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Sep 26 2013

How many habitable planets are out there?

By ‘habitable’ I mean planets that are neither too cold nor too hot but occupy a sweet spot that would be conducive to life as we know it existing. By combining actual data, from the Kepler space observatory launched by NASA in 2009 to look for Earth-like planets orbiting other stars, with statistical analysis, scientists have come up with an estimate of 15-30 billion habitable planets in our Milky Way galaxy alone. That seems like a lot, even if that number is tiny compared to the estimated 1011 stars in the galaxy.

When one factors in that there are about 1011 galaxies in our universe, the potential number of Earth-like planets is pretty large, around 1021, leading to the possibility that we (i.e., sentient life forms) are not alone in the universe.

So, to ask once again Enrico Fermi’s famous question first posed back in 1950: Where is everybody?

If life exists elsewhere, the reason we have not made contact with them may be because their technology is too primitive to make contact with us or the planet is too distant to do so, because of the limits posed by the speed of light.

Of course, there is always the possibility that the occurrence of the first self-replicating molecule that is necessary for natural selection to do its work and start the drive to more sophisticated life forms is so astoundingly tiny that our Earth is the only place in which the odds were overcome.

I personally feel (with no basis whatsoever) that there is a good chance that life in some form exists elsewhere is the universe but that we will never be able to overcome the difficulties posed by distance.

So while we may not be alone in the universe we will never truly know if we are or not.

25 comments

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  1. 1
    trucreep

    I don’t think video games are your thing, but there was a GREAT series called Mass Effect that took place in the Milky Way, and a cool feature was being able to travel to different systems and explore or read about the various planets. There’s a race called the “Volus” that developed in a “pressure-cooker” type environment, and they were ammonia-based life-forms. It always reminds me that life out there may be so different from us that we may not recognize it.

    Thank you for letting me express my nerdiness B]

  2. 2
    Chiroptera

    Of course, there is always the possibility that the occurrence of the first self-replicating molecule that is necessary for natural selection to do its work and start the drive to more sophisticated life forms is so astoundingly tiny that our Earth is the only place in which the odds were overcome.

    Considering how on Earth, life seemed to have appeared right when it could (right after the end of the Late Bombardment period), I think that it’s probably very likely that it’s pretty easy for life to arise and that it’s probably pretty common.

    On the other hand, on Earth it took a couple of billion years to advance to the eukaryote state, then a billion years or so before complex multicellular organisms came about, so maybe “complex” life might be rare, never mind wondering whether it may be unlikely that one evolutionary path leads to what we would recognize as “intelligence.”

  3. 3
    Trebuchet

    Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.

  4. 4
    colnago80

    Paul Davies of the Arizona State University has argued that one cannot make assertions as to the prevalence of life in the universe based on the possible large number of theoretically habitable planets. If one argues that the number of independent chemical reactions that have to occur can lead to a probability that is far lower then the number of such theoretically habitable planets. Of course, it’s the assumption of independence that allows these small probabilities.

    Back in the 1990s, there was an interesting Internet debate between Carl Sagan and Ernst Meyr on this issue where Sagan argued that intelligent life might be relatively prevalent in the universe while Meyr argued that it most likely was quite rare, possibly 1 instance per galaxy or less. I have to go with Mayr at this point in time as, even on this planet, the evolution of intelligent life was not a slam dunk. Sans the asteroid collision that wiped out the dinosaurs, we wouldn’t be here, although paleontologist Dale Russell has argued that the Troodons might have evolved into intelligent bird like creatures is they had not bit the dust during the collision..

  5. 5
    Dave, ex-Kwisatz Haderach

    Universe, The

    4. Population: none.

    It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is zero, and that any people you may meet from time to time are merely the products of a deranged imagination.

  6. 6
    psweet

    Given the ease with which even reasonably complex organic molecules are created in likely conditions and the apparent speed with which life appeared here on Earth, it seems inconceivable that ours is the only planet with life. And given how life has impacted the atmosphere here, it seems likely to me that we’ll have proof of life on other planets within a few decades (assuming we can afford to keep producing new telescopes). A whole atmosphere full of something reactive like molecular Oxygen is a pretty big signal.

    Fermi’s question, on the other hand, assumes not only life but intelligence, with a particular technological outlook, and the motivation to attempt communication with their “neighbors”. Merely using radio technology isn’t going to cut it, the distances are too large and the signal strength to communicate across a planet or even a solar system isn’t likely to be sufficient for interstellar distances.

  7. 7
    PeterG

    I heard about a possible avenue of SETI that would look for the waste of a spacefaring civilization. I found the idea pretty compelling, as it’s something we could do with today’s technology.

    NewScientist had a recent review of some other proposals:
    http://www.newscientist.com/article/mg21829112.100-alien-megaprojects-the-hunt-has-begun.html?page=1

  8. 8
    invivoMark

    Faulty logic. There could be an infinite number of inhabited planets even if they are a fraction of the total number of planets (just as there are an infinite number of odd numbers, even though they are half as numerous as whole numbers).

