Billions of Earths?

So Carnegie Institution of Washington’s Dr Alan Boss has recently caused quite a stir by suggesting that there could be billions of Earth-like planets in the Milky Way alone. As author of The Crowded Universe, Boss’s thoughts on the prevalence of Earth-like worlds is well known. So is his speculation sheer audacity, or does he have a point?

Well, consider our solar system. We’re around an average star, and by the copernican principle, there’s nothing especially unusual about our place in the Universe. So our average star has yielded two gas giants, two ice giants, four terrestrial planets, three moons which could be easily large enough to be thought of as planets and a host of dwarf planet sized objects (I’m going to count the 8 most interesting). So ignoring orbits and suchlike, we have four giants, seven terrestrials and eight dwarfs.

If we truly are average, then there are over 200 billion stars which could have a similar average number of planets in the Milky Way. Even if we’re (incredibly) conservative and assume that only half of those will have any planets at all, and only half of those with planets will have some terrestrial planets, that’s still 50 billion stars which could have terrestrial planets. How many will be Earth-like? Well, we have 1 in 7 right here. Let’s keep with the conservatism and assume that each of those 50 billion stars has only one terrestrial planet and keep to our 1 in 7. That still gives 7.14 billion stars with an Earth-like planet.

Of course, this is a toy estimate — and by no means a serious attempt at a guess… We don’t really have enough data to make any informed estimates, but… 7 billion is not a small number. We’ve seen hundreds of stars with dusty disks around them and we’ve barely scratched the surface. Planet hunting is a relatively new endeavour, but we keep finding more. We don’t even know how to look for any potential interstellar planets either. Plus, let’s not forget that we’ve recently found that the Milky Way is even larger than we realised.

In short, I would tend to agree with Alan Boss. I don’t see why the Milky Way shouldn’t contain billions of Earth-like planets, and indeed trillions of terrestrial planets. Boss also raises the valid point that if a planet is potentially habitable, why wouldn’t it be inhabited? Let’s see what Kepler finds once it’s launched!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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10 Responses to Billions of Earths?

  1. Anonymous says:

    The more interesting question is, in the instances where life does exist and develops into intelligent life and on into a technologically advanced civilization, how often do those civilizations survive the tendency to destroy themselves or their environment. Are not the seeds of our own destruction within the very qualities that have allowed us to flourish?

  2. invaderxan says:

    Well, it’s difficult to define, but we’ve seen life on Earth show remarkable intelligence and even social structure, without showing any trace of technology or civillisation.
    Take dolphins, for instance. It’s frequently said that they’re among the most intelligent animals on the planet. However, in an analogy to your own example, they simply don’t have the dexterity to develop technology the way we do. That doesn’t impact upon their inherent intelligence though — simply their ability to apply it.
    In turn, some species might have both the intelligence and the dexterity to develop technology, but have no reason or desire to do so.
    If this discussion is moving on to the question of how intelligence develops with the ability to apply it, I suspect that would open up a whole array of different questions.
    And I hope you like the book. I certainly did. It complements Lovelock’s Gaia quite well. :)

  3. I believe intelligent lfe may be common, but technology and civillisation may not be.
    Well, what’s your definition of “intelligent life”? Depending on what you mean by “intelligent”, one could say that there are hundreds of species of intelligent life.
    At any rate, I could conceive of a number of possible scenarios in which a human-level intelligence did not develop past a stone age civilization. For example, if there were no easily-domesticated plants or animals. Or if the species’ physical form discouraged it. For example, a flying species would have difficulty bringing tools from one place to another, and might be limited to simple stone tools that can be produced in situ and abandoned.
    I’ll have to check out that Lonely Planets. :-)

  4. invaderxan says:

    Actually, I forgot something… By the copernican principle, if there’s nothing special about our place in the Universe, then by inference, there’s nothing special about the conditions which life requires to evolve to a level of intelligence.
    (I’m pretty sure that one’s started arguments before! :)

  5. invaderxan says:

    This is, of course, a very good point. It isn’t just metabolism, a lot of human physiology is about clearing up all the free radicals our brains create, and our expanded cranium is also why childbirth is surprisingly hazardous for humans. But this does fall into the trap of assuming humanoid life — which is by no means the only path (and almost certainly not the best path) for evolution to follow.
    Rare Earth… It’s a fair theory, and well thought out. But I’ll see your Rare Earth by Peter Ward and raise you one Lonely Planets by David Grinspoon. :)
    As for evidence of alien visitation, I refer to my original statement — I believe intelligent lfe may be common, but technology and civillisation may not be. Personally, I’m agnostic to the idea of alien visitation without further information. There are too many unknowns for an informed opinion.

  6. Well, up to a point. However, intelligence also comes at a high cost. Brain tissue is VERY expensive. Something like 25% of an average human being’s metabolism is devoted to feeding those three pounds of brain tissue. Indeed, it’s been argued that Neaderthals may have died out, indirectly, due to their larger brains. There is evidence that this caused their children to develop a bit more slowly and nurse for longer, lowering their birth rate, allowing us slightly-smaller-brained sapiens to, in effect, outbreed them.
    Also, for complex life to evolve, especially high intelligence life, requires a certain degree of long-term stability, which, for example, it’s been argued, requires a large moon to stabilize axial tilt.
    Have you read Rare Earth by Peter Ward? His argument is that microbial life is probably extremely common, while multicellular life is probably rare, and intelligent life even rarer. I don’t know enough about the topic to come to an informed judgment on his arguments, but a lot of his points do seem logical.
    There’s also, of course, the question of the lack of any evidence of alien visitation at any point in our planet’s history, which would be odd if intelligent life is common – civilizations tend to expand, after all. Although, on the other hand, I rather suspect that interstellar colonization would be economically unfeasible, confining intelligent species to their home systems.

  7. invaderxan says:

    I don’t know about intelligence. Even intelligence simply seems like a logical evolutionary path — an intelligent predator will be more successful and outcompete it’s rivals, for instance.
    Technology and civillisation are, IMHO, the things that probably need the most specific conditions.

  8. I bed there is life on these planets, but intelligent life is quite rare. I wouldn’t be surprised to see plant life and primitive organisms all over the place.
    I think for intelligent life to evolve, more specific conditions are required.

  9. invaderxan says:

    Re: 1 in 3
    1 in 3? Wow… That’s a huge number. Thanks a lot for the link! :)

  10. Anonymous says:

    1 in 3
    Hi InvaderXan
    A recent paper suggests that ~30% of sun-like stars have super-Earths with periods of 50 days or less…
    …which potentially means 1/3 of red-dwarfs have planets in the right place to be Earth-like. Whether that’s how they turn out is another question we can’t yet answer of course.

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