Talk:Planetary habitability/Archive 1

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Has anyone else noticed that someone has replaced the word "the" with "Squirrel"? This is also means that words like "there" become "squirrelre".

Good article though, despite the prank. This was vandalism- it already has been reverted. --Adam ^(talk) 17:09, 23 January 2006 (UTC)

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I think this is a good and useful addition. Thanks very much Serendipitous. Still needed:

• A section in part two on atmospheric and ocean chemistry. What building blocks are necessary etc.
• Some less generic pics.

Marskell 18:54, 1 September 2005 (UTC)

Just pleased to help. Glad you found my contributions useful!

And I quite like the red dwarf pic :-)

Serendipitous 20:21, 1 September 2005 (UTC)

I think this would only need a paragraph dedicated to why gas giants are unsuitable (or not: I think I read somewhere that we know very little about them so why not?) and maybe get a few more sources, although I reckon reliable sources in that kind of field are scarce. Then I'll just give you the thumbs up :-D Jules LT 20:44, 1 September 2005 (UTC)

Should it not be pointed out that data for extra solar planets is skewed towards large planets in close eccentric orbits due to them being far easier to spot.--Pypex 23:22, 17 September 2005 (UTC)

Pypex's point has been incorporated. Marskell 23:13, 22 September 2005 (UTC)

Gas giants

You're right, it never actually states "gas giants do not have life and can not host life on satellites." Who knows, satellites of gas giants maybe where it's at; or the atmospheres of giants may be full of life, unknown to us. Perhaps below section 2 headline there could be a general paragraph about this. Marskell 00:00, 2 September 2005 (UTC)

Another point which may prompt an addition by someone with more knowledge than me is what might be called a "Mother Jupiter". I believe it is established that binaries and Jupiter-sized planets can orbit in the HZ of stars. The satellite systems of the Jovian planets suggest that a Jupiter-mass or super-Jovian planet could "mother" a host of habitable planets in the HZ. This would allow planets with a day/night cycle around red dwarfs.

astro-biochemistry

I've started a very incomplete and probably incorrect in places section on the chemistry required for life as we know it. It needs a lot more research and time but it's two in the morning now and I need to sleep. Please tear it to pieces if you feel it is warranted.

Good night. Serendipitous 01:08, 2 September 2005 (UTC)

Reads fine to me and I now honestly think every major issue is covered in some form. The length and detail is in keeping with the rest of the article; I just tweaked it slightly. Marskell 11:14, 2 September 2005 (UTC)

Thanks. I'm glad I was able to help. Just to say, I've had a password foul-up so I've changed my monker a bit. Serendipodous 16:09, 2 September 2005 (UTC).

"If it is radically tilted..."

The Milankovitch cycles of axial tilt and climate fluctuation suggest to me that, in fact, a planet with a radically titled axis of rotation is as likely to be habitable. I feel it is likely that the circulation would be altered in such a way as to moderate the seasonal effects at high latitudes resulting from radically tilted axes. Milankovitch cycles show that increasing global warmth results during periods of higher axial tilt, and this would eliminate ultra-severe winters whilst allowing more activity at high latitudes and permitting homeostasis.

Whilst I see that a very low axial tilt might make life impossible to achieve, radical tilt I feel is less likely to do this.

Perhaps "radically unstable" is better than "radically tilted" though the next point covers that. At some point the tilt (75 degrees? 85?) has to impact negatively on habitability wouldn't you say? If the tilt were parallel to the ecliptic could you imagine homeostasis being achieved? I suppose if it's observed below that sufficient heat transfer to the night side of a tidally locked planet is conceivable perhaps even in this case adequate circulation could be achieved. Anon, if you have an outside source detailing this that would be helpful. Marskell 11:45, 7 September 2005 (UTC)

This article was helpfully supplied to me by the good people at Universe Today:

[1]

Hope it helps!Serendipodous 16:52, 9 September 2005 (UTC)

