Rare Earth

Tal Cohen writes: "It is said that one of the most important skills a physicist needs is the ability to quickly make "back-of-the-envelope" calculations. For example, Jan Wolitzky (in Jon Bently's "Programming Pearls") tells about Enrico Fermi, Robert Oppenheimer, and the other Manhattan Project brass who were behind a low blast wall awaiting the detonation of the first nuclear device from a few thousand yards away. Fermi was tearing up sheets of paper into little pieces, which he tossed into the air when he saw the flash. After the shock wave passed, he paced off the distance traveled by the paper shreds, performed a quick "back-of-the-envelope" calculation, and arrived at a figure for the explosive yield of the bomb, which was confirmed much later by expensive monitoring equipment." Read on to find out what this has to do with the unusual characteristics of Earth, and how they could influence our search for life elsewhere in the universe. Rare Earth: Why Complex Life is Uncommon in the Universe author Peter D. Ward, Donald Brownlee pages 368 publisher Copernicus rating 7 reviewer Tal Cohen ISBN 0387987010 summary Maybe we are alone, after all.

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.

Comments

Re:The problem with all these equations...

[taliver]

I'm so glad I clicked reload before making this exact comment.

One additional thing, however, if what we even consider to be alive. Andromeda Strain really piqued by interest when one of the scientists showed a watch, a lit candle, and a rock, and claimed that each could be considered alive (the rock is just moving very slowly, the flame certainly has all the proper characteristics).

I'm betting we'll end up calling anything "alive" if we can't predict it's future behavior exactly. This would also mean that once we understand simple cells, we might not consider them "alive" anymore.

Influences, agendas shouldn't matter with facts

[stoolpigeon]

It seems interesting to me that someone critical of this idea uses the fact that they were influenced by the work of a creationist as a method of arguing against them.

What in the world does this have to do with anything?

Isn't this the very worst kind of thinking?

"You're idea can't be correct. There are other people who share this view and I don't agree with other things that they think."

Guilty by association.

Look at the argument for the argument.

Why did that deserve a footnote? I am guessing to fair warn those who might be terrified to find they had been suckered into 'agreeing' with a creationist.

The evolution/creationism debate on many fronts has devolved into a mess. There is a lack of honest exchange in favor of turning one's back to any argument or information.

Not very scientific.

Oh- and I predict this thread for the most part turns into a major conflagration.

.

Re:Influences, agendas shouldn't matter with facts

[Jbrecken]

It seems interesting to me that someone critical of this idea uses the fact that they were influenced by the work of a creationist as a method of arguing against them.

What in the world does this have to do with anything?

It calls into question the validity of the creationist's work. If a scientist has a personal agenda, he's more likely to grab a zebra hypothesis that supports his position.

Re:Influences, agendas shouldn't matter with facts

[stoolpigeon]

The idea that a person does not have an 'agenda' is preposterous when put into such broad terms.

The assertion basically is that his world-view is his agenda. You cannot live and act without some world-view. (regardless of whether or not you are conscious of it)

All human beings have basic presuppositions that they work with. In this kind of theorizing this is especially so as 90% of the work (as is mentioned in other posts) is guess work.

This guy is no more apt to 'go after' something than anyone else. And hopefully as facts come to light- those will prove of disprove his hypothesis. Opinions can influence research but they cannot change facts.

.

Skeptics, *yawn*

[mblase]

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.

So what? Science is science, and all that anyone is doing in this subject is educated guesswork. If an author or influence had a 'hidden, Earth-is-random agenda motivated by strong atheistic and humanist views,' would that make his science automatically invalid as well?

Just because someone's science is motivated by pre-existing beliefs doesn't automatically make his science bad. This is just prejudice, end of discussion.

Re:Skeptics, *yawn*

[jnik]

Here's the problem. Once upon a time Thomas Aquinas wrote about how wasn't it really nifty and cool that the science of the time and the theology of the time just really meshed in a fantastic way and they all got together so beautifully and pointed to each other and supported each other.

Then the science started changing, or more accurately progressing and refining itself. And the theologians felt threatened and tried to push down the science.

Now along comes Hugh Ross. Who's saying isn't it so wonderful that our modern understanding of the cosmos gets along so great with "our" theology and points the way to...

