# Talk:Rare Earth hypothesis

Rare Earth hypothesis was a good articles nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
 March 23, 2007 Good article nominee Not listed
WikiProject Physics (Rated B-class, Mid-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B  This article has been rated as B-Class on the project's quality scale.
Mid  This article has been rated as Mid-importance on the project's importance scale.

## New section: Enough time elapsed since the big bang for evolution to occur

I've added this straight away because it is topical and users will expect to see it somewhere in the discussion of Fermi's paradox.

Perhaps discussion might be useful about where is the best place to mention it in the wikipedia discussion of Fermi's paradox, or about how to express it. I thought here in the rare Earth article. But if that's not right, surely it needs to be said somewhere, if so where? Any comments or suggestions? Robert Walker (talk) 10:23, 19 April 2013 (UTC)

Hi Robert and many thanks for your contributions. It is difficult to know where the best place for your comments to be placed. Perhaps it is time for articles such as Rare Earth hypothesis and Fermi's paradox etc; to be merged? Whether anyone has the time to do this is another matter - but I do think it is worthy of discussion. Best regards, David J Johnson (talk) 10:39, 19 April 2013 (UTC)
Hi David, thanks, glad it is useful. Yes, I can understand why you'd want to do a merge, so everything is in the same place. On the other hand, as you say, it's a lot of work for someone, and when reading the articles I felt it works well to have them in separate places with a summary in the main article. Also, the Rare Earth Hypothesis, and Great Filter I felt deserve separate articles. But then, as you say it is difficult to know where is the best place to put anything that doesn't exactly fit in an obvious way into any of the existing separate articles, and isn't substantial enough for an article of its own. I don't have an answer to that. Robert Walker (talk) 22:39, 20 April 2013 (UTC)

I don't see the article as arguing anything even remotely relevant to the Rare Earth Hypothesis. The article states that evolutionarily speaking, it takes a few billion years for RNA molecules to evolve from collections of nucleotides. Such a process need not be confined to the earth- the article even gives the list of requirements for the evolution of these nucleotides, which is essentially only water, heat, and simple organic molecules. The implications- that Earth was likely seeded by primitive bacteria instead of developing them on its own, actually imply that Earth is not unique. They do not imply that the Earth is one of the first to have been seeded, just that it was likely seeded at about the same time as most other current planets with life. It does refute the likelihood that there are many civilizations more than a few million years ahead of us developmentally, but this is irrelevant to the Rare Earth Hypothesis. Without reason to believe that this seeding and development occurred much more quickly on Earth than on other planets, it is highly unlikely that the Earth contains the universe's first intelligent civilization. If there are editors that feel that there are reasons to argue such, please place them in this section as it does not in itself imply that Earth is the first. It merely implies it is one of the first, and that there are probably very few civilizations more than a few million years more advanced than humanity. Gondor2222 (talk) 09:07, 5 May 2013 (UTC)

I concur that this section shouldn't be in here. It doesn't imply Earth is rare. Furthermore, to be quite frank, I think the whole idea is crap. There are 3 'papers' all from the same guy (single author). Only two seem to have been published. The only one where was can see the reviewer comments, the reviewer comments were negative:
Reviewer 1:
"The only claim I am making is that the data analyzed in this paper and, for that matter, any comparative-genomic data I can think of do not provide any evidence in support of an early, extraterrestrial origin of life. Accordingly, I believe that terrestrial origin around 4 billion years ago should be taken as the null hypothesis."
Reviewer 2: "This paper is an example of how not to analyze data. "
Reviewer 3: "I do not see any striking arguments for panspermy in this work ... Whether the dependency is of the exponential form, however, remains to be seen. ... Discussion of minimal genome in this regard is a red herring. First, the Author misreads what is in the minimal-genome literature ... Second, no one ever said that the minimal or ancestral genomes have evolved by spurious assembly of 300 genes ... Third and most important, all this is not relevant to Author's own argument: the genome to discuss is not minimal one, but that of LUCA (last universal common ancestor). The latest reconstructions of LUCA gene content, notably Pubmed 12515582 and 16431085, come up with 600–1000 genes ... I find it counterproductive to dismiss the Lynch-Conery theory in one sentence – at least in the sentence that directs to the Charlesworth-Barton paper, as if it is the last word on the subject. ... With regards to absolute time scale, however, these theories may not be even that helpful, because the step from these general hypotheses to constant vs variable evolutionary rate would not be trivial."
Furthermore there are papers to the contrary:
"in spite of the many uncertainties involved in the estimates of time for life to arise and evolve to cyanobacteria, we see no compelling reason to assume that this process, from the beginning of the primitive soup to cyanobacteria, took more than 10 million years."
Two papers by one guy, at least one of which had negative reviews from 3 out of 3 reviewers, does not inspire much confidence.
Furthermore, it seems to go against arguments of the rare earth hypothesis, that suggest the early galaxy would have been inhospitable due to low metallically - as you need time for stars and such to synthesize heavier elements. I'm going to leave this comment up for a few days, and if there are no objections, delete the relevant section. 129.194.8.73 (talk) 09:40, 9 January 2014 (UTC)
And upon further consideration, the evidence is even weaker. 2 publications are cited. 1 of which 3 out of 3 of the peer reviewers disagreed with the conclusions. The other publication does not actually support the claim of a 10 billion year timespan, but merely proposes a mechanism (rather vague and speculative in my opinion) by which life may have started.
So in conclusion, we have one guy who only got one paper published, and it was published with open reviews, and the reviews were all negative (ie, it normally wouldn't be published).
And of course, again, it doesn't support the idea that Earth is rare in the galaxy.2001:620:600:3800:226:4AFF:FE0A:3CE2 (talk) 10:45, 9 January 2014 (UTC)

## Removed the citation flag

There are plenty of citations now. In fact, the article opens with one. Ray Van De Walker (talk) 18:40, 18 December 2008 (UTC)

## Non neutral POV

All the arguments and explanations presented only with one point of view. That of our Earth.

If you have arguments and explanations from the perspective of another inhabited planet, I'm all ears. Marskell 09:18, 16 November 2005 (UTC)
Nice joke, but what he's saying is similar to the carbon chauvanism counterargument used to counter the Rare Earth hypothesis. We cannot simply assume without proof that carbon-based life is the only type of life possible. Ever seen the alternative biochemistry article? Andreus 18:46, 11 July 2006 (UTC)

Fantastic work here, guys. (Wayne Hardman)

I'm not clear how the issue of NPOV is relevant to this article. If this article is presenting the fundamentals of the Rare Earth hypothesis, which is a sensible hypothesis (id est, a hypothesis sensible people can get behind, though still allowing other sensible individuals to disagree with it), to claim it is "POV" is trivially true. The hypothesis by definition is proposing a specific point of view, just as, say, an article about the Democratic or Republican party would carry a "POV" simply by stating the guiding principles of those political parties. Thus to say that the article is POV is a meaningless attack. And in a subtle way, it is a disqualifier of the hypothesis itself.
As an aside, I've noticed that lately, an awful lot of articles are being labeled "POV" simply for acertaining facts—facts about people's opinions about specific subjects and topics. Question: If an article is explaining what a specific group of people think, how can it avoid being POV? Just a thought. --TallulahBelle 01:32, 19 August 2006 (UTC)
A problem I've had with this article is that it lists 'facts' supporting the hypothesis without adequately presenting other interpretations of the evidence. For instance, the section The galactic habitable zone states, The orbit of the Sun around the center of the Milky Way is almost perfectly circular, with a period of 226 Ma, one closely matching the rotational period of the galaxy. The Sun's orbit is so perfect that it has remained clear of the spiral arms of the Milky Way over its entire 4.6 Ga lifetime. I added one reference (there are many others available) that shows that other astonomers interpret observations to mean that the Solar system passes through spiral arms on a regular basis. -- Donald Albury(Talk) 12:21, 19 August 2006 (UTC)

I read the book some years ago, and am looking for it now in order to tweak the article. Reading the article more carefully, the key assumption made by the Rare Earth hypothesis remains unstated, and that key assumption is that, since intelligent life takes billions of years to develop, such development can only be successful in environments (planets) that have remained very stable over those billions of years. That stability, the hypothesis goes on to say, is statistically very improbable.

I think the problem with the article as it stands now is that it is stating the Rare Earth hypothesis almost as if there had been a teleological determinism to the events described—that is, it is stating things as if some higher power had deliberately but subtly made the universe the way it is just so that we humans would develop.

But from what I recall, the hypothesis is statistical in its approach: It is basically saying that the planet we humans live on is the result of wildly improbable events that created a stable environment suitable to our brand of intelligent life. The hypothesis, from what I recall, wasn't saying that these wild improbabilities were the result of a higher power—it was saying that in a large enough statistical sampling, any odd-ball example can be found. The upshot is that, for the theory, life on Earth is the result of a combination of several statistical anomalies that in no way imply a higher power controlling or influencing events.