It does help, thank-you. I updated the page accordingly. The one thing I would like is a very reliable source on the number of red dwarfs. Wiki itself is I think in contradiction on the point and in reading you here anywhere from 70 to 90+. Marskell 16:45, 13 September 2005 (UTC)

References

I have just created a proper reference section. I personally don't like the main body link simply directing you to the bottom of the page. Thus, everything is double-linked: the links as they were remain and everything gets noted again at the bottom as primary or secondary. The primary sources are described as they would be in a normal bibliography while the secondary sources are just given a short description (and arranged alphabetically based on that description rather than their occurence in the article).

In general, we need a few more references. If this gets up to two dozen odd references, including six or eight primary I think I'll ship it off to peer review and eventually make a Featured Article request. Marskell 17:14, 23 September 2005 (UTC)

Orbital period blunder

My apologies. I made an error in my "Red Dwarf habitability" section. I claimed that a habitable planet orbiting Proxima Centauri would have an orbital period of slightly over a day, but that guesstimate assumed (rather stupidly) that the star in question was our Sun. I had my suspicions, and checked on a science forum. As it turns out, Prox's substantially smaller mass dramatically decreases a planet's orbital speed, so its orbital period is actually about 6.3 days.

Again, sorry. Serendipodous 17:24, 8 October 2005 (UTC)

Marskell, do you think...

...that my definition of planet article is good enough for peer review? I've added a number of references, and finally managed to excise some of the article's blatant POV. What do you think?Serendipodous 16:39, 16 October 2005 (UTC)

Red dwarfs and extreme day-night cycles

It would seem that the habitable zones around red dwarf stars are within zones of captured rotation. If the planet in question has a thin atmosphere, then temperature differences between day sides and night sides would be extreme (example: Mercury or the moon). But if the planetary atmosphere is relatively thick, then differences of temperature would be less severe (Venus) -- but only with strong winds and severe storms that themselves would make sessile life difficult even if temperatures were moderate. --66.231.41.57 03:53, 17 December 2005 (UTC)

Binaries

Sheee-it. How have I stared at this so long and not added anything about habitability in binaries? If anyone has good sources, note them here first and someone can have a go making the addition. Marskell 11:37, 20 October 2005 (UTC)

Here's a decent source.[2] There are other forms of binary habitability, but this is a good start.

[3], and here's another, a quick overview. I'll look into serious works later.

[4]

Here's an old Hubble shot of a gas giant being ejected from a binary system. I'd like to use it for my "definition" page, actually.

Serendipodous 14:56, 20 October 2005 (UTC)

Sol Station lists a bunch of primary sources in describing, though a touch heavy: http://www.solstation.com/habitable.htm. Marskell 15:05, 20 October 2005 (UTC)

OK, I've added. Not incredibly detailed but should suffice. The article just tips the scale at 30k... A nice length. I just want one pic that's relatively original (I think detailing the habitable zone--I've seen a couple on-line, but copyrighted). Get that pic and then off to FA. The peer review, predictably, generated little, though I notice that someone gave it a top to bottom copy edit, presumably via the review page.

Anyhow, is there anything forgotten? Marskell 16:59, 20 October 2005 (UTC)

Just an aesthetic quibble, but you might want to combine the binary and red dwarf articles into a separate section about "alternate stellar systems". Also, you never make explicit why blue and red supergiant stars are unsuitable for life, viz that they only live a few hundred million years. Plus, since the addition of the "Red Dwarf habitability" section, your comment about red dwarf habitability under spectral class is slightly redundant. Sorry; I'm a niggler. You begin to underderstand why I endlessly go back to edit my own work. Serendipodous 17:57, 20 October 2005 (UTC)

