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Rare Earth
But expensive monitoring equipment which can confirm the calculation does not always exist, and hence in some fields, our entire knowledge is based on back-of-the-envelope calculations and rough estimates.
Take, for example, the following question: "How many intelligent civilizations, capable of radio communications, currently exist in the Milky Way galaxy?". The worthwhileness of search projects (such as SETI) is closely related to the answer to this question. The number of positively known civilizations is exactly one: the human civilization. And yet, many scientists believe, or at least believed until recently, that the actual number is far, far higher.
This belief was based on various estimates, such as the calculation proposed by Frank Drake, now known as "The Drake Equation." This equation was popularized in Carl Sagan's remarkable TV series, "Cosmos". Sagan himself believed the calculation's result, and was one of the founders of SETI.
Drake's equation is easy to understand. Take the number of stars in the galaxy (about 200 to 300 billion, based on generally accepted estimates), and multiply it by: the percentage of stars that are similar to our Sun in the energy output and stability; the percentage of stars that have planets (since not every star has any); the percentage of planets orbiting their star in a proper distance (so they could hold liquid water, a necessity for maintaining life); the percentage of planets with liquid water on which life actually evolved; and finally, the percentage of life-bearing planets in which intelligent civilizations (i.e., those that can communicate by radio) eventually came to be. All in all, there are five or six factors in this product.
(Note: In my own copy of the book (2nd impression), page 267 states that "a good estimate for the number of stars in our galaxy [is] between 200 and 300 million" - one letter misspelled, and wrong by three orders of magnitude. I do hope the authors' actual calculations were based on the correct value.)
But what values should be used for the various percentages? Drake (and Sagan) chose what they considered to be a conservative approach, and estimated that only about 1 in 10 stars has any planets; only 1 in 10 planets is in the proper orbit, and so forth. Despite the conservative approach, the results were encouraging, indicating that there are thousands of intelligent civilizations in the Milky Way, and probably millions of them in the whole universe. Thus they concluded that there is intelligent life out there, in all likelihood; now we only have to look for it.
In their book Rare Earth, published by Copernicus Press in 2000, Peter Ward and Donald Brownlee point at Drake's (and other physicists') mistakes in a long and depressing discussion, a discussion that took the wind out of more than one SF author's sail.
The book presents what the authors call "the rare Earth hypothesis": simple (bacterial) life is very common in the universe; complex life (multi-cellular life forms, or animals -- let alone intelligent life) is very rare. The first part of the hypothesis is easy to understand, and few scientists will argue with it: indications of simple life were already discovered on rocks originating on Mars, and even here on Earth in conditions that were, until recently, considered completely hostile to life (such as temperatures higher than 100 degrees Celsius, in which 'extremophile' bacteria were found to exist). The second part is the interesting one, and it suggests that the existence of simple life does not necessarily lead to the evolutionary development of complex life, for any number of reasons.
Drake's mistake was basically in the assumption that all it takes for a planet to develop life is being in the proper distance from a proper star. The truth, Ward and Brownlee suggest, is that we have to look at each and every attribute of Earth, and re-estimate its importance for supporting life. Drake's equation is a statistical calculation, but with no other example for life, we're doing statistics with N=1.
Well then, what are the special attributes of Earth that we have to take into account when attempting to run this calculation?
- Proper distance from the star. If a planet orbits its sun too closely or too far away, liquid water would not exist. There isn't much margin for error here: a change of 5 to 15 percent in Earth's distance from the Sun would lead to the freezing, or boiling, of all water on Earth.
- Proper distance from the center of the galaxy. The density of stars near the center of the galaxy is so high, that the amount of cosmic radiation in that area would prevent the development of life.
- A star of a proper mass. A too-massive star would emit too much ultra-violet energy, preventing the development of life. A star that is too small would require the planet to be closer to it (in order to maintain liquid water). But such a close distance would result in tidal locking (where one face of the planet constantly faces the star, and the other always remains dark -- as with the moon in its orbit around Earth). In this case one side becomes too hot, the other too cold, and the planet's atmosphere escapes.
- A proper mass. A planet that is too small will not be able to maintain any atmosphere. A planet that is too massive would attract a larger number of asteroids, increasing the chances of life-destroying cataclysms.
- Oceans. The ability to maintain liquid water does not automatically imply that there will be any on the planet's surface. It looks like Earth acquired its own water from asteroids made of ice that crashed here billions of years ago. On the other hand, too much water (i.e., a planet with little or no land) will lead to an unstable atmosphere, unfit for maintaining life.