I think it's understandable why a lot of people are very, very nervous about him and anyone who's backing his institute. I don't entirely discredit the argument from design, but it's really more of an interesting philosophical twist on things than anything else. Scientifically I dislike Ross' approach because it potentially restricts scientific inquery to "approved" channels. Religiously I dislike it because it tries to pigeonhole god into a particular "gap" in the cosmos and when our scientific understand expands to fill the gap, suddenly god seems useless. I sorta think of god as having better things to do than hanging around as a mystical incantation to fill holes in our scientific knowledge, and I think science has better things to do than trying to back up a theological perspective.

This doesn't preclude discussion and research into the scientific/theological interaction, but I'm highly suspicious of checking scientific research against theological conceptions.

Hummel's Galileo Connection is a pretty good read on the subject, BTW.

seti

[Anonymous Coward]

apart from SETI being a huge waste of resources

I disagree with your statement. The people at Seti are investigating an area of science that simply has a low probability of success in a given lifetime. Does this mean it should not be done? It kinda reminds me of the people who play the lottery, you have a low chance for winning, but hey, if you do, it changes everything. and the longer you stay at it, and the more wavelength-space you cover, the better your chances get. Besides, SETI gets alot of money from private sources these days.

My personal opinion (and thats all this is, MY OPINION) is that SETI is not a waste in time and resources. Are they "LIKELY" to find anything? probably not, but ALOT of people feel that the payout if they do is great enough to continue to do it.

-hommiefro

Re:The problem with all these equations...

[bravehamster]

More important, if there are all these aliens out there, why haven't they visited us? Either

a) They can't. (starships impossible)
b) Nobody wants to. (Prime directive, or 'They're made of meat!')

I think far more likely is:

c) They don't think anyones here, because they assume nothing could live in a atmosphere full of such a corrosive poison gas as oxygen.

It's quite possible that they've checked our solar system out and dismissed it, because they had their own "Drake Equation". Maybe they also are working from a dataset of 1 and assume that life must develop like them.

Other factors

[MuValas]

- there needs to be a country named "USA"
- there needs to be a state named "Michigan"
- there needs to be a city named "Grand Rapids"
- there needs to be a woman named "Jackie" that
is of Norwegian heritage
- there needs to be a man named "Don" that is of
extremely mixed heritage
- they have to meet and marry
- they have to have two previous children, one
5 1/2 years old and female, one 8 years old and
male
- and then they have to all to take a trip to
a place called "Florida", with its especially
fertile air.
- "grandparents" must live in this place, and
these beings must take care of the aforementioned "kids" for an evening.
- both "Jackie" and "Don" must be in the mood.

and *then* you get me.

Since this is obviously amazing unlikely to ever
occur again, I have therefore proved that no
one in the world exists but me.

Ta-daaaaa!

Me.

(Gosh its lonely)

Re:The problem with all these equations...

[the_consumer]

There's another option both of you are overlooking:

d)Not only have they visited us, but they are us!

Re:If there were intelligent life on other planets

[murray.steele]

Why? We're not there....

Slide rule and back of envelope calculations

[Anonymous Coward]

Fermi's skill is not that unusual. When the slide rule was taught and used, almost anyone could make quick back of envelope calculations. It was an important skill to master because it aided in the use of the slide rule. To get the decimal point right with the slide rule, you had to have an idea of what the magnitude of your result would be. A slide rule will not fix the decimal point for you. That is up to you.

it's all about time...

[passion]

OK, sure, I can buy the argument that throughout time there must be thousands of civilizations in the universe that are capable of radio contact... but that's stretched out over the lifetime of the universe.

Not all civilizations will last forever, not all will go into space and continue propigating, not all have invented their radios just yet. After all, we just celebrated the 100th anniversary of a trans-atlantic transmission...

What if aliens had turned their sattelites on our speck in the sky just before our signals went out in the air... What if they died off millions of years before life started evolving on this planet? What if we're the first life to exist in the universe (not ruling out that others could evolve, just that they haven't yet).

We don't know shit about this, and we won't until our Zefram Cochrane comes along and helps us reach to the other stars.

Sauce for the goose...

[fractalus]

I've seen this on all sides of the debate.