• Combine the binary and red dwarf articles into a separate section about "alternate stellar systems." Instead, I think I'll change Suitable stars to Suitable star systems so that the Binary bit makes more sense under the descriptor. Something better than "Other considersations" could be come up with; indeed we could remove red dwarf habitability from the section and let it stand alone.
• Make explicit why blue and red supergiant stars are unsuitable for life. Yes, implied but not directly addressed. I will add a sentence.
• Comment about red dwarf habitability under spectral class is slightly redundant. "Conscious redundancy" for an important point. The first reference points to the later section as an expansion. Marskell 16:17, 21 October 2005 (UTC)

More stuff

I keeping thinking "everything stated" and then something else occurs. We should probably mention:

• Gravity and habitability (presumably under mass matters). I can't find a single source though, beyond generic, sci-fish references ("high-grav planets will have stocky aliens" etc.).

* Importance of magnetic field in protecting planet (this could probably get a single sentence in mass matters as well).

Thinking it through, this probably needs a dozen more references. There's much stated as if obvious (Mars probably geologically dead) and even some unsourced stats (red dwarfs may have light dimmed by 40%).

Finally, anything that reads too much like an editorial aside is being moved to the new Notes section. Marskell 15:59, 23 October 2005 (UTC)

Primary source

I tracked this down: [5]. I, uh, understand it perfectly ;). Red Dwarf section could be re-jigged slightly to accomodate the source given that we over-rely there on article. Marskell 11:41, 24 October 2005 (UTC)

I don't understand what was wrong with Ken Croswell :(. I have one of his books; he's Harvard trained and is a writer for the New Scientist. Was I wrong to trust his judgement?Serendipodous 14:46, 5 November 2005 (UTC)

Not at all! I wasn't suggesting removing him or anything only that two sources is better than one for a longish section. Marskell 14:55, 5 November 2005 (UTC)

Thank you

To all of those who have worked on this article, I thank you. It is very well structured and very informative. Keep up the good work. Zhatt 23:01, 27 October 2005 (UTC)

Brown dwarfs

I added the statement that brown dwarfs are not considered stars, because the first paragraph under red stars could be confusing, stating that over 70% of stars are red dwarfs, then stating that brown dwarfs are like more numerous -Satori ^(talk) 00:05, 12 November 2005 (UTC)

True. Thx. Marskell 10:03, 12 November 2005 (UTC)

Two references/note tags with number 12

{{mn|Mass-gap|12}} — Yaohua2000 17:34, 14 December 2005 (UTC)

Fixed. Marskell 09:41, 15 January 2006 (UTC)

Sources

What is the difference between a primary and secondary reference? I want to add some things, but I'm not sure how they would qualify in this article's reference list. Mred64 21:07, 17 January 2006 (UTC)

Primary sources are always immediate in the sense that authorship and research is assumed to be identical: Mary suggests that 5 % of people have green eyes because Mary has actually performed a controlled experiment documenting eye colour or, say, handled an archeological piece that deals with eye colour. This info will be presented in a university research paper generally. Secondary sources will state that "research from Mary indicates that 5 % of people have green eyes," e.g. secondary sources present the research of others that they deem correct without having actually performed the research. This info will be presented in a magazine, journal or book generally.

For instance, the HabCat is primary: two researchers sat down and collected, tabulated and compared data about nearby stars. Rare Earth is secondary: two researchers developed a thesis by analyzing data others had collected, tabulated and compared. Secondary sources are not "worse" or "less accurate" than primary sources--indeed they are the foundation of sources like Wikipedia because info presented in a "secondary" sense has been examined by at least two (groups of) people, and subsequent reviewers know the info isn't out to lunch. All well and good that X Research Team has found evidence of ceremonial burial amongst Neanderthals (in a primary sense) but have the editors and peer-reviewer team at Nature or Science deemed it accurate enough (in a secondary sense) to present it with authority? If it has appeared in Nature or Science then it passes the bar for inclusion here.