Over the weekend I'll reread the material I have and rewrite the article to remove that cloying feel the article currently has. As an aside, I find the theory in its distilled form fascinating. And it is the only hypothesis that satisfactorily answers the Fermi paradox, at least to me. --TallulahBelle 13:46, 19 August 2006 (UTC)

Whether the argument is teleological or statistical, there are many assumptions in the argument. When there are prominent published reliable sources that contradict or call into question the assumptions made in the argument, those other views must be included to achieve balance. Personally, I think that there are other possible explanations for the Fermi paradox that do not require Earth to be 'rare'. The only constraint that the Fermi paradox seems to impose is that technological civilizations engaging in interstellar exploration or travel or, pouring energy into space, are very rare at this time. One possibility is that a sufficiently advanced civilization won't waste energy by spewing it into space. I also think that there is considerable doubt whether Earth will ever send people or machines more than very short distances outside of the Solar System, and I would not be surprised if space exploration is eventually abandoned as too expensive. In any case, we need much more data before we can draw more than the shakiest of conclusions. -- Donald Albury(Talk) 17:01, 19 August 2006 (UTC)
No, Ward and Brownlee make no mention of teleology. Guillermo Gonzalez, who worked with them and is quoted approvingly and at length in the book, does apparently make an "argument for God" in the Privileged Planet. W & B were apparently unaware he published intelligent design articles—the story is there in one of the reviews.
In any case, I don't think the problem here is an implied teleology, but simply that the article has been presenting the book's arguments without qualification as Dalbury is suggesting. Rare Earth is fascinating, but they consistently cherrypick arguments that support their position and present overly definitive statements as to what planets/systems/galactic regions cannot support life. To some degree the book is a polemic. The anon who was editing this, however, would simply take "A cannot B" and repeat it here without qualification. I tried to remove this pattern in the first few sections but, as often happens, left it for some other wiki-work because the page is so damn long. This, BTW, is why the balance tag is there, and I don't think it should be removed yet—the page needs to be carefully scoured. If and when you have a go at this, it would be nice if you could shave off ten K. Marskell 17:15, 19 August 2006 (UTC)

I disagree strongly that the article is POV-tainted. (a) it's obviously describing a theory which of necessity involves describing the arguments for that theory, and (b) contrary viewpoints are presented. This is an excellent example of a well-researched thourough article. I petition the Wikipedia Gods to remove the POV tag. Jberkus 20:12, 8 October 2006 (UTC)

Done. BTW, the second paragraph vectors people to the opposite, the principle of mediocrity. I think that takes in the larger scale NPOV problem. And let's face it, we just don't have the observations to say how rare the earth is yet so it's got to be opinion if you are going to have an article at all. Keith Henson 00:18, 25 October 2006 (UTC)

Great page, wonderful. I would like to see a section discussing the capacity of intelligent life to master interstellar communication. Since we beleive we can 'hear' across the galaxy, it seems plausible that we may have a 'conversation' with another intelligence long before we meet them, if they exist. Drake included a term covering the amount of time the life could communicate. Rare earth doesn't seem to. In Drake's time this term was a mere 10 years, now it's about 50 years and growing.

Wikipedia is not really a discussion forum.m Its about writing articles according to wikipedia format.

198.59.49.177 (talk) 19:47, 1 August 2009 (UTC)

Sorry, I forgot to sign the above post. Sean7phil (talk) 19:49, 1 August 2009 (UTC)

By the way, on the wikipedia compliance of the article: It is clearly written well within the guidlines.

67.176.38.74 (talk) 20:04, 1 August 2009 (UTC) 1/3/1961 67.176.38.74 (talk) 20:05, 1 August 2009 (UTC)

Sean7phil (talk) 20:07, 1 August 2009 (UTC)

## Fact check.

Someone needs to fact check this as the "99% of stars are red dwarves" statistic conflicts with at least two other Wikipedia pages. 'fraid I don't have time or expertise tho.

There appear to be a lot of other facts in here that need checking too, or at least proper qualification of how certain they are. Despite recent advances we still know very little about what it takes for a planet to become Earthlike, stating that there must be this or that condition is probably inappropriate. Bryan 04:55, 23 Mar 2005 (UTC)

But surely the assumption that a whole series of improbable events are required to create an Earth-like planet is a premise of the Rare Earth hypothesis. Once you qualify this by saying these events are not necessarily so improbable, and may not all be required anyway, then you have created a different hypothesis - something like the Not Quite So Rare Earth hypothesis, perhaps. Having said that, the point that the Rare Earth hypothesis is based on assumptions rather than known facts should probably be stated more clearly in the intro. Gandalf61 20:25, Mar 23, 2005 (UTC)

I think it must be mentioned that the planet itself must have oceans in which life will be created. Because life can emerge only in liquids but not in gas (because molecules interact with each other pritty rearly) or hard state (because molecules do not interact at all). Also the planet must have a magnetic field to repel the radiation. See Mars for a good example - it has no magnetic field and the atmosphere and (eventualy) any water is blown off by the solar wind making it a desert planet.

## Interaction with other bodies???

I don't deny that an impact with Earth probably occured, but I don't think that it is necessary for plate techtonics. In Earth's beginning the interior is very hot, and heat from the core began pushing it's way to the surface as a mantle plume. The plumes would form numerous volcanoes and hot spots (like Yellowstone) that would begin to seperate the crust with three huge cracks, or rifts, at 120° (the Red Sea, Arabian Sea, and the East Africa Rift System are examples of three rifts). As the crust began to push against itself, it would eventully begin to fold and split with one side going below the other forming many subduction zones. Volcaninc arcs would begin forming on the overlying basin. As the thicker volcanoes on a subducting plate approaced the subduction zone it would be accreted to the overlying plate and a new trench would begin forming on the other side of the volcano. This method of accretion formed the cratons of the continents.

If you want a good example, boil a soup of some kind (or anything else that would form a lot of bubbles) and watch how the heated bubbles will be grouped together between the rising "hot spots" of the liquid. BRO_co03 23:43, Mar 30, 2005 (UTC)

This is not an assertion on our part, but rather we assert that the authors of the hypothesis assert it. I don't doubt your objection has been raised elsewhere — you might want to add something about it to the criticism section if you can find an authoritative source about it (otherwise I'm afraid it's only original research). Deco 03:34, 31 Mar 2005 (UTC)
Isn't the current thinking that plate tectonics been "rebooted" with each major supercontinent, anyway? That's what I'd gotten while researching this previously(as an amateur sci-fi author), but neither supercontinent nor plate tectonics mentions it. If it's still true and we can get a ref it might be good for those pages as well. Darekun 00:52, 18 October 2007 (UTC)
Venus does not have plate techtonics, yet it had (and may still have) a hot interior. As Venus shows, simply having a hot interior does not automatically produce plate techtonics, so there must be another factor. Something had to account for Earth having plate techtonics, yet its twin Venus does not, the slight size difference does not between the two does not seem sufficient. A giant impact that created the moon has got to have had a major effect on the Earth's structure.

It has actually been proven that early Mars had plate tectonics (google "Mars once had plate tectonics" for reference), so plate tectonics certainly do not require any kind of extremely improbable event to start. It is quite possible that Venus lost its plate tectonics because the heat dried the olivine out, making it more frictional, or because the heat made the plates too plastic and sticky to be tectonic, or a combination of both. And the reason why worlds like Earth and Mars need plate tectonics to not freeze over is because they are so far from the sun they need greenhouse gases to sustain liquid water. A geologically dead world could have liquid water if it was in the REAL goldilock zone, not the chilly zone Earth and Mars orbits inside. The lack of volcanoes along with some atmospheric loss would protect such a world from runaway greenhousing. On the other hand, a much more geologically active world could sustain plate tectonics with less insolation than early Mars had, which in itself is still less than Mars gets today due to the dim early sun. Multiple paths to habitability.— Preceding unsigned comment added by 95.209.39.191 (talkcontribs)

Better to say that some scientists have interpreted certain observations to mean that Mars may have had some kind of plate tectonics in the past. Others disagree. You can find a full range of opinions by googling "tectonic Mars". -- Donald Albury 16:45, 25 March 2011 (UTC)

I have changed 99% to 70% for the percentage of Red Dwarfs while noting 5% for G stars. See for instance:

[1], 3.3 or [2]

I have never heard the 99% figure. Dwarfs period, perhaps (the sun is technically a dwarf, after all, just not a red one :)? Marskell 15:29, 11 Apr 2005 (UTC)

I changed it to 90%, based on the Spectral classification article. But maybe this is wrong too. Ben Standeven 03:23, 16 Jun 2005 (UTC)
Looks like 70% are M class and 20% are K class. Of course, the article doesn't mention the dwarf/giant divide, which is also significant. Ben Standeven 15:14, 19 September 2006 (UTC)

## Position of our Solar System in the Milky Way

I was taught that our sun is in fact located in the spiral arms of the Milky Way. This page says the Sun would have to be located outside of the spiral arms to avoid events that would destroy life on Earth.

