Now, I have not read this book, and therefore I am not fully versed in their arguments. I plan to read the book and make a more detailed argument, but I wanted to lay down my base hypothesis first. Also, Wikipedia does an ok job at summarizing their argument.
So, before we get to their equation, let's look over some data:
- It is estimated that there are 200-400 billion stars in the Milky Way Galaxy.
- It is estimated that there are about 100 billion (1011) galaxies in the universe.
- Galaxies tend to contain between ten million (107) and one trillion (1012) stars.
Now let's make some guesses. In the famous Drake Equation, it is estimated that half of all stars will have planets. So that gives us a more manageable number of 1010 stars with planets. For the sake of argument, let's say that our solar system is fairly typical, and maybe the average number of planets in a solar system is around 6. I think I am being generous to the Rare Earth-ists here, as a solar system could easily have tens of planets, not to mention life-supporting moons. That gives us 6 * 1010 planets in the universe. Disregarding the Drake estimate of 2 planets per star supporting life, let's say that, I don't know, 1/8 of planets support complex life.
So, we have finally arrived: 60,000,000,000 planets could support life. Sixty billion seems to be far from the estimate of 1. If my estimates have an error of 99.9%, there would still be 7,492,500,000 planets supporting complex life. Of course, some of that may have died off already, and most (statistically) we would never be able to find. (If you read the numbers above, and you're good with visualization, you should already have realized the size of the haystack in which we are searching for a proverbial, uh, 60 billion needles.)
As I have said, I will in fact be making a more detailed argument later. However, I assert that the vast immensity of the universe makes the Rare Earth theory extremely unlikely, statistically. And I haven't even broached the subject of carbon-based vs. non-carbon-based life or the general assumptions of what a 'life-supporting planet' requires.
QED.
Like you, not having read the book, the only information I have for now is the Wikipedia article that you linked to. Judging from a quick skim of the article, it seems that the argument is mainly that life is probably unique in the Milky Way galaxy. And indeed if you look at the "Rare Earth equation", which is analogous to the Drake equation, you will find that the terms reference the Milky Way, not the universe as a whole.
ReplyDeleteGiven your stated data, that reduces the area of study by a factor of 11, which isn't insignificant. I think that narrowing the search area to the Milky Way is perfectly reasonable as signals from any galaxy not within the Milky Way's gravitational well would be far too feeble for our telescopes to pick up and you'd have to be an incredible optimist to think that our descendants will every visit another galaxy that is outside our galaxy's gravitational well that is not the Andromeda galaxy, which will likely collide with us in a few billion years.
Of course the biggest problem with predictions of life's abundance is we only have two major data points: there is life on earth and we have not had any credible instances of communication with an intelligent species originating from off our planet. This leaves quite a range of possibilities to be rationalized...none of which can be proven or disproven with the data, though hopefully we can improve this with the coming ability to do infrared readings of exoplanets' atmospheres.
And just to preempt you on the topic of non-carbon based life forms, carbon is both more plentiful and more chemically interesting than other potential elemental substrates (carbon's heavier cousin, silicon comes to mind). I admit that it is possible that "carbon-based" life may not be nucleic acid/protein/carbohydrate/lipid based as terrestrial life is, but I'd be willing to bet that the vast majority of life in the universe has some sort of essential carbon chemistry.
However, like you, I'd bet that complex life exists elsewhere in the Milky Way and perhaps even intelligent life. In fact, perhaps the reason behind the Fermi paradox is that the vast majority of intelligent social life forms resemble dolphins, who we don't suspect of being anywhere close to developing radio technology or rockets simply on the basis of negative effect that water tends to have on electronics and the way dolphins' bodies are adapted to their environment (no opposable thumbs).
Well said.
ReplyDeleteFirst, after reviewing my math.. I don't know what I was doing. I wish I had my original notes. Half of 10^20 is in fact not 10^10, it is 5 * 10^19. So, we have 3 * 10^20 planets in the universe. And 3.75 * 10^19 planets capable of supporting life. Ouch. Exponents will get you every time.
But, let's limit our search to the Milky Way and take the conservative estimate of 200 billion stars, which is probably more appropriate due to the large number of stars in the galaxy's center.
So, using my estimate of 6 planets per star, we get 600,000,000,000 planets (remember, half of stars have planets). That would give us 75 billion possible life-supporting planets in our galaxy.
My corrected math seems to make this ridiculous, with statistics heavily stacked in favor of complex life on other planets. Intelligent? Still around? Statistics don't offer much for that. I'll have to repost with better math, and look into the book to get a better picture.
Thanks for the thoughts!