In this vein, much of what people actually read to gather information on a topic is neither primary nor secondary--it's tertiary (3rd-level) information. CNN or Wikipedia will collect information from secondary sources and present it as truth, trusting the diligence of the second-level source. On this page (and on good, often featured articles in general) editors try for a mix of both primary and secondary sources to ensure we're a good tertiary source. OK, find some research papers detailing "whatever" but also find some secondary sources presenting "whatever" in a matter of fact way.

Finally, I must say, please familiarize yourself with the footnoting system before adding sources on this article. By all means add sources (it's going to hit the main page in five days and the more accurate the better). But as it works now if you just throw in an external link between note 22 and 23, somebody (moi, for instance) well have to change 23 to 24 and so on and so on... Any more specific questions you can ship to my talk page if you like. Cheers, Marskell 23:35, 17 January 2006 (UTC)

OK. I think I'll wait 5 days. I have little bits and pieces I can add here and there, but I'll need to double-check the cites (having two astrobiology books and a subscription to Astronomy eventually makes everything one big blur in terms of sources) and then see if I would have to alter the references section. Thanks. Mred64 04:23, 18 January 2006 (UTC)

Dangerous Spiral Arms

What do they mean about "Dangerous spiral arms" in this article? I don't think the spiral arms of a galaxy swing around like a circular saw blade, destroying life. Seems invalid. --Shanedidona 04:13, 23 January 2006 (UTC)

From the source at the beginning of the section: Keeping out of the way of the Galaxy's spiral arms is another requirement of the Galactic Habitable Zone.

The density of gases and interstellar matter in the spiral arms leads to the formation of new stars. Although these spiral arms are the birthplaces of stars, it would be dangerous for our solar system to cross through one of them. The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.

Luckily, our Sun revolves at the same rate as the Galaxy's spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often. I'll add this briefly. Marskell 08:04, 23 January 2006 (UTC)

Mistakes regarding carbohydrates and hydrogen bonding

It seems like the article is describing the covalent bonding between hydrogen & carbon atoms incorrectly as a "hydrogen bond", which is something altoghether different, being a medium strength attraction between partially-charged atoms typically in solution. In fact, the whole sentence is questionable - carbohydrate metabolism isn't life's (only) fuel even on earth. The article seems to imply that it will be a universal characteristic of life.

Good points. I'll change them. Serendipodous 13:31, 23 January 2006 (UTC)

Galaxy: Black Hole and Neighbors

As a non-expert just reading and thinking along, I have doubts about two statements regarding favorable positions within a galaxy:

• Why is it good to be far away from the central black hole of a galaxy? A star revolves around the center of a galaxy according to total mass - which is in no way influenced by the form that mass takes on - whether it´s lose, visible matter or an extremely dense singularity. What is the danger of a black hole, really? I read recently that for the orbit of earth, it wouldn´t matter one bit if the sun suddenly collapsed into a black hole. All that changes is the loss of radiation, making this a cold place. However, no radiation from stars other than its own sun is a good thing for a habitable planet, is it not?
• The article claims that other stars closer to our sun might hurl objects from the Kuiper belt into the inner solar system. Okay. Then again: The article also explains how gas giants may both protect earth from comets (and thus from disastrous impacts) AND attract comets into our orbit (thus supplying early earth with much needed water). Would a neighboring star not similarly do it both ways - hurl objects into the inner system as well as pull some other objects out that might otherwise fall in?

--212.202.162.151 17:00, 23 January 2006 (UTC) (EMaraite from the German wikipedia)

My intuitive and (as far as I know) the scientific answer to Black holes is: don't want to be sucked toward them and ripped apart :). I'm free to be corrected, but if there were suddenly enough mass at the centre of the solar system to form a black hole the Earth wouldn't exist for long.

But that is not how Black Holes form at the center of Galaxies or solar systems - they do not "suddenly" pop up! And anyway: Any sun with a mass above the Chandre...lalala limit (long Indian name), that is more than about 1.5 times the mass of our sun, will eventually collapse into a black hole, without any gravitational impact on its planets. True enough, if you are very close to a very massive black hole you might feel a noticeable difference in gravitational pull - straing your sun or making your orbit skewed. However the mere existance of a lot of mass at some distance is no problem at all - it doesn´t matter whether the mass is in one black hole or distributed among a few million stars (which is a galaxy).