I don't have a reference handy, but my understanding is that spiral arms are not permanent structures, but rather density waves, so that stars move in and out of spiral arms. -- Donald Albury(Talk) 10:52, 20 July 2006 (UTC)
Here's a site stating the 'density wave' theory for 'Grand Design' spiral galaxies. And this page says that the sun does indeed pass through spiral arms on a regular basis. -- Donald Albury(Talk) 11:15, 20 July 2006 (UTC)
I think you are confusing two types of "in". The sun is located "in" the spiral arms of the Milky Way, in the same way that a football is located "in" a team of football players, or a nice little cottage may be located "in" a grove of trees. And yet it is not "in" any particular one of them. --68.0.124.33 (talk) 20:15, 17 March 2008 (UTC)
I don't think the OP meant that. In fact, "The Solar System (and, therefore, the Earth) is within the Orion–Cygnus Arm", according to that WP article. So the Sun is currently within a (minor) spiral arm. --Roentgenium111 (talk) 20:01, 7 November 2011 (UTC)

## What's being done here

The new greatly expanded article is presenting too many statements of fact rather than qualified arguments (it's also rather verbose). Small example: "The central star cannot be a multiple star system, for which the stability of planetary orbits is problematic". We have no business saying it "cannot be". See the binary star heading on Planetary habitability.

That said, a great deal of work has gone into this (by an anon I think). I feel we just need to step back and examine the language we are employing. Marskell 11:27, 26 June 2006 (UTC)

"The central star cannot be a multiple star system, for which the stability of planetary orbits is problematic" describes standard thinking in astrobiology since the 1960s. Moreover, the sentence asserts that planets that are part of multiple star systems are less likely to support complex life, not that complex life is impossible on such planets.

Plenty of exoplanets have actually been detected in binary and multiple star systems, just check the catalog.— Preceding unsigned comment added by 95.209.39.191 (talkcontribs)

### Repeat

I agree. Serious accuracy/POV problems. The anon is probably not coming back soon. Please be bold and fix it. Deco 01:53, 10 July 2006 (UTC)
"Complex life requires water in the liquid state." is intended to describe Ward and Brownlee's thinking and should be read in that spirit. No one can pretend that that sentence is a statement of fact. It would be hopelessly tedious and pedantic to preface every statement with "Ward and Brownlee state that...", "...believe that...", "contend that...", and so on.
The intent of this entry is not to describe the universe and its history, but to summarize the book Rare Earth, noting concurring academic voices and giving due deference to critical ones.
It seems to me that Ward and Brownlee are describing the conditions necessary for the evolution of Earth-like life, and no other. This is perhaps the hypothesis's great strength and weakness - should it be emphasised more at the top of the article? SheffieldSteel 21:39, 26 February 2007 (UTC)
Do you have a source that says that? What you are proposing may be original research. -- Donald Albury 03:47, 27 February 2007 (UTC)
I don't have a source that says anything. I was just asking a question, really. I'm interested in whether the Rare Earth hypothesis actually says something about the probability of life occuring on other worlds per se, or about the probability of there being other planets which are sufficiently similar to Earth to support life as we know it. Not having access to a library, I can't check the original source, and I think the article could be clearer on this point. Hey, I'll give it another read. Maybe I'm missing something. SheffieldSteel 01:56, 28 February 2007 (UTC)
Sorry, I don't know the answer. -- Donald Albury 22:46, 28 February 2007 (UTC)

## Metallicity problem

To be honest, I am unconvinced by the "stars-with-low-metallicity-cannot-support-terrestrial-planets" argument. Take a look at Tau Ceti which has anything from 70% to 22% the metallicity of Sol yet astronomers have found ammounts of asteroidal/comet-like material exceeding Sol's nearly tenfold. They say this makes it very likely that the star supports terrestrial planets. Of course, having that many asteroids and comets would be adverse to life in other ways but consider that a star possibly having only a fifth of the Sun's metallicity has that much solid, asteroidal matter around it - I think that's bound to cause some doubt in the theorems surrounding metallicity and its relation to terrestrial planet formation. Consider the Zeta Reticuli binary system, for instance - both stars of which are believe to have 60-70% of Sol's metallicity - they were ruled out of the HabCat project because of this, I belive. Evidence garnered from Tau Ceti could suggest terrestrial planet formation around Zeta Reticuli might be more likely than we think.

Asteroid impacts could force some lifeforms to evolve intelligence, like severe climate change caused our ancestors to evolve from Australopithecus to Homo by forcing them to invent their way out of the disasters.

There are other serious issues that I have with the Rare Earth hypothesis, such as its carbon chauvanism.— Preceding unsigned comment added by 95.209.39.191 (talkcontribs)
I hold no brief for the "stars-with-low-metallicity-cannot-support-terrestrial-planets" position. While it is standard in the contemporary literature, Rare Earth included, it could well be mistaken and I am happy to acknowledge that. Truly low metallicity, by the way, is not 20% or 50% of Sol, but more like <10% or even <1%. And it appears that there are many galaxies, and many parts of the Milky Way, whose metallicity is a tenth or even a hundredth of that of Sol. The purpose of this entry is, again, not to describe the "truth" about the universe, but to summarize the book Rare Earth and its critics.
I would welcome someone adding 200-500 words to this entry describing the "carbon chauvinism" position.
Abundance of asteroids, comets and dust around Tau Ceti does not imply there are any terrestrial planets around it. Look at our asteroid belt: there's much dust there, but no planet. Moreover, all these asteroids combined would be less even than our Moon. And this is why there is an asteroid belt instead of planet - there is not enough material to coalesce into a single planet. The same situation may be around Tau Ceti - there is much dust there, but only because actually there is not enough to create planets. If planets were formed, they would coalesce most of the dust, and we would see much less dust there. Grzes 13:22, 27 September 2006 (UTC)
I think your logic doesnt work; if your objection is that since there was not enough material to form a planet in an asteroid belt, a system with not enough such material could not form a decent-sized planet, then surely finding a star with many times more such material could in fact form such a planet. Now whether the dust there seen means no such object in fact did get created there is anyones guess for now (and pure speculation), but in either case it would seem to disprove the argument that insists that lack of material, due to low metalicity, is a cause for this not to happen. In any case, I think the common explanation on asteroid belt is the interference from jupiter preventing creation of a single body there; and this is discussed under a different argument, on orbital stability and allowable positions of gas giants. --78.3.2.208 01:17, 25 August 2007 (UTC)

We know that metals (minerals) are essential to higher forms of life on Earth but does that automatically mean that a non-metallic life form is impossible?

97.124.158.15 (talk) 00:14, 2 August 2009 (UTC) 97.124.158.15 (talk) 00:16, 2 August 2009 (UTC) Sean7phil (talk) 00:18, 2 August 2009 (UTC)

In astronomy, 'metals' means any other element then hydrogen and helium, so in that sense non-metallic life is impossible yes. EmilTyf (talk) 16:17, 25 March 2014 (UTC)

## so that the star will only gradually, if at all, drift into a spiral.

It seems counterintuitive to the point being made to not have made this edit. I hope someone will confirm.

This site indicates that the solar system passes through a spiral arm about every 100 million years. -- Donald Albury(Talk) 11:10, 20 July 2006 (UTC)

So our solar system has a less than optimal galactic orbit, but one that has not proved catastrophic.