- A constant energy output from the star. If the star's energy output suddenly decreases, even for a relatively short while, all the water on the planet would freeze. This situation is irreversible, since when the star resumes its normal energy output, the planet's now-white surface will reflect most of this energy, and the ice will never melt. Conversely, if the stars energy output increases for a short while, all the oceans will evaporate and the result would be an irreversible greenhouse-effect, preventing the oceans from reforming.
- Successful evolution. Even if all of these conditions hold, and simple life evolves (which probably happens even if some of these conditions aren't met), this still does not imply that the result is animal (multi-cellular) life. The evolution of life on Earth included some surprising leaps; two worth mentioning are the move from simple, single-cellular life to cells which contain internal organs, and the appearance of calcium-based skeletons. It appears like the first of these leaps took more time than the evolution from complex single-celled life to full-blown humans.
- Avoiding disasters. Any number of disasters can lead to the complete extinction of all life on a planet. This include the supernova of a nearby star; a massive asteroid impact (like the one that probably caused the extinction of dinosaurs, and 70% of all other life-forms at the time); drastic changes of climate; and so on.
There are also a few attributes that seem, at first, to be completely unrelated to life and not required for its development. Ward and Brownlee argue strongly for the importance of the following attributes:
- The existence of a Jupiter-like planet in the system. Apparently, Jupiter's large mass attracted many of the asteroids that would have otherwise hit Earth. Could life evolve in a system with no Jovian planet? On the other hand, too many Jovian planets, or one that is too large, could lead to a non-stable solar system, sending the smaller planets into the central sun or ejecting them into the cold of space.
- The existence of a large, nearby moon. Luna, Earth's moon, is atypically large and close. Both of Mars's moons, for example, are minor rocks by comparison. What does this have to do with life? Well, it turns out that Luna kept (and still keeps) Earth's tilt stable. Without Luna, the tilt would have changed drastically over time, and no stable climate could exist. If the tilt would have stabilized on a too-large or too-small value, the results could also be disastrous; Earth's tilt is "just right."
- Plate tectonics. Surprisingly enough, it seems like plate tectonics are required for maintaining a stable atmosphere. Plate tectonics play an important role in a complex feedback system (explained in detail in the book) that prevents too many greenhouse gases from existing in the atmosphere. No other planet (except maybe for Jupiter's moon Europa) is known to have plate tectonics. Is this a rare phenomenon, but required for life?
The bottom line is that many additional factors must be added to Drake's equation. One must keep in mind that as any term in such an equation approaches zero, so too does the final product. For most terms, we have no way of reliably estimating their true value, but it seems like at least some of these values are extremely low.
Two important things should be noted about this book. First, about what it does not contain: although I am sure many people will see the Rare Earth Hypothesis as another proof for the existence of a god, this notion of a proof is completely unrelated to the authors' ideas. The hypothesis claims that the conditions for creating complex life are rare; but we know for a fact that at least in one case, all the required conditions were met. Additionally, anyone who insists on taking the ideas of this book as a proof for god's existence will also have to accept the authors' prepositions about the age of the universe, the age of planet Earth, and more importantly, the theory of evolution.
Second, about what the book does contain: the book discusses at length all the issues I've listed above, and more. The problem is that sometimes one gets the feeling that these issues are discussed in too much detail, and the authors tend to repeat themselves, or to delve too deep into some of the less-important aspects of their theory. This is certainly not your common popular-science book; it relies on very up-to-date research results (including some results that were not even published when the book went to press). The writing gets technical on many points in astrophysics, biology, chemistry, and geology (as well as the new field of astrobiology, of course). Over 25 pages of bibliography and references are included.
The theory's weakest point, however, is obvious. The authors admit (after 281 pages of discussion) that their base assumption was that every complex life-form would be similar in many ways to life on Earth: "We assume in this book that animal life will be somehow Earth-like. We take the perhaps jingoistic stance that Earth-life is every-life, that lessons from Earth are not only guides but also rules. We assume that DNA is the only way, rather than only one way" (p. 282).
For me, reading this book was a fascinating and awe-inspiring experience. The most important conclusion (apart from SETI being a huge waste of resources) is an unavoidable cliché, which the authors avoided presenting directly, even though it stares into the reader's face from every page and each paragraph: What we have here is rare, maybe even unique. We should try a little harder to make sure it survives.