Ergo: In my best judgement, black holes per se at the center of any orbit do not cause any trouble. --15.195.185.77 15:38, 25 January 2006 (UTC) (i.e. EMaraite again)

I was not here suggesting that that is how they form. It was a hypothetical answer to a hypothetical scenario. Marskell 18:21, 26 January 2006 (UTC)

Re Oort cloud, Kuiper belt objects etc. I'd suggest this: most of the bodies that could've crossed the planetary orbits and impacted have largely already done so. One comet hits us (or Mercury or Saturn) and there's one less to impact in future. After a point in a system's evolution, what remains, remains in stable orbits. With or without a star to disturb it, something could still come out of the Oort cloud, but if a star were disturbing it continually bodies that would otherwise carry on about there business have a greater chance of accidentally getting knocked in. Make sense? Marskell 18:22, 23 January 2006 (UTC)

The center of a galaxy contains is considered "dangerous" because there is a higher chance of supernova and other energetic phenomena, I think. And I can't remember why they think the central black hole is harmful, maybe another radiation thing (black holes can create X-rays). I can't say I have a certain answer to the star question, but this is just what I think would be a good answer from what I know. It somewhat comes down to timing. When Earth is forming, comets are like harbingers of life because they bring water and some organic molecules. At this stage, a gas giant planet almost constantly flings the numerous comets and planetesimals into the developing inner solar system. Of course, some bodies will probably collide with the gas giant, but there's so much material that many objects will collide with the inner planets. Now though, there is less material. However, impacts obviously wouldn't be helpful at the moment since life is well and good right now. Jupiter currently protects the inner planets from the "leftover" comets and also can't interfere with the Kuiper Belt or Oort Cloud, because it just isn't that big. However, a nearby star is a different matter. A close star could not only influence the protoplanetary disk in the early solar system, but even now. A star too close to the solar system now could disturb millions of objects in both the Kuiper Belt and the Oort Cloud. I guess the problem with a close neighboring star is that it would cause continual impacts much longer than a gas giant would, and perhaps delay life. Mred64 23:39, 23 January 2006 (UTC)

Re Black Holes: a) The center of the galaxy being a busy place with all sorts of dangers is not dependent on there being a black hole or not. b) Only very small black holes (much lighter than planets!) emit high energy radiation. The heavier the black hole, the less and less energetic its radiation. Hence the term "Black" hole

Re Oors cloud: Hmmm - ok, could be, but we´re all guessing and picking, aren´t we? Any references? --15.195.185.77 15:38, 25 January 2006 (UTC) (i.e. EMaraite again)

On black holes: I didn't mean to say that a black hole in the center of the galaxy caused that stuff, I just meant to say that the center of a galaxy just is bad given our current "understanding" of habitality. To your part b) yeah, I guess you're right.

Oort Cloud: I can give a reference for the statement that a current nearby star would be harmful (most of the rest of my comment was just extrapolation, although I bet someone has made that argument before if we want to look really hard for it). Here we go. Got a quick and dirty cite for it. Dorminey, Bruce. "Dark Threat." Astronomy July 2005: 40-45. I still have the original magazine and article if that ends up going into the article and/or we need more information for a reference.

