≡ Menu

One Hundred Billion Trillion Habitable Planets

Alan Boss, whose new book The Crowded Universe will soon be on my shelves (and reviewed here), has driven the extrasolar planet story to the top of the news with a single statement. Speaking at the American Association for the Advancement of Science’s annual meeting in Chicago, Boss (Carnegie Institution, Washington) said that the number of Earth-like planets in the universe might be the same as the number of stars, a figure he pegged at one hundred billion trillion.

A universe teeming with life? Inevitably. The Telegraph quoted Boss on the matter in an early report on his presentation:

“If you have a habitable world and let it evolve for a few billion years then inevitably some sort of life will form on it,” said Dr Boss.

“It is sort of running an experiment in your refrigerator – turn it off and something will grow in there.

“It would be impossible to stop life growing on these habitable planets.”

Few Centauri Dreams readers would disagree with the notion that life may be common in the universe, but what about intelligent life leading to technology? That’s a far greater challenge, and Boss notes that our own civilization will be unlikely to exist in another 100,000 years. The odds on our running into another civilization at roughly the same stage of development are vanishingly small. Let’s see what Kepler finds. The planet-hunter lifts off in a scant three weeks on a mission Boss believes will find a habitable terrestrial planet within four years. How we would accomplish the unmanned mission to study this world that Boss refers to is something we continue to speculate about on Centauri Dreams.

Apropos of Boss’ comments, our man in the maritime antipodes, Paul Titze, sends along this memorable quotation from Christiaan Huygens, who wrote of these matters in 1695:

What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock’d with so many Herbs, Trees and Animals, and adorn’d with so many Seas and Mountains! And how must our wonder and admiration be increased when we consider the prodigious distance and multitude of the Stars?

Comments on this entry are closed.

  • kurt9 February 17, 2009, 12:04

    100 billion trillion habitable planets? Are you sure? Plate tectonics? The Eukaryote?

  • Ronald February 17, 2009, 12:13

    It seems a bit overly optimistic to me to suggest that there would about one Earthlike terrestrial planet *with life* for every star in the universe.
    The fraction of sunlike (solartype) stars in our MW galaxy is about 1.5 to 7 %, depending on the strictness of your definition. Some galaxies may have a higher %, others lower.
    One in a hundred would be fantastic, even one in a thousand still very abundant.

    BTW, despite the fact that I am Dutch, I did not know this great quote from Christiaan Huygens, what a visionary!

  • Ronald February 17, 2009, 12:16

    The number of stars in the observable universe, BTW, is estimated between 30 and 70 times 10^21.

  • James M. Essig February 17, 2009, 13:06

    Hi Paul and Maciej;

    One hundred billion trillion habitable planets! That is potentially one trillion within the Milky Way alone. Indeed, the Milky Way and perhaps the visible universe could be teaming with life, and ETI civilizations could be very common.

    In the event that most or nearly all of these potenitally habitable planets do not contain ETI civilizations, the potential number of settlements for humans is awesome. I like to consider the huge number of persons that such a huge number of habitable planets support.

    Let say that Earth can somehow support 100 billion persons through going completely green, sustainable, and energy efficient by using solar, wind, ocean wave, and hydroelectric power generation. Then the number of human beings on this totality of 10 EXP 23 planets would be a huge 10 EXP 34. If each person lived on average one thousand years planet time, in 5 billion years, the number of human beings brought into existence would be 5 x 10 EXP 40 which is as great a number as the number of human red blood cells it would take to equal the mass 25,000 Earths, but in a volume of about 7 times greater yet.

    Upon we humans first setting boots on an extrasolar planet, the whole of humanity is going to be jubulent. I believe that reports of such potentially huge numbers of habitable planets is going to spur on the race to get to another star system sometime during the first half of this century. I can not think of any thing that would unify the human race more.



  • Brandon Bell February 17, 2009, 14:13

    The Silence that Fermi questioned is actually not such a puzzling issue in that we have likely under estimated the difficulty of signal detection and the frequency and duration of tech civilizations. My intuition, not worth much of anything except maybe as a guide toward some hopefully interesting story material, tells me that extant civilizations will have reached ‘post-organic day one’ , essentially a post-singularity society. But before this sounds too far-fetched, I merely mean that a society will have created self-replicating, sentient, spacefaring offspring that are no longer limited by organic life-spans in order for a civilization to live past either their initial technological phase (and remain technological instead of merely agrarian) or the habitability of their home planet.


  • Didac February 17, 2009, 16:04

    It’s very difficult to make prediction with only half a biosphere in mind (half in the sense that we have an idea of the first 4 billion years of Earth’s biosphere, but very few certainities in regard to the next 2-4 billion years in future). But with that sample of 4 billion years we have:

    – 30% of time (1000-1500 million years) of complex cells.
    – 15% of time (1000-1500 million years) of complex organisms.
    – 0.00000005% of time (50 years) of space-faring civilization.

    Anyway, in 1023 planets there were 1015 space-faring civilizations. But 1015 is a number enough diluted of samples (number of ‘simultaneous’ civilizations / cubic megaparsec) to explain Fermi’s paradox.