I would say the Sun has a typical orbit for stars in spiral galaxies outside of the central core. Stars close to the core move faster than the spiral arms. Orbital speed decreases with distance from the core, so stars furthar out move more slowly between spiral arms, until at some radius from the core, the orbital speeds of stars approximately match the speed of the spiral arms. Even further out, the orbital speed of stars is less than that of the spiral arms, and you could say that the spiral arms catch up with stars and pass around them. As the extra mass in spiral arms tends to pull stars towards it both when the star is approaching and when it is receding, stars spend more time in spiral arms than would seem the case just looking at the width and spacing of spiral arms. All in all, I would say there is nothing special about the Sun's orbit in the Galaxy, and if staying out of spiral arms is optimal, there are very few stars that qualify, and the Sun isn't one of them. -- Donald Albury(Talk) 14:59, 20 July 2006 (UTC)
I'm not an astrophysicist, so please clarify:
Is it true that most of the stars in the Galaxy -- even the ones currently inside a spiral arm -- typically manage to stay outside an arm for 100 million years at a time?
So then $f_g$, the fraction of stars in the galactic habitable zone, would be quite large -- perhaps 1/2 ?
Or is it true that the Sun is one of the few stars that manage to stay outside an arm for millions of years at a time -- that most stars either get stick inside an arm, or else rapidly travel across the gaps between arms in less than a million years?
So then the assumption that "$f_g$ is small, 1/10 or less" is still true, even if we loosen the criteria to "stay outside a spiral arm for millions of years at a time", rather than "always stay outside the spiral arms"?
--68.0.124.33 (talk) 20:52, 17 March 2008 (UTC)
First, note that our solar system is in a minor spur rather than (as previously supposed) the Orion spiral arm of the galaxy; so anything you've read about being in a spiral arm is sort-of out-of-date.
Second, note that a "spiral arm" is actually a pressure-wave moving through the galaxy, rather than part of the population of stars (or, indeed, of normal (as opposed to dark) matter), in the same sense that a storm front isn't part of the geography through which it passes, although the fact that a geographic location is subject to such fronts is important to conditions in that location. Just so, the fact that spiral arm pressure-waves pass through a stellar neighbourhood is important to conditions in solar systems in that neighbourhood, even though the solar systems shall be left behind by the pressure-wave in time. -- Eddy, 14:45ish, 30 Aug 2009 (UTC)
"Orbital speed decreases with distance from the core," [pedantry ahead] actually, the orbital speed (m/s) doesn't vary much between the central bulge and the outer rim (a central fact in the case for dark matter); however, the orbital angular speed (radian/s) consequently drops off in inverse proportion to distance from the centre, which is all that's actually needed for the case you present. [/pedantry]
So, taking as given that the spiral arms move at a consistent angular velocity, and supposing (possibly unrealistically) their resulting range of speeds of propagation to straddle the stellar speed of the disk, solar systems near the hub would tend to overtake the spiral arms, those near the rim would tend to be overtaken and, somewhere in between, there would be star systems which keep pace with the spiral arms, so are either always in or always out. The Rare Earthers maintain that only stars in this last category are capable of developing interesting life; their critics claim that our solar system isn't even in this category, so it can't be a prerequisite of life.
Quick back-of-the-envelope calculation: Milky Way tells me that Sol orbits it roughly once per 220 Ma, while the spiral pattern does so once per 50 Ma, so any given feature of the pattern over-takes us once per 65 Ma or so (or passes us once per 40 Ma or so if we're going in opposite senses – is that what Milky Way means by its "(negative rotation)" annotation on the 220 Ma datum ?). There are four arms in the pattern, so that would imply passing (backwards) through a spiral arm once per 16 million years (or head-on once per ten million years). Which is significantly more often than the hundred million years mentioned in the criticism section. Not sure what to make of the discrepancy here. -- Eddy, 15:17ish, 30 Aug 2009 (UTC) —Preceding unsigned comment added by 84.215.6.188 (talk)
A possible resolution to my back-of-the-envelope discrepancy could be that passing through a spiral arm is innocuous when the solar system is in a region of low density of interstellar gas; the pressure wave might then ride round us. If this is the usual case, and being in a high density region only happens one time in about six (or ten), then we'd get once per hundred million years, as asserted by critics. -- Eddy, 15:45ish, 30 Aug 2009 (UTC)
Also, to address the section heading, a direct corollary of drifting only slowly into a spiral arm (or other high-peril zone) would be spending a long period there and only drifting out slowly; which is likely worse that passing rapidly through (albeit that's "rapidly" w.r.t. geological time-scales). Instead of a drastic thinning of the gene pool, it might simply sterilise the planet ! -- Eddy, 15:45ish, 30 Aug 2009 (UTC) —Preceding unsigned comment added by 84.215.6.188 (talk)

## n sub e

I'm going to cut/paste the quote right from the article so that the mistake becomes readily visible: "The following discussion is adapted from Cramer.[31] The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates N, the number of Earth-like planets in the Milky Way having complex life forms, as:" "It calculates..." Who is the specific referent to 'it'? Drake or Ward and Brownlee. For those not familiar with intricacies of the discussion, a specific reference should be made. So for revision try: "Ward and Brownlee calculate..." This would clarify the sentence considerably, again for those not familiar with the discussion.StevenTorrey (talk) 18:06, 29 September 2011 (UTC)

[32]


The article says:

• $n_e$ is the average number of planets in a star's habitable zone. This zone is fairly narrow, because constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus $n_e$ = 1 is a likely upper bound.

This is a problematic paragraph for two reasons. (1) The time required for complex life to evolve is already covered in $f_l$. and (2) $n_e$ cannot have an upper bound of 1 since in our own solar system, there are two planets in the habitalbe zone: Earth and the Moon. Aelffin 22:45, 13 August 2006 (UTC)

Nice point. And double planets probably are not rare either, as we have a second pair in the Solar system, Pluto and Charon (unless, that is, you don't accept Pluto as a planet). Now, let's find some sources that make those points. :) -- Donald Albury(Talk) 09:21, 14 August 2006 (UTC)
Can you provide a reference that states that the moon is a planet? And no, Pluto is not a planet  ;-) - SigmaEpsilonΣΕ 20:11, 31 August 2006 (UTC)
Well, Pluto was a planet when I posted my comment. :-) Also note that the first proposal, which was rejected in favor of the one that reduced Pluto to a dwarf planet, would have made Charon a planet. As for the Moon being a planet, see Double planet, Welcome to the double planet at the European Space Agency, and here, an argument that the Earth-Moon system will eventually become a double planet. :-) -- Donald Albury 23:53, 31 August 2006 (UTC)
After reading all three articles, I still come to the conclusion that Earth and the moon are not a double-planet system.
• The double planet article specifically states that scientific consensus is against considering the moon part of a double-planet system; especially considering the IAU's recent decision.
• The ESA article only mentions "double planet" once, and has no scientific basis for that claim.
• The last link you give is just one person's (your?) letter to the IAU
- SigmaEpsilonΣΕ 01:27, 1 September 2006 (UTC)
Not my letter, and I guess my smileys were not prominent enough. In any case, 'double-planet' has been used many times to describe the Earth-Moon system (just Google for it), even if it is not scientifically valid, and the letter merely points out that the Earth-Moon system will eventually evolve to the point where it does meet the IAU definition of a double planet. -- Donald Albury 08:56, 1 September 2006 (UTC)
Perhaps in a few billion years we can reconsider the Rare Earth Hypothesis, but for now $n_e \le 1$ is still not contradicted by our solar system. As far as this section of the talkpage is concerned, RE is valid. - SigmaEpsilonΣΕ 10:27, 1 September 2006 (UTC)

Leaving aside pedantic definitions of what is a "planet" and what isn't, there are now 4 bodies in the solar system that "we" are pretty sure harbor (pun!) significant quantities of liquid water: Europa, Enceladus, and Ceres, and Earth. But alas, not Luna. "We" are also pretty sure there used to be lots of water on Mars, and suspect that there might be liquid water on Titan, which has actual lakes.

So would that make $n_e$ = 4 for our system, or is there something other than "large quantities of liquid water" involved in that factor? --68.0.124.33 (talk) 13:00, 7 April 2008 (UTC)

Yes, there is something other than "large quantities of liquid water" involved in the factor ne: furthermore, "large quantities of liquid water" isn't really part of this factor (it's sort of fpm, with a tacit assumption that water's a reasonable proportion of what's not H, He or Li), as this is the number of planets that happen to be in the right place for any water (that they happen to have) to be liquid, without regard to whether the planet is a gas giant or a mud-ball (that's fpm's job).
Note that a proper estimation even of our solar system's ne could give more than the four you mention, if there are bodies in the solar system that happen to have no H2O but on which it would have stably been liquid had it been present. Then again, lack of H2O now is a fairly good indicator of not qualifying, since our theories of solar system formation generally predict that it was present on all rocky bodies on formation; if it's since gone away, conditions obviously weren't right for it to stay liquid.
It seems the Rare Earthers take the simplifying assumption (also seen on Habitable zone) that water will only stay liquid due to being the right distance from the star, which Enceladus and Europa (at least) contradict by exploiting an alternative source of energy (tidal heating due to orbiting a gas giant). So their limiting of ne to planets in orbits in the right range does indeed underestimate the number of suitable bodies on which water would stay liquid.
The rationale (hinted at on the page) for estimating ne to 1 is roughly that any planet forming in the habitable zone would mop up most of the proto-planetary material in that range of orbits, leaving none for a second. This argument is flawed if a gas giant could form in such an orbit and have several water-and-rocky moons of reasonable size; which seems eminently possible, considering what we know of the moons of Jupiter and Saturn. -- Eddy, 16:30ish, 30 Aug 2009 (UTC) —Preceding unsigned comment added by 84.215.6.188 (talk)

## Rotation?

The giant impact theory hypothesizes that the Moon results from the impact of a Mars-sized body with the very young Earth. This giant impact also gave the Earth its axis tilt and velocity of rotation (Taylor 1998). Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable.