  9. 9
    AsqJames

    Merely using radio technology isn’t going to cut it, the distances are too large and the signal strength to communicate across a planet or even a solar system isn’t likely to be sufficient for interstellar distances.

    The answer is clearly a space based Aldis lamp system. We’ve already got a handy local light source with sufficient power to bridge interstellar/intergalactic distances – the sun. All we need to do is place a remotely controlled venetian blind directly in line between the sun and whatever star we’re trying to communicate with and we can start blinking away in Morse code.

    The biggest problem would be keeping the ‘blind’ in the correct position for an extended period of time. If we could overcome that challenge, and given that by some estimates there are up to 100 potentially habitable planets within 30 light years of us*, we could conceivably have an answer from a similarly advanced civilisation within the lifetime of a person alive when we start transmitting.

    Although if their scientists have to go through some alien equivalent of parliament/Congress/UN to get funding approval it may delay things a wee bit.

    * – If it’s feasible and worth doing, we may as well put a whole bunch out there to increase our chances.

  10. 10
    AsqJames

    Just to add…

    Wow! How cool is this: A list of every star within 50 light years which is visible to the naked eye.

  11. 11
    sailor1031

    Sorry the Universe is NOT infinite. Go back and check your cosmology text again.

    As for “habitable” planets we have no idea what forms of life may exist in the Universe let alone what their necessary conditions and evolutionary histories might be. What is inhabitable to us may not be to some species we don’t even know about. Shoot – We can be pretty sure we don’t know all the lifeforms existent on this planet.

  12. 12
    jamessweet

    Yup, I agree with your last two paragraphs.

    There are a lot of pretty big unknowns in the Drake equation, but my gut feeling is that we are probably not alone in the galaxy (though we might be), and we are definitely not alone in the universe. But as you said, the distances are too vast. Physical contact is extremely unlikely, and an active interstellar civilization is essentially out of the question*.

    IF, and here’s a big if — IF we are only at the beginning of the timespan a typical technological civilization can expect to be doing technology on this level or greater, then radio contact might be a reasonable possibility. If radio-capable civilizations tend to last for hundreds of millennia, then it’s not unreasonable to imagine a typical one might spend some of that time engaged in Active SETI-type programs. And if that’s the case, and the galaxy really is rife with intelligent life, then it’s not too much of a stretch to imagine that a lot of those civilizations might, during their several hundred thousand years of existence, find themselves pointing a radio telescope at a star that happens to be pointing a radio transmitter right back at them.

    There are so many IFs in that previous paragraph, any one of which could be reasonably attacked by a skeptic. And even if all of those pieces fall into place, still all we get is a “Hello, we’re here!” message. And that’s the optimistic prediction.

    * I leave an asterisk here, because it’s JUST conceivable that some sort of race of very long-lived beings, i.e. intelligent machines, might manage to set up “regular” interstellar travel and commerce (“regular” on their time-scale, that is). The universe is big enough and expected to go on long enough that this possibility can’t be entirely ruled out, but it seems pretty damned implausible.

  13. 13
    dickspringer

    Wrong. Odd integers can be associated in one-to-one correspondence with all integers. Therefore, the number of odd integers in the same as the number of integers. Infinite sets work that way.

    Integers cannot be set in one-to-one correspondence with real numbers. There are more real numbers than integers.

  14. 14
    Rob Grigjanis

    Seems to be infinite

    We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe.

  15. 15
    invivoMark

    You’re correct. I was sloppy in my language. But there can still be an infinite amount of inhabited planets in an infinite set of planets, where a fraction of the planets are inhabited.

    My post would be correct as: “just as there are an infinite number of odd numbers, even though they intuitively seem like they ought to be half as numerous as whole numbers.”

  16. 16
    Frank

    There are a lot of zeros when you write out 10^21. In the one system that we know in some detail, one planet is definitely habitable, one isn’t that far away from habitability (and might have been habitable in the past), and a few moons might be (or might have been, or could be in future) habitable.

    I seriously doubt that we’ll hear from anybody else in my lifetime (or even the lifetime of H. sapiens—we’ve been around for just a blip in time, and within a hundred years of having radio, we came plausibly close to destroying ourselves with a nuclear war), but I agree that we are probably not alone. It’s fun to think about the possibilities.

    I can imagine a solar system rather like ours, except that “Mars” is a bit bigger, has a substantial atmosphere, and an axis tilt such that one hemisphere always faces the “Sun.” Life could conceivably develop on both planets, but would be vastly different. Assuming they overlapped in time, physical distance could be overcome by technologically advanced civilizations, but would other barriers be too great?

    What about instead of an earth-moon system, there were a double planet where both members were roughly the same in size and composition? This doesn’t seem implausible (to the non-cosmologist, at least). Similar lifeforms could develop on both. Again, assuming that civilizations overlap temporally, and communications were established, would politics take over? Imagine a cold war where the USSR is literally from another planet.