  • Hiro February 17, 2009, 19:24

    The number is too high, I think it’s way overestimated. I would be really happy if our galaxy has one million earth-like planets or more. Still, I don’t buy this estimation.

  • Benjamin February 18, 2009, 2:26

    I like the Huygens quote.

    The implication of there being this many habitable planets seems to be that ETI is vanishingly, invisibly rare, for us not to have detected even one identifiable signal in what has become a fairly sophisticated search for them. I am very hopeful that there is ETI out there, and I think such a hope is reasonable within the bounds of what we know; but for there to be such an extraordinary profusion of life in the universe seems excessively hopeful, given that we have not yet seen any of it.


    I would be jubilant just to know anything at all about an extrasolar habitable planet. But to mount a mission there with the aim of colonisation is something I cannot imagine is plausible or advisable within the next century; intensive study and observation would be a must, and I say intensive in the sense of being, perhaps, the very most intensive burst of scientific activity mankind has ever conducted. But if we do not have experience from the Moon and Mars, of setting up bases in hostile environments on other celestial bodies with limited support from earth… it would be a waste of resources to colonise an extrasolar world, at least until we have experience and technology developed in our own backyard.

  • James M. Essig February 18, 2009, 10:16

    Hi Benjamin;

    Thanks for your insights.

    You make some excellent points. Going back to the moon by 2020, and then on to Mars a decade or so later, I hope will jump start the global communities desire to reach ever further outward.

    I read in a recent issue of Popular Science or Popular Mechanics Magazine that NASA is developing 40 KW to 80 Kw nuclear reactors that would be burried about 5 meters below the lunar surface and which could run for 7 to 8 years between refueling an anticipation of powering lunar settlements and outposts. So nuclear energy I hope will facilitate our settlement of the Moon and Mars.

    As for the extrasolar worlds, I think you are likely entirely correct with the exceptions of the possibility of some practical unforseen break through in faster than light travel.

    If we can learn how to break matter down directly into pure energy, then the concept of the interstellar ramjet will be limited to gamma factors imposed only by friction and drag and extreme gamma factor induced cosmic rays and dust impacts.

    I will settle for a visit to the moon by 2030. and anticipation of what further dreams may come. By then I will be 68 years old.



  • ljk February 18, 2009, 10:52

    Huygens quote comes from his grerat work titled Cosmotheoros,
    which was first published in 1698, just three years after his death.

    Thanks to the Internet, the entire work is online in English here:


    Note that while Huygens was at the forefront of imagining life on
    other worlds, he generally assumed that any intelligent alien beings
    would have to be similar to humans, as they would need hands
    with fingers to perform complex functions.

    Humans, bound to this one little planet for the vast majority of our
    relatively short history as we have been, continue not to truly
    appreciate just how big the Milky Way galaxy is, let alone the
    entire Universe (to say nothing of a Multiverse).

    We have barely begun to explore the outer worlds of our own Sol
    system, let alone even the nearest neighboring star system. We
    have found just over 330 exoplanets so far, most of them giants
    bigger than Jupiter circling very close to their stars. And most of
    them we haven’t even seen directly yet – and the few we have
    are just little dots of light, nothing more.

    An entire ETI mission could be wandering the Planetoid Belt and
    we would have no clue about it, as hiding from us in even near
    space is not a difficulty. ETI could be on the far side of the Moon
    and be safe from our senses and sensors at present, without
    needing cloaking devices or any other kind of fancy concealment

    Most of our SETI programs have been fairly sporadic since the
    first modern one conducted briefly by Drake in 1960. The vast
    majority have also limited themselves to the radio spectrum.
    Since our best bet is to find beings more advanced than us,
    searching for radio messages may be like the old addage of
    a primitive tribe trying to find other tribes by their smoke
    signals or drums.

    So with this in mind, plus the possibility that there are kajillions
    of inhabitable worlds in the Universe, we should be neither
    surprised nor disappointed that we have not been contacted by
    ETI yet or come across some sign of them.

    There is a grand adventure ahead of us. This is why I support
    both an expansive SETI program and getting some kind of
    actual interstellar probe mission on its way by 2050 or at least
    by 2100. Unless we get a visit or a message, these are our best
    methods of finding extraterrestrial life.

  • george scaglione February 18, 2009, 10:53

    thank you guys all of the above is very interesting.but i do not myself buy so large an estimate. ben and jim, also good stuff and thank you. although i do not like it i have to agree that the estimate of back to the moon by 2020 is also probably not right on!2030 might indeed be more like it and then naturally on to mars from there!! my guess would be if all goes well that then that puts us on mars by 2050! lol i’ll be 101 years old! think i’ll be selected to be a member of the first crew to go!!?? :) respectfully guys your friend george

  • andy February 18, 2009, 16:20

    I could buy an estimate of 1 habitable planet per star, provided that this was not restricted to Earth-type worlds. After all in our solar system alone there are at least half a dozen more potentially habitable worlds other than Earth. The vast majority of them are ice moons.