That would be cool if you could look that up, as I frankly don´t have the time and resources to check anything, really. I am just nitting and bitting from memory! --15.195.185.75 09:17, 26 January 2006 (UTC) (EMaraite)

Re b) (Black holes) "much lighter than planets!" No, by definition they are incredibly more massive than planets. Black holes can only form in the presence of incredible mass accreting in one spot through gravitational collapse. The centre of the galaxy is indeed a "busy place"--which is believed to have a black hole at its centre making it busier. Should this not be mentioned? Re "guessing and picking;" we do have 28 sources which you might look through. Honestly, I'm not sure the point of the post here. Specific issues generally get addressed on this talk page but I'm not sure what specific problem is. Marskell 23:12, 25 January 2006 (UTC)

Although this discussion now does not bear directly on the article and my initial question anymore, I think I should point out that you are indeed wrong on 2 counts: 1. There is nothing in the definition of a black hole that requires a large mass. While the usual way that black holes are formed involves the collaps of former stars more massive than our sun, it is entirely possible for there to be much smaller black holes; for example, many are thought to have originated very shortly after the Big Bang (primordial black holes), to have slowly deteriorated since, which may now be near the end of their existence at a mass as small as a mountain range, emitting a burst of light (and thereby losing more energy/mass), until they disappear (Steven Hawkinng´s "white hot black holes"). 2.: The center of a galaxy is busy not because there is a black hole, but because of the way that galaxies are formed in the first place (and well, because it IS the center). So here´s the core of my initial question: What makes the relative proximity of a black hole as such dangerous? As long as you keep a distance of a few light years, I really think you ought to be save. (The likelihood of coming into too close contact with other stars is a whole different matter). --15.195.185.75 09:17, 26 January 2006 (UTC) (EMaraite)

Re Black holes: I'm going to do some backtracking through some books and magazines I have. One of them had a picture of the galactic habitable zone (similar to the one in the article) and talked briefly about the supermassive black hole at the center of the galaxy and what it meant for "habitability". As a sidenote: did the source for the galactic habitable zone talk about the effects of the supermassive black hole? I also think the argument about radiation in black holes is true, or at least the basic part. While we don't see planet-mass black holes, something about the energy/radiation and a smaller surface area over which it was spread makes it more energetic. But don't trust me on this, I can't remember what the book/article said, so I'm going to look to find it.

I think the "guessing and picking" was more directed to my response about why another star close to the sun was bad, since I didn't have a reference initially. Mred64 03:49, 26 January 2006 (UTC)

Correct. Sorry if I caused any confusion on that. --15.195.185.75 09:17, 26 January 2006 (UTC) (EMaraite)

You might see our own page Supermassive black hole for the type believed to lie at our galactic centre. If "a few light years" is a sufficient safe zone I'd be surprised but am more than willing to be corrected.

To be clear, yes any amount of matter could at least in theory form a black hole if compressed enough (Earth mass compressed to a micron would presumably form a black hole...) but gravitational collapse simply does not occur with bodies of such size. From David Darling: "in theory, any mass if sufficiently compressed would become a black hole. The Sun would suffer this fate if it were shrunk down to a ball about 2.5 km in diameter. In practice, a stellar black hole is only likely to result from a heavyweight star whose remnant core exceeds the Oppenheimer-Volkoff limit following a supernova explosion [6]." As far as I know, primordial black holes remain theoretical and for the purposes of our discussion on the galactic centre we're discussing black holes in the traditional sense: i.e., super-massive bodies that have accreted mass over time by cannibalizing nearby stars. I'd just offer an intuitive question: if the Sun's effective gravitional pull extends to the Oort cloud (roughly a light year) what is the safe zone for a body millions to billions Solar masses? How many stars have been "sucked into" the Milky Way's black hole, at 2 million plus stellar masses? Two hundred thousand? Twenty million? Enough, I'd suggest, that calling it a "danger" is a fair analysis for this article. Marskell 09:42, 26 January 2006 (UTC)

Ok, you got it mostly right this time  ;) and anyway, I mentiones small black holes only in response to Mred64 who remembered that black holes might create x-rays - which is true only for the hypothetical small ones (Hawking radiation), but not for the supermassive. However, the article on Sagittarius A*, a radio source associated with the milky way´s central BH, explains its emenating strong radio waves (NOT x-rays!) like this: "The observed radio energy emanates from gas and dust heated to millions of degrees while falling into the black hole." Gas and dust falls into the BH because it is just that: gas and dust - and not defined bodies on kepler orbits. Maybe that gas heating and radiation is the or a reason why a huge BH nearby is dangerous? I haven´t read anything directly to that effect, but would be willing to accept such an explanation.

