz they probably would have used radio in their prehistory!!

  25. amphiox

    Will this explain some of the dark matter and dark energy that we are trying to understand?

    Not likely to any significant degree. Consider that the mass of the sun is about 1000x the mass of Jupiter, and that currently observable baryonic matter constitutes 4-5% of the cosmos, while dark matter seems to constitute about one third (so there’s almost ten times as much dark matter by mass). Even if planets outnumber stars by 10 to 1, or even 100 to 1, the total mass of all the planets would still be just a minuscule fraction of the total mass of all the stars.

    Not to mention that when one talks about the effect of the gravity of a star on other stars (and by extension the gravity of a group of stars, ie a galaxy, on other groups of stars, which of course is how we figured out that there actually is dark matter out there in the first place) the gravity of the “star” that we observe is actually the combined gravity of the entire star system – everything in that system that orbits the same center of gravity will contribute to the gravitational effect of that system on anything outside of it. And that means that we are already automatically including the effect of any planets that are already orbiting stars when we measure the effect of that star’s gravity on distant objects. And the total contribution of the planets in a star system to the star system’s total gravity is probably less than 1%. (Again, Jupiter is only 0.1% the mass of the sun, and Jupiter by itself is more massive than everything else in our solar system, all the other planets, dwarf planets, asteroids, comets, etc, combined).

    And therefore the only planets that could contribute in any way to the measured dark matter gravity are the rogue planets, and these planets would have to outnumber the stars by at least 1000 to 1 to even just equal the gravitational effect of the stars. To contribute a significant portion to the dark matter effect, they’d have to outnumber the stars by over 10000 to 1.

  26. amphiox

    So, we need the closest exoplanet in the habitable zone and hope that it is not already occupied.

    Not necessarily. Who says that planets are absolutely required for space colonization? Assuming no lightspeed breaking, if you actually have the life support technology to send a colony ship to another star system, you actually don’t need planets anymore. You can build space habitats at will. (A space habitat is just an interstellar spaceship without an engine.)

  27. amphiox

    All else is secondary, tantamount to party conversation, unless you don’t mind the human race being wiped out.

    The human race is likely going to be wiped out no matter what, eventually. But we’ll have a longer run if we get off this rock. Possibly a MUCH longer run.

  28. amphiox

    and while Dyson Spheres or stellar beacons are visible over interstellar distances, again we haven’t searched much for these. And there are likely more efficient means of energy production – black holes anyone?

    In agreement with the last sentence here, I actually think Dyson Spheres are probably very unlikely. If you really think about what a Dyson Sphere entails, you realize that at the heart of the conception lies a glaring anachronism.

    The energy generation/collection technology envisaged for it is decidedly 20th Century (passively collecting stellar radiation via surface area absorption) while the construction and engineering technology needed to actually build such a thing is well up in the 30th Century range.

    In other words, if anyone out there has the capacity to build a Dyson Sphere, it is highly unlikely that they’re going to need to use anything so crude.

    So unless they’re building it as an art project, or children’s toy….

  29. Jess Tauber

    Yeah, your dirtside civilizations will be mighty impressed with the billions of bound and rogue planets out there, as places to potentially settle in and call home. My belief is that truly advanced civilizations (if the term even applies) will have gotten beyond the need to keep their feet (or whatever) on the ground. Perhaps they might even convert themselves to something else (say, coherent systems of tachyons or other superluminal materials). At that point visiting anyone anywhere just entails knocking on the door. Why haven’t they contacted us? Busy off making universes, I’d guess- but an occasional Christmas card would be nice. :-)

  30. Daffy

    Years ago A.E. Van Vogt wrote a short story about aliens invading Earth—they knew to come here because they alone had discovered a “secret” method of detecting stars with planets. It was generally assumed back then that we would never, ever know if other stars had planets at all. What a long way we have come since then. Some smart people behind all this—kudos to all of them!

  31. Chief


    Barring the theoretical space plus (or minus), ie star trek etc, realms used for communications, We humans have only the electromagnetic spectrum in which to draw from. This includes radio, light and then the higher freguencies of gamma and Xray. All of which we do scan to a degree and should someone be communicating in this band, I feel what we would notice something interfering with our own data in use.

    Note that any advanced communications will be limited to certain frequencies as mimics by natural sources will swamp out or degrade the signal over long distances of travel. We try to focus our receiving equipment to be most sensitive over the bands where there is the least amount of interstellar noise generated. (of course SETI has giga band receivers being used to try to keep an open mind on the search.

