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!