Question: Why is the Earth's rotational speed attributed to the impact? Mars has a rotational speed very similar to Earth's without any evidence of a massive impact in the past. If the impact is what gave Earth its relatively rapid spin, what gave Mars its? Nik42 05:22, 18 August 2006 (UTC)

The Earth's rotation was much faster in the past (10-hour day?), and the Moon was much closer. Tidal friction has both slowed the Earth's rotation and pushed the Moon's orbit out. The close similarity of Earth's present rotational speed to that of Mars is a temporary (on a geological time scale) phenomenon. However, you are right, the impact theory does not explain why Mars rotates in just over 24 hours while Venus rotates in just over 243 days. Either other processes can give a planet a relatively fast rotation, or Mars also suffered an unidentified major impact, which implies that such impacts are not rare. -- Donald Albury(Talk) 23:14, 18 August 2006 (UTC)

## Stellar Flares In Red Dwarfs

The page in the section discussing the problems with red dwarfs, fails to mention the fact that many if not most of them periodically undergo stellar flares, where they emit out hundreds of thousands of times more x-rays than they normally do. Such flares would bake any planet close enough to the star to support life. I don't know if this was mentioned in the book, but it's an additional factor.

## Correct me if I'm wrong.

Under "Rare Earth Equation" :

"And if any factor is near zero, so is the product of all factors."

This is not true. With the extremely high orders of magnitude in the calculation, a near zero factor does not imply a near zero product. For example: 1 x 1030 * .000000000000000000000001 = 1 000 000 —The preceding unsigned comment was added by Stottpie (talkcontribs) 01:43, 8 December 2006 (UTC).

## So what is consensus?

We can speculate all we want, but what is current scientific consensus as to the validity of the Rare Earth hypothesis? More generally, what is the most probable scientific resolution to the Fermi paradox? Neither article answers any questions about consensus, compared to an article like intelligent design which mentions in the introduction what the consensus is. 76.19.173.43 23:38, 1 January 2007 (UTC)

I think the best answer at this point is, We don't know. Both Drake's calculations and the rare earth hypothesis are attempts to quantize the probability of other planets supporting intelligent life by making guesses at the probability of necessary conditions. It is only in the past 15 years that astronomers have found evidence of extra-solar planets, and what they have found has required wholesale revisions of assumptions about how planets form and evolve. At the same time, here on Earth assumptions about the conditions that will support life are having to be revised as living things are found in more and more 'extreme' environments. We will have to learn a lot more about the universe before we can assign realistic probabilities to all the factors in Drake's equation. -- Donald Albury 01:57, 2 January 2007 (UTC)

## Planet formation

I have restored the following phrase, "Given that rocky planets tend to form closer to their central stars," as this article is a discussion of the "rare Earth hypothesis", and not an article on the current state of theories about planet formation. I think we can work on making it clearer that it was widely accepted at the time the "rare Earth hypothesis" was developed that rocky planets formed closer to a star and gas giants formed relatively further from a star (and that planets tended to stay where they formed), while recent discoveries have shown that extra-solar gas giants may be found so close to their stars that their orbits are as short as a few days, thus calling into question all the previous assumptions about planet formation and history. -- Donald Albury 23:40, 8 January 2007 (UTC)

## 7 Ga in Evolutionary "pumps" paragraph.

The Evolutionary "pumps" paragraph states: "There is no evidence whatsoever of life until 3.8 Ga, when the late heavy bombardment ended, marking the end of the Hadean eon. Over the next 3.2 Ga, there is no evidence, other than a few possible worm tracks,..."

As I read it these numbers add up. So there is no real evidence for the first 7 billion years. As the earth is not that old: is this a typo or is there another starting point? Pukkie 11:21, 8 March 2007 (UTC)

It was stated confusingly. The first number needs to be read as "years ago", not as years since the Earth formed. The Earth formed 4500 million years ago and the Hadeon eon ended 3800 million years ago. The next number, however, is the span of time from the end of the Hadeon until what the book claims is the first real evidence for life, at about 600 million years ago, or 3200 million years after the end of the Hadeon. See Precambrian for a chart of the timeline. I've edited the article to eliminate (hopefully) the confusion. -- Donald Albury 00:25, 9 March 2007 (UTC)

## Evolutionary pumps

"Even if all of these above conditions are met, complex life does not necessarily evolve. There is no evidence whatsoever of life until 3.8 Ga (3800 million years ago), when the late heavy bombardment ended, marking the end of the Hadean eon. Over the next 3200 million years, there is no evidence, other than a few possible worm tracks, of life more complex than the protists; if there were proto-nematodes or other small soft bodied organisms, they left no fossils.

The terrestrial fossil record is thought to show that a complex ecosystem, consisting of many niches, each filled, has been attained several times, the first being just after the Cambrian Explosion. The theory of Punctuated equilibrium argues that: *Once a planet has an ecosystem whose niches are all filled, the rate of evolutionary change drops considerably; *On Earth, the time required for evolution to fill all niches (to reach equilibrium) has been relatively short compared to geological time. An "evolutionary pump" is any mass extinction event that results in many empty ecological niches, thereby speeding up evolution. Such events, which can place all of a planet's complex life at risk, include a sudden change in the energy put out by the central star, a collapse of the magnetosphere, a sudden change in a planet's spin rate or axial tilt, a nearby supernova, gamma ray bursts anywhere in the galaxy (perhaps resulting from merging neutron stars), and any rapid and drastic change in climate or ocean chemistry. Rare Earth focuses on two candidate evolutionary pumps, global glaciation, and bolide impacts."

This needs citations, it does not come close to my understanding of punctuated equilibrium and neither does it match with the Wikipedia article it links to on the subject. TimVickers 02:57, 22 March 2007 (UTC)

Okay, there's two main problems I see:
1. It's out of date: The "worm tracks" of the Ediacaran fossils are much better studied now.
2. I believe Gould did talk about an evolutionary pump, but not as part of punctuated equilibrium.
There's also a few minor problems:
1. "speed up evolution" is a somewhat meaningless term. Better to say "speed up the evolution of new species" or something like that.
2. Collapses of the magnetosphere had better not put all complex life at risk: There's one due to happen any time now. Temporary loss of the magnetosphere happens every time earth's poles flip, and they do this at something along the lines of every 20,000 or 200,000 years - I fear my memory is a bit off.
3. How are these sudden changes in rotation and axial tilt supposed to happen in isolation? I suspect if either did happen, they'd be the least of our worries due to the other effects of whatever caused it.
There's not a lot I'd be willing to trust in that section. Gut it and rewrite it from scratch. Adam Cuerden talk 07:36, 22 March 2007 (UTC)
I started looking through The Structure of Evolution Theory for any references to pumps or filling ecological niches and punctuated equilibrium. I thought these statments seemed appropriate p-916: "The basic logic and formulation of punctuated equilibrium does not proceed beyond the level of species treated as independent individual, or "atoms" of macroevolution. All biologists recognize, of course, that extensive ecological interactions bind each "atom" to others in complex ways. The "bare bones" structure of punctuated equilibrium does not include specific claims about ecological aggregation made of species as component parts. In this sense, punctuated equilibrium operates as a "null hypothesis" of sorts, by regarding each species as making its own way through geological time, with interactions treated as important components within the set of backround conditions needed to explain the particulars of any history. In this sense, punctuated equilibrium treats time homogeneously, and species as independent agents; the theory therefore includes no inherent, or logically enjoined, predictions about the nature of temporal clumping in the ecolgical interactions among species." Regards, GetAgrippa 19:56, 22 March 2007 (UTC)

## GA nominee.

I moved the GA nominee template that was placed into this section to the top of the page. Homestarmy 15:23, 23 March 2007 (UTC)

## References

Just a suggestion here: the references in this article would be much more useful if the articles followed the WP:CITET guidelines. This would allow readers to follow the statements in the text directly to their sources, allowing for much easier cite checking and further research.--Margareta 17:02, 22 March 2007 (UTC)

I'll try to help out. —Viriditas | Talk 09:53, 7 July 2007 (UTC)

## GA fail

Seriously under-referenced, also needs in-line cites for what references it does have. An article this length would need many specific references, particularly for the sweeping generalisations made in the text. Discussion of evolution so inaccurate I moved it to the talk page. TimVickers 16:36, 23 March 2007 (UTC)

Thanks TimVickers for your efforts in the editing of this article and trying to maintain the credibility of the facts. It is editors such as yourself that give me hope that Wikipedia will one day be considered totally reliable and a gold standard for encyclopedias and Wikis. GetAgrippa 18:37, 23 March 2007 (UTC)