Post Scriptum: A news item in the November/December 2001 issue of the Skeptical Inquirer (Vol. 25, No. 6) states that "David Darling, an astronomer who is a critic of the Rare Earth hypothesis, has revealed that one of the strongest influences on the authors, a young [...] astronomer who they acknowledge in their preface 'changed many of our views about planets and habitable zones', has a hidden, Earth-is-unique agenda motivated by strong 'intelligent design' religious views." That astronomer, Guillermo Gonzalez, published several articles in Connections, a quarterly newsletter published by Reasons to Believe, Inc. In one of these articles, co-authored with the creationist scientist Hugh Ross, Gonzalez writes: "The fact that the Sun's location is fine-tuned to permit the possibility of life [...] powerfully suggests divine design."
Darling published these findings, along with a detailed point-by-point scientific critique of the Rare Earth hypothesis, in his book Life Everywhere: The Maverick Science of Astrobiology . Skeptical Inquirer quotes Darling as saying, "What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is."
For more about this book, please see this page. For additional book reviews, please visit Tal's bookshelf. You can purchase Rare Earth from bn.com. Want to see your own review here? Just read the book review guidelines, then use Slashdot's handy submission form.
How about the magnetic field of the earth? Is it known yet whether most other earth-like planets have as intense a magnetic field, or is this property rare as well? I understand that the surface of our planet is shielded from a lot of bad radiation by the magnetic field.
"If you're thinking what I'm thinking, you're right." -
---- El diablo esta en mis pantalones! Mire, mire!
>If a planet orbits its sun too closely or too >far away, liquid water would not exist. There >isn't much margin for error here: a change of 5 >to 15 percent in Earth's distance from the Sun >would lead to the freezing, or boiling, of all >water on Earth.
What about subterranian water?
The ground is constant at about 60 degrees, summer or winter. On a planet with thinner atmosphere, that water might be liquid even when surface water boiled
___
It's the end of my comment as I know it and I feel fine.
Isn't it possible that life doesn't necessarily have to be water-based, carbon-based, or in need of a sun or planets at all? I forget which novel I read it from (it was years ago), but there was a sci-fi author (Asimov?) who put forth the idea that maybe there could be an intelligent life form that is electro-magnetic based.
Let's expand our thinking and loosen up the requirements a bit!
I went to the city because I wished to live without deliberation.
The problem is, these guys are talking about what it takes to creat (Spock voice) "Life As We Know It".
Who is to say there aren't all kinds of life, that can flourish under completely different conditions than what we have here on Earth, or in our solar system?
And I love the "Earth is exactly the way it needs to be to support life on Earth" bit. Well, duh!
This is tripe.
First off, Drake's equation was meant to map our ignorance, not as a serious attempt to enumerate the number of planets with intelegent life, just as if someone asked me how fast a car I'd never seen or heard of was, I might answer "take the distance from where it stops to where it stops and divide by the time it takes" as a (slightly) more informative way of saying "I don't know." Then I suppose these clowns would come along and say "But the driver might have taken side trips! What if he forgot his sun-glasses and had to go back for them? You aren't accounting for acceleration/deceleration time! What about the wind?", etc.
Secondly, "the moon is vital to life" is one of those science fiction plot ideas that predates science fiction. It comes in many forms, but I've never seen one that doesn't beg the question (we couldn't have evolved without the moon because the moon causes X, we are the only example of us we have, and evolved with X; therefore we needed the moon to evolve). It sometimes makes me wonder at the sagacity of whoever coined the term "lunatic."
Third, many of the things they drag in are by no means established (and several are in fact in doubt). For example, we don't know where the Earth got its water, so we can't say if the process is common or not. We have only detected large extra-solar planets because that's all we know how to look for. We don't know that a stable climate is needed for the evolution of complex life (some argue that an unstable climate is required, lest you get stuck at a local addaptive maxima.
Anyway, I could go on, but you get my point: this is tripe.
-- MarkusQ
I have two ideas regarding this.
First, as a civilization approaches the technological level necessary to travel to the stars, they also have a myriad of opportunities to kill themselves off. I.E. nuclear war, designer viruses etc. As technology increases, (at least here) we are coming to the point where more and more dangerous technology can be used by the single deranged individual.....
If an evolutionary model is used, I think most species would have a crazy or two who might end up causing their own extinction. We are very near this point. Imagine either nanomachines, or plague as the easiest self replicating disaster.