I'm a religious person; I believe in a creator. Does that mean I agree with all the creationist wackos out there who don't know how to do good science? Nope. Does it mean I look skeptically at atheistic scientists who look at something they don't understand, can't explain, and pronounce there must be some mysterious non-divine explanation because they've already decided there's no God? Of course I expect them to back up their science.

Right now science doesn't have good explanations for exactly how macroevolution works. Religion doesn't have good explanations for the apparent age of the universe. Everybody should just fess up and admit they don't know the whole story, quit pushing dogma, and work on finding honest answers.

But hey, I'm religious and therefore biased.

Re:Sauce for the goose...

[eyeball]

I think what's missing in most creation vs. science arguments is the allowance for the posibility that both sides could be wrong.

Re:The problem with all these equations...

[km00re]

It's also possible that we are the most advanced civilization to develop so far. How's that for a depressing thought?

Major flaw: increased number of variables

[angel'o'sphere]

By increasing the number of variables in the "Drake equation" the authors make one major error: a lot of the variables they introduce are very close related:

E.G.
a star of proper mass and:
A constant energy output from the star
are close related to each other IF the star has a similar age like our sun e.g.

I mean: if the star is similar old like the sun and has similar mass like the sun, it will have a similar and constant energy output, like our sun.

For the planet the following variables are not independened from each other or even depend on the variables related to the sun above:
Proper distance from the star.
A proper mass.
Oceans
Plate tectonics.

IF the planet has the proper mass THEN the planet will have plate tectonic. EXCEPTION: the planet is FAR older than our earth.

IF the planet has the proper distance from the star AND the planet has the right mass THEN the planet WILL have oceans.

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

I doubt that. IMHO the approach should be other way around. We shoudl look how many variables indeed are only different expressions of the same basic principle.

There are several astrophysicians which strated to study and make models for solar system creation. They describe how a solar system is comming to existance like this:

You have a big cloud of "dust". Depending on the distance from the galactic core and super novae around that area you will have there a defined mixture of heavy elements and lighter ones.

During star forming the mixture is slowly compressed by gravity ... during this phase you have a sorting of all elements by weight.

Basicly the same process like in a mixture of liquids and sand and lead in a hot pot: lead sinks to the bottom of the mixture, above sand will settle, then you have the hot water and on the surface you have the oil.

Now imagine you have a dust and gas cloud as big as our solar system. The center is several thousand degrees hot, but FAR from ignition.

There will be several hot spots where bodies are forming. The closer the bodies are to the center, the more heavy elements will participate in the forming.

IIRC some 10 years ago an astrophisics got a nobel price for crafting such star system forming models.

He proofed that our solar system only had one way in "condensating" into planets and that is the way it is visible now.

Well, of course we could have the Venus a bit farer away and Mars a bit closer.

Earth then would probably not exist but Mars would be bigger.

Same for the outer planets, there could be one more or one less. But the distribution of mass from the inner side of the solar system to the outer side would be very similar.

And it only depends on two things: total size of the dust and gas cloud forming the solar system and total amount of heavy elements in the cloud.

Bottom line, if two dust clouds are similar enough (-> size of sun which is ignationed is similar) and in the same distance to the galactic core ( -> distribution of heavy elements is similar) they will condensate to similar solar systems.

If you take ten sun like suns I bet that ALL have planets and that 3 have one or more in the distance of the Venus/Earth/Mars belt.

And those planets will in the size of Mars to Venus. Because there is NO WAY in forming any other planets in any different size or any different distance. (If the system has enough iron and other heavy elements)

There are further variables which are supported by weak arguments: a big moon.

Sure, a big moon stabilizes the rotation axe.
Sure, it might deflect incomming bigger rocks.

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.

This was recent hsitory! In terms of the age of the earth.

The same for the proper distance, a final quote: 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.

The earth was some hundred million years ago totaly covered by ice. There is a recent story about that in scientific american, it was covered here on /. Live allready existed at that point. And it survived under the ice plate as the ocean was warm enough by vulcanic activities. Not only bacteria but high evolved live like crabs etc.

There is absolutely nothing preventing that, to be the normal way in other solar systems. E.G. if Alpha Centauri has a Earth sized planet as far away as Mars ... it could just be a ice covered ocean world like our world was 400 million years ago. Habouring live, of course.

Regards,
angel'o'sphere