    Obviously this is pure speculation and is based on projecting the properties of the one civilization we know about onto the hypothetical. Could evolution on another planet lead to an advanced civilization with no penchant for violence? As I said, there are a lot of zeros in that exponent, and it’s fun to think about the possibilities.

  17. 17
    sc_770d159609e0f8deaa72849e3731a29d

    paleontologist Dale Russell has argued that the Troodons might have evolved into intelligent bird like creatures is they had not bit the dust during the collision..

    If there was intelligent life- intelligent by our definitions and intelligence we could perceive- 80 million years ago, say, might it not have been one of the many species that did not leave fossils? If there had been intelligent life which had lived and advanced enough to build a civilisation as like ours, what signs of civilisation would remain 80 million years later, even if we were looking for them and could recognise them?

  18. 18
    lpetrich

    Yes, the Drake equation: N = Rs * fp * ne * fl * fi * fc * L
    When Frank Drake proposed it, we only had a clue about Rs. Now we are getting some clues about fp and ne.

    The rest of the parameters, fl fi fc L, are still very difficult issues.

  19. 19
    CaitieCat, in no way a robot nosireebot

    That’s so cool.

  20. 20
    sc_770d159609e0f8deaa72849e3731a29d

    Odd integers can be associated in one-to-one correspondence with all integers. Therefore, the number of odd integers in the same as the number of integers.

    How far is that a matter of how the question is put? We could say

    Odd integers can be associated in one-to-one correspondence with all integers. However, any odd integer n has n-1 smaller integers and when set in a one-to-one correspondence with all integers the smaller odd integers do not match all the whole integers. Therefore there must be more whole integers than odd integers;.

  21. 21
    Greg

    SC_…..,

    You are incorrect. Any set of numbers that can be put in a 1 to 1 correspondence with the positive integers (natural numbers) is an infinite set, and is equivalent to any other set that is in a 1 to 1 correspondence with the positive integers. The natural numbers, odd integers, negative numbers, and rational numbers are all infinite, and are all the same order of infinity.

    Greater than and lesser than don’t apply to infinite sets. Positive odd integers and positive even integers are infinite sets. Normal math would say positive odd numbers < positive integers, but that's wrong.

    Infinity + a finite set = a finite set + infinity = infinity.

    Sometimes this gives me a headache.

  22. 22
    DsylexicHippo

    “If life exists elsewhere, the reason we have not made contact with them may be because their technology is too primitive to make contact with us or the planet is too distant to do so, because of the limits posed by the speed of light.”

    What about the third possibility that they are far, far more advanced in technology than us and are simply not interested in making “contact” with primitive life forms like us? Perhaps they are content in observing us making fools of ourselves from a far distance. Humans have only been around for a blink of an eye relative to the cosmological timeline. I think we are deluding ourselves by assigning the “primitive” label to others when we have just begun scratching at the surface that science has to offer us.

  23. 23
    left0ver1under

    Where is everybody?

    If life exists elsewhere, the reason we have not made contact with them may be because their technology is too primitive to make contact with us or the planet is too distant to do so, because of the limits posed by the speed of light.

    Why the assumption that Earth is not the first planet with life capable of technology that can emit radio signals? It took over eight billion years for the sun and Earth to form, and a billion years before life to begin on Earth after that, and three billion to get to today. It could easily be that all life elsewhere is no more complex than plants or reptiles.

    And why the assumption that life capable of communication exists at the same time we do, or existed at the right time in the past for their signals to reach here now? If intelligent life took the same time to arise that we did, then chances are their signals will get here long after we’re gone and vice versa.

    Even if you knew who and from where a corked bottle was thrown in the ocean, and you knew the ocean currents, you could wait on that ocean’s shore your entire lifetime and likely never see it. And finding such a bottle by accident while on weekend seaside vacation is even less likely.

  24. 24
    StevoR : Free West Papua, free Tibet, let the Chagossians return!

    Good article – thanks Mano Singham.

    I suspect based on the history of life on life – one species capable of sending radio signals into space and travelling to the nearest astronomical neighbour in person a few times breifly back in the late 1960′s early 1970s that while habitable planets and life may be relatively common, intelligent life or at least technologically advanced intelligent life may well be very rare.

    For most of Earth’s existence the most sophisticated lifeforms have been bacteria and for almost all the rest the pinnacles of ervolutionhave been dinosaurs and diprotoodons. Life with advanced mathematical minds may be are indeed and this , I have a feeling, answers Fermi’s paradox.

    However, I would really really love to proven wrong.*

    * Provided that proof does NOT come in the form of a hostile alien invasion! ;-)

  25. 25
    StevoR : Free West Papua, free Tibet, let the Chagossians return!

    PS. I think this youtube clip :

    http://www.youtube.com/watch?v=Ln8UwPd1z20

    ‘Carl Sagan – Cosmos – Cosmic Calendar’ really puts us and the mayfly evanescence of our time on this planet in great perspective.

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