2nd, perhaps other life does exist, but is not motivated by the explore and conquer ideal.
A xenophopic or non-curious species.
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I was about 3/4 of the way through writing a new novel when "Rare Earth" came out; since "Rare Earth" contradicted pretty well every premise I'd based on the novel on, I was pretty freaked--until I actually sat down and read through the book. I shouldn't have worried. The basic arguments put forward in "Rare Earth" are each consistent and compelling; the problem is that each challenge to the development of life presented has its own solution(s), which they ignore. For instance, they maintain that plate tectonics is essential for the maintenance of an atmosphere; this is manifestly untrue, because neither Venus nor Mars have plate tectonics, and both planets have atmospheres (albeit unlike our own). In fact, when you examine Venus, it turns out to have something that may fulfil the same role as plate tectonics: "coronae" which are upwellings from inside the planet that form ring-shaped volcanic chains. So an Earth-like planet with coronae is quite conceivable, even likely. The authors of "Rare Earth" argue fallaciously by assuming an exact match to the Earth to be required for life, then running through a laundry-list of reasons why such an exact match is rare. But an exact match isn't required; not even an inexact match. My new novel posits planets in orbit around brown dwarfs (failed stars bigger than Jupiter but smaller than the smallest red dwarf). In researching the book I became convinced that such exotic environments (which may be the rule rather than the exception in our galaxy because brown dwarfs are at least as common as lit stars) are perfectly fine environments for the development of life: for sunlight, substitute infrared radiation and intermittent visible-light flares from the dwarf; for plate tectonics, substitute tidal stretching by the dwarf; for a Jupiter to protect against cometary impacts, substitute a smaller and more impoverished Oort cloud. The list goes on and on--for every supposed "requirement" of the Rare Earth hypothesis, there's at least one, usually many, alternatives.
His argument was if there were a number of intelligent alien civilizations in our galaxy - then there was a good statistical probability that some would be much more advanced than us. If they colonized the galaxy at moderate sub-light velocities (say 0.1c) then they would have colonized the entire galaxy in about 10^5 years. So if there were many extraterrestrial civilizations intelligent aliens should be here by now (he assumed that UFO phenomena were not produced by aliens).
This stuff is on the web - but I have forgotten the URL. Google "Fermi's question" and you should find it.
There's strong evidence that the earth was once an iceball yet life not only survived, it had an unprecedent and unmatched explosion of diversity after the Thaw.
The problem with the earlier models is that they only considered the incoming solar radiation and the ice. Shortly after the oceans froze over, the surface temperature near the equator was -50F and stayed there for many thousands of years.
But the earth (and any tectonically active planet) has volcanoes. Volcanoes release greenhouse gases, notably CO. According to one estimate I saw on the Discovery Channel (IIRC), the CO level hit _10%_ and the surface temperature was something like 150F before the ice started to melt. (Remember that the conversion from ice to water takes a *lot* of energy, and there was only poor thermal coupling between the hot atmosphere and frozen ocean.) Once the ice started breaking up, there was a cascade effect that lead to a thousand years of acid rain as the CO was washed out of the atmosphere.
And after the Thaw, we had the Cambrian(?) Explosion, the transition from the simple single-celled organism (the only life that could survive under the shattered sea ice) to multicellular life.
This begs the question - is an "iceball" stage a necessary precondition to multicellular life? If it is, and the fact that most life-bearing planets will have an iceball stage since stars become brighter over their lifetime as main sequence stars, then a key part of their argument is invalid. Life-bearing planets will have ice-ball stages, and multi-cellular life will appear after the Thaw.
As an aside, one thing that's unique about the solar system is the unusually high level of metals for a system of our age. Maybe complex life requires these metals, and we're a few billion years too early.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
MarkusQ, big kudos to you for the first correct, appropriate in context, use of "beg the question" I have ever seen on this site. It is misused *vastly* more often than it is used correctly, so it's a relief to see it right for once. Way to go.
Light cup, beer drink, thin so chain, neck turtle fat, man I won't say it again
But: how important is a stabilized rotatino axis?
During earth history the planet flipped its rotation axe several times by 180 degrees. Yes, what is now north pole was then south pole.
Um, I have never heard of this happening. I know the magnetic poles swap places from time to time, but the rotational poles have never done so, AFAIK. If they had, it wouldn't just be North and South switching, it'd be Easy and West as well!
"If you're thinking what I'm thinking, you're right." -