The article on Supermassive black hole however seems to me abiguous about the question whether it is dangerous, or at what distance it is. It mentions two of the reasons why I think it is not that big a deal as long as you are on an orbit at some distance: tidal force is low (you could enter into it safely without noticing much, let alone being torn apart); and it should have trouble finding matter to suck in because most eligible matter has enough angular momentum to stay outside on a "safe" orbit.

So all I want with the article: Explain, why the center of a galaxy is dangerous - because the BH alone won´t do the trick. You have to mention the presence of more matter (increasing the danger of collisions), the hot gas falling into it, some tidal forces and what not. I think I shall attempt to do that... maybe during the weekend... --212.202.162.151 15:43, 26 January 2006 (UTC) (EMaraite, who also thinks he should eventually set up an account on the english wikipedia...)

I have expanded the section. Marskell 16:53, 26 January 2006 (UTC)

Appreciation for Earth

This article not only was very intuitive, but gives the reader an extreme sense of appreciation of our home planet. As we all know there is an over whelming number of star systems Universe wide. Earth is so very rare.

I always viewed volcanic and tectonically active planets as a "hostile" environment. But to discover these attributes are a requirement for life... Amazing, how Earth has successfully balanced a ‘safe enough' environment from a hostile one, adds to her rareness.

--206.223.21.130 18:04, 23 January 2006 (UTC)Jason Miller junkmail@millerfam.org

Read Rare Earth if you haven't! It is a beautiful thing to consider. Marskell 18:24, 23 January 2006 (UTC)

New sections

So after yesterday we have two new small sections, "Gaia" and the "Privileged planet." The latter appears to be a strawman (Guillermo Gonzalez has apparently deployed Rare Earth to argue for a creator BTW) and I'm not sure if it belongs. However, one option is to make a final level two section "The Earth in context", or maybe "The pale blue dot" which could be a place-holder for the two sections, i.e., having evaluated Planet hab we make one final comment on Earth's rarity. It needs to be short though--we're over 40k now. Thoughts? Marskell 07:25, 24 January 2006 (UTC)

Hi. Sorry if I shouldn't have added that on "PP" - I see it was edited away, and when I did my edit, I hadn't realised how much of a process underlaid the article. However, regardless of whether you accept Gonzalez/Richards' conclusions, the hypothesis (that there is a link between habitability and the ability for intelligent life to make observations) is an interesting one, and one that as yet hasn't been refuted. I would have thought that if this article is to be up-to-the-minute, this is an interesting perspective.--Exiled from GROGGS 12:18, 27 January 2006 (UTC)

Axis and procession

The article states that "smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead". This is incorrect. The earth is kept warm by radioactive materials releasing energy. The earth would have cooled to be solid within a few million years otherwise. It is the presence of heavy radioactive materials that allows the earth (or other planets) to be gealogicaly active.

I also am not certain that, in general, a moon would have any positive affect on reducing precession - indeed the moon is the main cause of procession! Mars, for example, rotates almost inline with the sun and has very little precession, so far as I am aware. Pogsquog 10:48, 24 January 2006 (UTC)

"All of the geological activity on the earth today is driven from this initial source of heat at the earth's formation, aided and abetted by continued radioactive decay of elements in the earth's interior" (James Madison University) [7]. Incomplete rather than incorrect. I will edit accordingly and should have done so already; I recall reading that up to 80% is now radioactive decay though I now can't find it. However, the analysis of surface-to-volume ratios remains sound I think. Marskell 12:04, 24 January 2006 (UTC)