Oh well :( (I nominated this article) I think this article will definetely be ready for renomination eventually though. :) --Occono 14:00, 26 March 2007 (UTC)

I am more or less the author of much of this entry. I can assure you skeptics out there that most of the unsourced atatements can be verified by simply reading Rare Earth with the help of the index. I am unable to do that at present. Also keep in mind that this entry is not so much about the "truth" of the Rare Earth hypothesis, as it is a summary of Ward and Brownlie's book. I trust you can all appreciate that that book is a fascinating one. The story is also a rapidly evolving one, and the book is already 7 years old.132.181.160.42 23:51, 18 April 2007 (UTC)

## Water Vapor and CO2

The article incorrectly states "CO2 is the primary greenhouse gas". Water vapor is: see http://en.wikipedia.org/wiki/Greenhouse_gas . Change to "CO2 is a principal greenhouse gas"

65.29.163.115 21:18, 10 July 2007 (UTC)

Actually, the article yet states that CO2 is responsible for keeping the planet warm, rather than the presence of the entire atmosphere. My astronomy texts say the presence of an atmosphere (of whatever gas) keeps the planet warm(ed). 68Kustom (talk) 20:43, 28 August 2008 (UTC)
Here is the exact passage:
The habitable zone is closely connected to the greenhouse warming afforded by atmospheric carbon dioxide (CO2) or other greenhouse gasses. Even though the Earth's atmosphere contains only 387 parts per million of CO2, that trace amount suffices to raise the average surface temperature of the Earth by about 40°C from what it would otherwise be [12].
I'm pretty sure this is not true. I recommend changing this passage to read "The habitable zone is affected by the strength the greenhouse effect of a particular planet." And does the book give the source for that 40°C figure? --MarkO (talk) 16:50, 25 February 2009 (UTC)

The greenhouse effect needs to be considered over the lifetime of the solar system. At times, it was known Earth was completely frozen over, and it is believed that that a steady build up of C02 in the atmosphere due to volcanic activity is what caused it to eventual thaw. But if Earth was a little colder (due to being slightly farther out), below the freezing point of C02, then C02 would never have built up in the atmosphere and Earth would have remained frozen. Mars today is often the freezing point of C02 in today's warmer sun, and Mars probably never had the amount of water forming a giant reflecting snowball as you had on Earth for cooling. Note, even today at the South pole it can get below the freezing point of C02, and presumable the Snowball Earth was colder than today.

I think that it can be considered "common knowledge" that water vapor is by far the dominant contributor to Earth's greenhouse effect. This is recognized by those on all sides of the debate about the primary reasons for Earth's current global warming episode (e.g. see http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/ and http://www-eaps.mit.edu/faculty/lindzen/222_Exchange.pdf for two notable authorities on different sides of the issue), and can easily be seen by the much greater abundance of water vapor in the atmosphere and the diminishing returns of increased CO2 (due to the atmosphere already being nearly opaque to the infrared wavelengths that CO2 readily absorbs, as discussed in the second source).

The content has been changed and the first source has been added as a citation for this change. Jbuczyna (talk) 16:25, 2 March 2011 (UTC)

I question whether it is "common knowledge" that water vapor plays a major role in the greenhouse effect. Moreover, there is no exemption in the verifiability policy for "common knowledge." If by "common knowledge" you mean what is understood or believed by a majority of people, then it is often wrong. -- Donald Albury 23:56, 2 March 2011 (UTC)

## Moon and plate tectonics

http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html So the claim that a large moon is necessary to initiate tectonics seems false, and the claim that its very unlikely without it is at least contrary to what little is known on the topic. --78.3.2.208 01:23, 25 August 2007 (UTC)

## Jupiter: Friend or Foe?

Jupiter: Friend or Foe? - In short: lack of Jupiter causes same number of bodies to enter inner solar system like normal jupiter. Less massive (Saturn class) planet in same place would attract more bodies. Rare Earth followers should claim that bodies only with mass multiple of Mjupiter properly shields inner solar system from space trash. ;) Not first hit to this hypothesis, not last... :> --Madcio 15:58, 26 August 2007 (UTC)

Oooch... I just noticed that already was incorporated in text... sorry :) --Madcio 16:33, 26 August 2007 (UTC)

## The Book

I think Rare Earth: Why Complex Life is Uncommon in the Universe deserves it's own article. We have articles for other popular science books. --The Radio Star 20:45, 25 September 2007 (UTC)

## I disagree

As far as I'm concerned, this is complete rubbish. This theory is too conservitive. think out of the box. Even if "earths" are rare, because there are so many sun like stars, they must be very common. T.Neo 12:55, 26 September 2007 (UTC)

Thanks for sharing your concerns about the Rare Earth hypothesis. However, a Wikipedia discussion page is not to discuss of the subject, but to discuss about the article. So please bring your concerns to a forum devoted to this kind of problems , not here. --Cyclopia 13:46, 26 September 2007 (UTC)

Yeah, You're right. Sorry. 17:53, 27 September 2007 (UTC) —Preceding unsigned comment added by T.Neo (talkcontribs)

## Why are there so few references to Rare Earth: Why Complex Life is Uncommon in the Universe in the footnotes?

? --Blue Flame 007 21:21, 10 November 2007 (UTC)

## Pseudoscience

The REH is typical pseudoscience. It is selective in its presentation of the facts (which is a very grave sin) and fails to gave adequate weight to alternative interpretations (it fails Occam's razor). In fact, the only hint that this is not the latest fundamentalist Christian attempt to invade science are the credentials of the authors. Altough I recognize that the article should in principle be about the REH and the book, I think more scepticism and caution is warranted.

To conclude with Drake: "[...] we are not remotely smart enough to hypothesize ab initio the system of the DNA-protein world, or even the RNA world. It was handed to us on a silver platter. This should be a strong warning that we are over our heads when predicting what might have taken place on other worlds."

82.72.87.196 (talk) 14:31, 25 February 2008 (UTC)

Thanks for your contribution, but the Talk page is a page to discuss the article quality and how to improve it, not to discuss your opinions on the article subject. --Cyclopia (talk) 08:40, 26 February 2008 (UTC)
I could have been clearer. I wrote that to emphasasise what the main criticism of REH is, and I think the article's criticism section gives undue weight to REH minutia while missing the bigger picture. In fact, some criticisms mentioned aren't technically criticisms at all like the stuff about Dawkins. Altough amusing, dragging in the highly tenuous multiverse-like hyptheses do not in any way contribute to the article, and certainly cannot be taken as serious criticism of the subject at hand.
The various and carefully selected observational facts advanced in support of the REH, like the (relative) stabalization of the Earth's axial tilt, or the concept of Galactic Habitable zones, etc..., have no proven or even plausible connection to the emergence of multicellular lifeforms.
In the book interpretations are stretched specifically to support their thesis. While reading the book I was startled to discover the authors apparantly were apparantly unaware of some basic facts, for example that venus is not tidally locked, or that the milky way's spiral arm rotation characteristics are different from disk star rotation periods - in fact the sun passes trough spiral arms several times in one galactic orbit. The list goes on and on... —Preceding unsigned comment added by 82.72.87.196 (talk) 19:47, 1 March 2008 (UTC)
If anything the REH degenerates into its literal meaning - exact replicas of the Earth are likely to be rare - but otherwise has no bearing on the question of "advanced" life elsewhere in the universe.
Kind regards,
82.72.87.196 (talk) 18:46, 1 March 2008 (UTC)

It has nothing to do with Christianity, fundamentalism, creationists, or what have you. I'm not sure why you pulled in something completely unrelated to make your point. I doubt that Peter Ward or Donald Brownlee are Christians, much less fundamentalist. Carl Sagan isn't a Christian either (if I remember correctly, he's an atheist). Although I'm not a fan of the Rare Earth Hypothesis myself, I'm not going to drag some unrelated argument and berate that while I berate the theory as well. 98.198.83.12 (talk) 19:49, 23 September 2009 (UTC)
Please, this talk page is not a forum on the subject, it is a discussion place for the article. Please read WP:TALK. Thanks! --Cyclopia (talk) 20:35, 23 September 2009 (UTC)

## What happened here?

Goodness, it's a shame that this page turned into a bloated free-for-all. It's become an ad-hoc place to add anything for or against the possibility of ET life, with no clear coherence.

I have been thinking of creating Rare Earth (book), focussed solely on the work that Ward and Brownlee actually published. But then, what would we do with this page? I don't want to create a content fork. We do have Extraterrestrial life and Planetary habitability (an FA) to absorb the best material here. Perhaps cut the extraneous here with an eye to those pages, and leave this page covering the book?