    I was thinking that with the increase of potential stars with planetary bodies, figuring out where to point becomes less of an issue, and distance and time of transmission is more relevant. (I think….)

    If you really wanted to overcome the distance and possible amount of data “particles” reaching us, maybe we should look for AM or FM modulation in Tachyon particles….

    Hmm. how to build a Tachyon generator without a lot of mass.

  32. MadScientist

    And yet SETI can’t find ET.

  33. Tim Gaede

    I hope we soon have an exoplanet with a real name rather than a designation. Perhaps such a planet would need to be Earth-sized and in a “Goldilocks zone”. Maybe the IAU would require sufficient data on the planet that is not just related to its size and orbit. I suppose the Terrestrial Planet Finder would have to be resurrected in some form in order for that to happen.

  34. Adrock

    We can’t see anything so there’s nothing out there? We couldn’t see germs four hundred years ago but as it happens, they’re all over the place.

  35. Jess Tauber

    Re.33.- if you look very very closely you will see ET, hidden right in the middle of SETI… 😉

  36. Brian Too

    Personally I think that the Dyson Spheres and Rings are unlikely. These constructions are gravitationally unstable around their host star. While any civilization capable of constructing such a device might also have suitable station-keeping technology, this is technically inelegant and energetically inefficient.

    Also, one thing about the Dyson objects that has always bothered me. They are supposed to use planetary resources to create a thin, strong, stable shell. However metals are by far the most suitable material for such construction. AFAIK most of the material in most planets is rock. Rock is strong in compression but very weak in tension and entirely unsuitable for creating thin shell structured objects. What am I missing? Unobtanium?

    @16. Ganzy,

    There were numerous Star Trek Voyager episodes exploring these themes. The series adopted the use of a Temporal Non-Interference rule to explain why the time travellers were not normally seen/known.

  37. Chris

    @16 and 36
    See http://xkcd.com/716/ for an explanation about time travel.

    @36 Brian
    The Dyson Sphere in ST:TNG was made of a carbon-neutronium alloy.

    Although another big problem is where to get all the material. Assuming a sphere the size of the Earth’s orbit and 1 m thick, this would need a volume of material of 2.7 x 10^23 m^3. Now if you made it out of iron with a density of 7860 kg/m^3, you would need 2 x 10^27 kg of iron. The Earth weighs 6 x 10^24 kg so you’d need about 350 Earth masses of iron. Possible but way outside our current capabilities.

  38. The universe is more peculiar the more we look at it. We, on the other hand, seem to be unique on this planet, and our development by no means inevitable. Flight, for instance, which seems so unlikely, has evolved again and again. Birds fly, bugs fly, even some kinds of mammals fly. But, so far as we know, city-building intelligence evolved only once, and that very recently. So I am not surprised by the failure of SETI. We are misapplying the principle of uniformity, I think. The laws are the same everywhere, but the results are gloriously different.

    Will we ever leave this beautiful island world and strike out across the the vacant ocean of space in search of new worlds to conquer? Probably, since as a species we seem to have an innate wanderlust. Long before technology made ocean travel safe and practical, humanity had spread almost over almost the entire Earth. We are more widespread, I believe, than any other single species. It seems unlikely that mere distance will stop us from spreading further, some day.

  39. Messier Tidy Upper

    [I’m trying to catch up with all the news that’s been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Here’s Part 1, and here’s Part 2.]

    This is part 2? I thought it was part 3! 😉


    Great news & fascinating field of astronomical inquiry. I love reading about this stuff. Thankyou BA. :-)

  40. tak

    great series! I’m new to this and was wondering if there’s a way to detect planets with Neptune-like orbits? Meaning planets further out which don’t transit the star that often.. and how are the planets counted whose orbit is not on a plane that allows us to detect a transit?
    This is just so fascinating and mindboggling.. I want more^^

  41. Dragonchild

    Estimate still seems too low.

    Bear in mind that for us to find a Jupiter-sized planet with a semi-major axis similar to Jupiter itself (one revolution every 11 years), we’d need up to 11 years of non-stop observation, not including duplication which would add another decade. Neptune takes over 160 years per revolution. Only two planets in our system could be discovered through current observation methods in reasonable time. Mercury and Mars are too small; everything else is too far out. Not complaining about the efforts, which are remarkable, but we gotta admit we’re still guessing here. If there’s one thing to be learned about Fomalhaut b, it’s that some planets orbit stars at over 100AU — more than twice Pluto’s distance from the Sun.