  • Denver February 18, 2009, 17:25

    James M. Essig Says, “Let say that Earth can somehow support 100 billion persons through going completely green, sustainable, and energy efficient by using solar, wind, ocean wave, and hydroelectric power generation.”

    Dude. Seriously. You’d be lucky to feed and clothe 3 billion. Living in a fantasy world is nice and all, but day-am I hadn’t realized how far the disease had spread.

  • James M. Essig February 19, 2009, 16:48

    Hi Denver;

    Thanks for the perspective.

    Going nice and green does not mean we need to limit our population. With low environmental impact and sustainable industry as well as the use of renewable energy, I am convinced that we can support 100 billion people on Earth.

    We already have 7 billion people on Earth, and I think just about all of them wear clothes. By the way, I am an advocate for free market capitalism but with appropriate governmental regulation and corrective measures so that we do not have another economic meltdown such as has ocurred in the housing market industry within the U.S..

    In short, going green will help the human race to grow rather then stiffle its growth or impose undue limits on its growth. The same applies to human populations that we will develop on extrasolar planets.



  • andy February 19, 2009, 16:55

    Living in a fantasy world is nice and all, but day-am I hadn’t realized how far the disease had spread.

    No it’s not a fantasy, because the Singularity is based on science not mythology, so what seems to be the application of magic pixie dust is actually totally reasonable. Honest.

  • MaDeR February 19, 2009, 18:39

    Why no one notice that Alan Boss talks about *habitable planets*, not *inhabited*? Yeah, I know that he thinks leaving planet in sun’s rays inside habitable zone for a few bilions years inevitably leads to life, but I am not THAT optimistic. Bad atmosphere and no life. Bad chemistry and no life. Venus is prime example of world that had chance to become habitable. And lost.

    From other side, we have example of almost-unhabitable world, Mars (“almost” make BIG difference). And vaguely promising Jupiter moon, Europa. And even more uncertain Enceladus or Titan.

    So maybe Alan is not THAT way off? In our system alone we had THREE planetary tries (with one succesful for certain) and at least three interesting moons in this regard.

    So… not hundred bilions in our galaxy, but hundred milions planets and who knows how many moons. Feh. Almost nothing. ;)

  • Ronald February 20, 2009, 6:37


    I agree with most of what you say, but for one issue: I think that population growth and maximum population size are not and must not be goals by themselves, but only means to achieve certain other goals, such as maintaining and advancing technological civilization and spreading our life and civilization through the cosmos.

  • Ronald February 20, 2009, 6:44

    @MaDeR: but he also states: ““If you have a habitable world and let it evolve for a few billion years then inevitably some sort of life will form on it,” said Dr Boss.”
    So, that indeed seems to refer to planets with life.

    But if you are right, and there are many potentially inhabitable planets (right temperature, water, some form of atmosphere), but still uninhabited by any life, that would in fact, as I have argued before, be good news to us humans as well: a lot of real estate for the picking (i.e. either direct colonization or terraforming) and no ethical issues with regard to inhabitant life, which, I believe, we should always treat with the utmost care and respect.

  • george scaglione February 20, 2009, 13:08

    ronald yes sir that would be good news indeed! only “problem” getting there.lol ! the place might be thousands of light years away! however on the positive side that is the kind of thing we spend time thinking about here. thank you very much your friend george

  • Ron S February 20, 2009, 13:55

    ‘Habitable’ is a slippery concept. If we were to land on Earth before the emergence of life and pull off our spacesuit helmets, we’d be dead in under a minute. That’s one version of ‘habitable’. If we want a planet just a wee bit less deadly we require it to be ‘inhabited’ so all that life can make the place more comfortable for us. That’s a different version of ‘habitable’.

    If we discover an Earth-like exo-planet, and if we want to go there, there is some question as to which version of ‘habitable’ would be best. Both have their pros and cons, and it would also depend on why we want to go there – explore or move in.

    Anyone else notice that habitable and inhabitable seem to be synonyms?

  • ljk February 24, 2009, 0:59

    Seeing another Earth: Detecting and Characterizing Rocky Planets with Extremely Large Telescopes

    Authors: Philip M. Hinz (UA), Scott Kenyon, (SAO), Michael R. Meyer (UA), Alan Boss (Carnegie-DTM), Roger Angel (UA)

    (Submitted on 23 Feb 2009)

    Abstract: The detection of lower mass planets now being reported via radial velocity and microlensing surveys suggests that they may be ubiquitous. If missions such as Kepler are able to confirm this, the detection and study of rocky planets via direct imaging with ground-based telescopes of apertures > 20 m is feasible in the thermal infrared. We discuss two cases for detecting rocky planets, the first via detection of molten Earths formed though an Earth-Moon like impact event, and the second via detection of planets around very nearby stars. These observations have the potential to give us a first look at a rocky planet similar to the Earth.

    Comments: White paper submitted to the Astro2010 Decadal Survey PSF panel

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0902.3852v1 [astro-ph.EP]

    Submission history

    From: Philip Hinz [view email]

    [v1] Mon, 23 Feb 2009 06:06:07 GMT (338kb)