Alternatives welcome, Marskell (talk) 19:41, 11 March 2008 (UTC)

What might be termed 'woolly cosmology bloat' appears to be a fairly common phenomenon on such articles (both in terms of coverage & in terms of poorly-sourced content). I don't think a second article on the book would improve matters. However, if you can substantiate that the book meets WP:NB & that an article on it covers all the relevant (non-bloat) issues raised in the article, I would not be averse to renaming it to being on the book. You would probably also need to tag the sections you're proposing merging into other articles with the appropriate templates, per WP:MERGE.
Yeah Iv'e read the book in its entirety (though not the article). The many worlds at the end is a bit of bloat and waffley while I actually welcome the intent and the idea it needs to be whittled down and rephrased quite a bit to maybe 3 or 4 sentences maximum I think. Perhaps i will come back and do that later but if i did i would have to add one sentence about string theory branes (in the multiverse) which is another alterntiave many worlds type explanation but trust me i wont waffle like the current paragraphs (in the plural: 4 of them). If anyone prompts me here with agreement I will come back and do that? (Rarely these days (no pun intended) do my balanced edits get reverted!) Leave me an encouraging note here. "Many-Worlds Interpretations" would be a better section and title and they would get the barest mention then (wikifying helps that too) trust me. Regards, 122.148.173.37 (talk) 14:52, 11 June 2008 (UTC)

## "Tidal lock rules out axial rotation." Huh?

"Tidal lock rules out axial rotation." Hooboy, even a first-year astronomy student knows that the Moon is tidally locked to Earth and yet rotates about its axis. That quote is just bad science, and I fixed it. 68Kustom (talk) 20:45, 28 August 2008 (UTC)

## Giant impact theory

The article refers to work suggesting that impacts of this type may not be rare, but it does not discuss the specific nature of the impact required to produce the moon. It had to be oblique, but not too oblique. The speed of the impact and the size of the two planets would presumably also be important.

The article states that without the moon the Earth's tilt would be chaotic. I have read that the Earth would go through periods when one side would permanently face the Sun. Is this widely accepted?

If correct, these points would substantially increase the importance of the hypothesised impact.

On another point, I do not think that the Dawkins comment is relevant to this article, as it is criticising the idea that the Earth is unique, not rare. Dudleymiles (talk) 18:08, 1 September 2008 (UTC)

It is widely accepted that for example Mars rotation axis undergoes large chaotic changes, and numerical simulations seem to indicate the Earth would do the same if the Moon was absent. (see for example this article, with references to scientific literature). As for "permanently", well, I guess it means the Earth could find itself in a situation like that of Uranus, with one pole constantly exposed to the Sun for about half a year, and another in darkness for the same period of time. This is however very different from a Sun tidal locking situation, which the Earth is unlikely to sustain. --Cyclopia (talk) 08:24, 2 September 2008 (UTC)

## Plate Tectonics

This article insufficiently (if at all) explains why Plate Tectonics are considered necessary for the formation of life. That's a rather critical point, considering the topic of the article... FusionKnight (talk) 19:05, 9 March 2009 (UTC)

I believe it's to do with the carbon cycle. Now my education on this matter is from watching documentaries on nature TV channels (which is why I'm not editing the article) but as I understand it, complex life (remember it's complex life that is supposed to be rare) needs time to form, but in the absence of plate tectonics, all the atmosphere's carbon dioxide will get fixed into rocks, either by chemical or biochemical processes, which would make conditions inhospitable to life; thus any life that did form would have insufficient time to become complex in the absence of plate tectonics. With plate tectonics, these carbon-bearing rocks get subducted and the carbon is released back into the atmosphere from volcanoes, providing a negative feedback loop that promotes environmental stability, allowing time for complex life to evolve. At least, that's the argument as I understand it. --Rogerb67 (talk) 14:16, 10 March 2009 (UTC)

Lifeforms could mine carbon dioxide from the ground biologically, just like plants on Earth get some nutrients from the soil.— Preceding unsigned comment added by 95.209.39.191 (talkcontribs)

'The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust[citation needed].' This seems perverse and wrong. Without plate tectonics there would be only oceanic crust and no continental crust. 92.235.165.32 (talk) 16:30, 17 May 2010 (UTC)

Wholeheartedly agree with this. Even a link to a http://en.wikipedia.org/wiki/Planetary_habitability#Mass would be a good idea to provide some context.--EvenGreenerFish (talk) 02:27, 31 October 2012 (UTC)

## Metals/Metallicity

I apologise if this has been covered elsewhere, but I can't see it. In the section 'The galactic habitable zone' the article states that (in astronomical terms) 'metals' means "all elements other than hydrogen and helium". But in the very next section, 'A central star of the right character' metals are defined as "elements other than hydrogen, helium, and lithium" (my emphasis). I know nothing about this subject, but I cannot see how those statements can be reconciled.

It may be merely evidence of inadequate patience on my part, but when I sample a scientific article that's way beyond my knowledge base and come across what appears to be a glaring internal contradiction, I tend to conclude there and then that the article is, at best, unreliable. Can someone who knows what they are talking about change one or other of the conflicting statements? (Or am I an idiot and they don't contradict each other at all?) Grubstreet (talk) 00:01, 13 March 2009 (UTC)

I suspect it's context dependent; from metallicity: "When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements." So whether lithium (and presumably beryllium) are bothered to be counted as "metals" probably depends on whether the small amounts present are important in the context. The context here may be lost somewhat since the original context would have been in the source documents. I don't think you need to conclude the article is necessarily very unreliable; in the context of this article, whether you count lithium as a metal is largely irrelevant as far as I can see: You're going to need reasonably large amounts of much heavier elements to produce life as it is on Earth. --Rogerb67 (talk) 01:14, 13 March 2009 (UTC)

## The Great Filter

I have proposed that The Great Filter be deleted or merged with this article. I can discern no real difference between the theories J8079s (talk) 02:32, 22 April 2009 (UTC)

Rare Earth refers to just the Earth and our past, the Great Filter is broader and concerns civilisation's future. --Michael C. Price talk 06:52, 22 April 2009 (UTC)

## Criticism

I took out the section on Dawkins restating the Drake equation. A lot of scientists will restate a certain position, and a restatement by another scientist isn't really a good way to make the point of the criticism. I suggest finding more and better criticism, as a restatement by a scientist who just mentioned the Rare Earth Hypothesis in a book about the silly debate between atheists vs theists is hardly a good way to note the opposing viewpoints. I understand that criticism sections are not seen as particularly helpful, but it would help to improve the article, I think. I also merged the Darling section into the main section of that part of the criticism. It was much too small to have its own section. 98.198.83.12 (talk) 22:20, 15 July 2009 (UTC)

## Life in the galaxy?

If one surveys stars, and their planetary systems, of 4.5 - 5 Byrs old; and then detects terrestrial exoplanet of say 1.5 earth masses, then one probably still has a magnetosphere - if we are typical (Copernican Principle). And if in habitable zone, then also look for oxygen signature in atmosphere, from very far distance (lyrs). So if photosynthesis for such a long time of 5 Byrs (stromatalites 3.5 Byrs ago for us), then would there be some probability of a species developing culture? Then consider large numbers fall out for our galaxy of 100-200 B stars. —Preceding unsigned comment added by 12.72.150.22 (talk) 05:24, 20 August 2009 (UTC)

On the same point I raised when you raised this on Talk:Abiogenesis -- do you have a WP:RS for this line of thought? This talkpage is for discussing (reliably sourced) improvements to the article, not for idle speculation. HrafnTalkStalk(P) 10:04, 20 August 2009 (UTC)

## Rare EArth Hompage

I placed the Deadlink Template on the Rare Earth Home page The Resident Anthropologist (talk) 18:52, 20 September 2010 (UTC)

## Probability of complex life

One should perhaps add that the probability that complex (multicellular eukaryotic) life would arise from simpler (eg prokaryotic) forms is also very low. First, it has been recently argued that even getting single cell eukaryotes involves a very low probability event, the acquisition of a mitochondrial-precursor by another prokaryote (see http://www.newscientist.com/article/dn18734-why-complex-life-probably-evolved-only-once.html). The recent Lane/Martin Nature article cited in this link suggests that mitochondria were essential precursors to other aspects of eukaryote formation. Second, the fact that it took over 1 billion more years for complex forms such as animals to emerge, and the rather complicated gene control mechanisms needed (which we still don't fully understand) suggests that the next step was also low probability. Thirdly, while it's yet unclear exactly what type of nervous system is necessary for any form of intelligence, it seems possible (in my view likely) that only mammals are intelligent, because only they have the necessary neocortical machinery. Finally it's becoming clearer (e.g. the work of Daniel Povinelli) that there is a large qualitative gap between human and mammalian intelligence; we do not really understand the nature of this gap, or what bridges it. I suspect that there is very little complex life in our galaxy, and very little human-level intelligence anywhere in the Universe. Paulhummerman (talk) 19:34, 10 May 2011 (UTC)

## Timescale

There is a report of an International Astronomical Union meeting, 'On a cosmological timescale, The Earth's period of habitability is nearly over' at http://spacefellowship.com/news/art11657/on-a-cosmological-timescale-the-earth-s-period-of-habitability-is-nearly-over.html which raises two points relevant to this article. One speaker said that increasing heat from the sun may make the earth unsuitable for advanced life in half a billion years. As it took 4.6 billion years for intelligent life to evolve, and if it had taken a few hundred million years longer it would have been too late, this may mean that the time it takes for intelligent life to evolve is a limiting factor, although I do not know of any source which discusses this point.