  42. amphiox

    And yet SETI can’t find ET.

    With all due respect to the efforts of the SETI people, right now SETI is akin to a colony of relatively intelligent ants trying to discover the existence of humans by trying to sniff the pheromones in the cubic metre of air around their nest.

  43. Jorge

    @8. Greg Fish

    I think it is an acceptable margin of error because it’s within a single order of magnitude.

    Having in mind that it was possible that the probability was around 0.001 or 0.0000001 (or even less… we had no idea after all), being sure that the probability is between 0.1 and 1 is pretty good indeed.

  44. Al

    @37 Brian Too: There exist Dyson sphere designs that are gravitationally stable (Dyson swarms, statites).

    What are rocks made of? Metal and semi-metal oxides. Plus, only the crust of planets is rocky – the interior of the Earth is basically a big ball of Iron and Nickel. This is because early in Earth’s formation when everything was molten, the heavier elements sank down to the bottom and the lighter ones (oxygen, silicon, etc.) floated on top.

    Plus, metal isn’t even the best construction material. Carbon fullerenes and graphene are superior building materials and carbon is relatively abundant in the Universe.

    The main objection to a Dyson sphere is, as Chris said, the sheer *amount* of material you’d need. Considering the solar system, you’d need to break up every planet to get enough material for even a thin shell.

    However, it’s quite possible that other star systems out there have a wealth of building materials. We’ve seen systems with a large number of huge planets. Plus, there’s always the possibility of towing in material from outside the star system.

    Personally I think it’s likely that at least a few alien civilizations have built Dyson spheres, but probably not enough for us to have a high chance of detecting.

  45. Deepak

    Given the enormous distance between stars and the constraints of the human race in terms of the speed, inter-galactic travel seems almost imposssible, at present. Would a more serious research on parallel universes makes sense? If multiple earths are in arrangement next to each other with different histories (wherein some of the earths may be fully empty!)then all we need to do is to find the door / window that leads us to the other earths to occupy!

  46. Nigel Depledge

    Gosh, I’m quite stunned at the number of comments about radio transmissions that fail to take into account the fundamental features of what a radio wave is.

    (OK, I’ve only read as far as the mid-twenties so far, so there might be something enlightening that I just haven’t reached yet.)

    First, IIUC, nearly all natural sources of radio waves have broad bandwidths. Artificially-produced radio waves (except in perhaps the first 20 – 30 years of our use of radio) have narrow bandwidths. Certainly a suffciently narrow-band signal would be worth following up with additional observation.

    Second, if a radio signal encodes information, there are only a very few ways of varying a radio signal. On/off keying, for instance (as in Morse code transmissions) would be pretty obvious, even if we couldn’t hope to decode the message’s content. Similarly, modulation of the frequency or amplitide of a signal would be obvious. A narrow-band signal with some kind of modulation on it would be a pretty good sign of ETI, but you’d have to question how intelligent ET is. After all, we’ve considered how we might encode a message to transmit to the stars, and we’ve concluded that the encoding would need to be pretty obvious (how about a repeating sequence of bits, where a positive frequency deviation represents one and a negative frequency deviation represents zero, in which the number of bits is the product of two primes – with this, we have encoded an image).

  47. Nigel Depledge

    Hmm, just realised some of my last post is not clear.

    So, to clarify – you’d have to wonder how intelligent ET is if you can’t decode the message. We’ve thought about how to encode a message in such a way that it’s reasonably easy to decode without any knowledge of who or what we are.

  48. andy

    Dyson swarms are vulnerable to Kessler syndrome. You’ll need to ensure all your satellites are supplied with enough reaction mass to do course correction, and make sure that when you fire your rockets you don’t impinge on any of the other satellites, etc.

    They just don’t strike me as something you’d really want to build, for a whole variety of reasons.


  50. Wow, am I ever happy I arrived to your website. Wicked stuff!

  51. Kevin J

    When it comes to life on other worlds my gut feeling is that while microbiol life maybe quite common, complex life may by quite rare, while intelligent life of our kind – life that can communicate beyond it’s home planet – maybe very very rare indeed.

    Our type of intelligence doesn’t seem to be an end product of evolution, but rather an off-shoot. There’s no real reason to believe that it evolves on all planets where life has begun, after all it’s only happened here once in 3 billion years of evolution.

    The other even more depressing thought, or maybe from another perspective amazing thought, is that life really is an improbable event and that it’s very very rare in the universe indeed. And of course if that’s true then this little blue world of ours is even more important and special.


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