The other (contrary) point is that the sun may not be the optimum type of star for life, as orange dwarves remain stable for tens of billions of years and are much more common than stars like the sun. Dudley Miles 22:31, 14 June 2011 (UTC)

## Ward and Brownlee

There seems to be a lot from them, but no refs. "Brownlee and Ward (2000), pp. xxi–xxiii." isn't a ref William M. Connolley (talk) 20:36, 27 May 2012 (UTC)

• The note refers to 'Ward, Peter D., and Brownlee, Donald, 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books (Springer Verlag). ISBN 0-387-98701-0', listed in the references. Dudley Miles (talk) 15:02, 28 May 2012 (UTC)
• I now see that another editor has corrected Ward and Brownlee shown the wrong way round, which is why it appeared an orphan ref. Dudley Miles (talk) 15:06, 28 May 2012 (UTC)
• (Edit conflict)That was me. This article was originally created almost 8 years ago as an exposition of the book, without any references. The first references were added six months later. Most of the editing since then has been focused on alternatives to the premise of the book, and I guess no one has been interested in adding in-line citations to the book. I could try to find a copy of the book, but I have other priorities. -- Donald Albury 15:14, 28 May 2012 (UTC)

Ah, thank you. I see now that the lede pretty well says this is W+B's theory; my only excuse is that I'd jumped in from another article William M. Connolley (talk) 15:51, 28 May 2012 (UTC)

## Oxygen Prerequisite for multicellular life

The article asserts that the discovery of multicellular life that does not require oxygen counters the Rare Earth argument that complex life requires oxygen, however I can't find anywhere that this argument has actually been made in the article. The only thing it mentions is a reference to a claim by Nick Lane that prokaryotes require a significant increase in available energy to evolve into complex life. No specific link to Oxygen is made. I have flagged this assertion because I would like to know where it originates. This needs to be spelled out. Otherwise any counter-argument in this context is meaningless. In any case, the counter-assertion is also weak in that it is worded such that it is unclear whether the species mentioned have "evolved" to fill a niche or that life could have originated there. --EvenGreenerFish (talk) 02:11, 31 October 2012 (UTC)

## Photosynthesis requires a day/night cycle ... ?

The article infers this in the statement that "Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable." I can't find anything to back this statement up. I always thought that what made photosynthesis viable was light along with availability of a solvent and organic compound capable of synthesizing to release energy - rather than dependent upon the speed of rotation of the planet ...

Furthermore problems with the statement: 1) Surely the day/night cycle causes daily variation rather than reduces it. Look at Mars which has a similarly rapid rotation to Earth 2) A planet with sufficient atmosphere could re-distribute heat regardless of the duration of the day (ie Venus has very little temperature variation)

There is nothing to say that photosynthesis could not be viable anywhere that water and carbon dioxide (or suitable substitutes) and sufficient light are available, in the case of water it may well be either groundwater or atmospheric.

I would prefer this statement was not used as an argument for Rare Earth without some sort of citation or discussion here I will remove it. --EvenGreenerFish (talk) 08:58, 21 March 2013 (UTC)

## Rotational Speed ... ?

Can anyone please decipher this babble ... and exactly what it has to do with the subject

Although the Giant Impact Theory posits that the impact forming the Moon increased Earth's rotational speed to make a day about 5 hours long, the Moon has slowly "[[3]]" much of this speed to reduce Earth's solar day since then to about 24 hours. The length of the day is still increasing as the Moon decelerates Earth's rotation. In 100 million years Earth's solar day will be roughly 24 hours 38 minutes, in 1 billion 30 hours 23 minutes. Larger secondary bodies, due to larger tidal forces, would decelerate their primaries faster, which could increase the solar day of a planet in all other respects like earth to over 120 hours within a few billion years. This long solar day would make effective heat dissipation for organisms in the tropics and subtropics extremely difficult in a similar manner to tidal locking to a red dwarf star.

Thanks --EvenGreenerFish (talk) 03:45, 26 May 2013 (UTC)

Further to this is a definitive claim that Mars has not had massive impacts. This is contrary to science opinion. See Martian_dichotomy#Single_Impact_Hypothesis and Hellas Planitia. Clearly this claim is inappropriate and in any case, the train of thought does not appear to lead in any definitive direction. --EvenGreenerFish (talk) 09:25, 27 June 2013 (UTC)

## REH and its "link" to creationism/intelligent design...

Perhaps the link between the Rare Earth hypothesis and intelligent design is significant, although the following sentence (tacked on to the end of the Anthropic Reasoning section) and its source don't seem to support that notion:

"Critics also point to a link between the Rare Earth Hypothesis and the creationist ideas of intelligent design."

https://en.wikipedia.org/wiki/Rare_Earth_hypothesis#cite_ref-49 (Frazier, Kendrick. 'Was the 'Rare Earth' Hypothesis Influenced by a Creationist?' The Skeptical Inquirer. November 1, 200)

This one sentence serves as the only introduction to this notion within the article. The reference isn't conveniently accessible and its notability should be called into question, as well as its bias and neutrality; it's the Skeptical Inquirer, after all.

The sentence also feels a bit out-of-place, or contrived. The sketchy reference kind of supports that idea; as if the editor went looking for a source that would support the REH/creationism "link" that he or she already wanted to include in the article. I'd suggest this line ought to be removed until it can be supported by competent references (and ideally, the notion itself should be fleshed-out a little more if it's going to be included in this article at all). — Preceding unsigned comment added by Waxxaw (talkcontribs) 16:13, 8 September 2013 (UTC)

## Request for context

Hey editors working on this. Would you please 1) include a section on how this hypothesis fits in the fields of planetary astronomy and astrobiology -- is this mainstream, FRINGE, somewhere in between? and 2) include content about that in the lead. I read the whole article and I have no idea if I have just been led off the reservation and into la la land, or if this is seriously considered by the mainstream. thanks! Jytdog (talk) 13:07, 6 April 2014 (UTC)

Ping, just to call folks attention to this. I do hope someone adds context to this article describing where it fits in the spectrum between mainstream and fringe. Thanks. Jytdog (talk) 13:07, 6 April 2014 (UTC)
i am happy to use your language - is it "held by a consensus of working scientists or a minority view"? Thanks Jytdog (talk) 01:14, 18 April 2014 (UTC)
There is no evidence for or against extraterrestrial life, therefore, how could anyone hold it as a consensus or a minority view? It's a thought experiment at this point. At Ohio State, Stanford, UC Berkeley and other schools, it's taught in astronomy and astrobiology classes in the context of the origin of life as well as the search for ET life and the Fermi Paradox. It is one possible explanation of many as to why we are "alone", and one explanation of many as to why we see life on this planet but not others. In these university classes, there is no consensus on whether it even applies because there is no good evidence one way or other. It could be true, and one can make good arguments for why it is true. On the other hand, one can argue that it is false, and one can make good arguments why it is false. So the question isn't whether it is mainstream or fringe, held by a consensus or a minority, it's an open question left to future science experiments and missions to bring back more data. I just don't even see how you could apply "mainstream" or "fringe" to this hypothesis. As a species, we aren't even intelligent to put down our weapons and look for answers. The irony is that it is seriously questionable whether intelligent life ever evolved on Earth in the first place. Viriditas (talk) 01:30, 18 April 2014 (UTC)
indeed Jytdog (talk) 02:04, 18 April 2014 (UTC)

## Timeline Image Error

Has anyone noticed that on the image depicting the timeline of life on earth, the word "anfibians" is present? Patrick of J (talk) 05:30, 4 September 2014 (UTC)

I've also noticed 2 other errors:

1) "moluscs" - "Mollusca" is the name of the phylum and "mollusks" are the creatures themselves.

2) "aracnids" - Misspelling of "arachnid."

I would advise whoever produced this image to please fix these errors and double check other names as well. The image looks professional, but the blatant misspellings obliterate any encyclopedic credibility.

Patrick of J (talk) 05:40, 4 September 2014 (UTC)