Re second point: "Mars, for example, rotates almost inline with the sun." I'm not sure what you mean. Mars' axial tilt is currently 25 degrees and precession is dramatic, ranging from 15 to 35 (though over 90 000 yrs). Marskell 12:15, 24 January 2006 (UTC)

Oops! Second point was wrong - looked up inclination instead of axial tilt :P I'm still not clear on how the precession would make a difference, however.. it would seem that the obliquity of the poles would be a more relevant factor? This varies over a 41000 year cycle for the earth. Pog 12:58, 24 January 2006 (UTC)

Thanks Pog. That section may indeed have some problems with due emphasis. You'll note that we definitely mention obliquity (axial tilt) in the second bullet point and again in reference to the moon but only in the mean sense and not in terms of variation. Also in re-reading it I notice "must have..." has been deployed thrice which is too didactic for this topic. I'll try to shake it up a bit soon (or feel free yourself). Marskell 17:03, 24 January 2006 (UTC)

I'm thinking of doing a reordering of the page

I think that the "Other Considerations" section would be better split between other sections. "Red dwarf systems" could be combined with "binary systems" to create a section on alternative stellar systems; "Galactic neighborhood" could be considered a stellar attribute. One of the principal attributes of a star, after all, is where it is. "Good Jupiters" could (loosely) be considered a planetary attribute, since the existence of a "good Jupiter" directly affects the evolution of a planet. Serendipodous 11:47, 24 January 2006 (UTC)

I could live with Alternative stellar systems. Above or below Planetary characteristics? I don't like the idea of going through most of the article without discussing planets directly but the Red dwarf business probably flows more naturally after the first section (or even as a part of it, but that would make for a massive section). I think Good Jupiters and and Galactic Neighborhood should be left where they are. Finally, while Gaia deserves mention given that the topic has some popular awareness, I think Privileged Planet may be Intelligent Design by the backdoor. I'd like to remove it but it was a good faith addition by an anon who I think subsequently got an account so I was hoping for other thoughts on what to do with it. Marskell 17:13, 24 January 2006 (UTC)

What do you think? I honestly think it works, but then I'm a bit anal when it comes to sections :). As for the "privileged planet" argument, creationist or not, the addition isn't very well thought out or explained, and I really think it should be ditched.Serendipodous 23:22, 25 January 2006 (UTC)

I think it's fine now, though the refs are going to have to be re-numbered. I think a section "Life and habitability" would work under other considerations. That is the degree to which the first emergence of life has a subsequent affect on habitability. This could incorporate Gaia and also theory from David Grinspoon. Marskell 09:59, 26 January 2006 (UTC)

First self-replicator

Perhaps a consideration for planetary habitability ought to be the pre-biotic conditions (reactions conditions - pressure, temperature, reactants etc.) conducive to initiating life aswell as the considerations taken - in this article -toward sustaining it. This would probably involve looking at self-replicator cells and the reaction mechanisms of DNA molecules.

I am not sure whether these points are suitable for inclusion in the article, as they involve more about the origin of life than a planet's suitability to sustain life, but still there could be room to look at some factors as regards a planet's suitability for initiating life (if it is at all possible that life can originate in such a way without recourse to God?). 62.249.242.232 20:18, 24 January 2006 (UTC)

For the time being, some one might might disambig an existing phrase in the text with origin of life. Marskell 17:33, 25 January 2006 (UTC)

Anon concerns over energy

This article has a serious problem: unfortunately, while I know the problem exists, I don't have the expertise to solve it. Earth does NOT solely rely on solar radiation for its temperature, but radioactive decay, heat from initial formation, AND solar radiation. (So I was told by a prominent astronomer.) This is a common misunderstanding promoted by drawing HZs around stars as if stellar radiation was the be-all and end-all. So, a section is needed on the heat budget of planets, something like stellar_radiation + radioactive_decay + residual_heat + gravitational_heating ~= black_body_radiation. I presume Mercury is a good example of stellar radiation dominating. Io is an example of gravitational heating dominating. Around a brown dwarf, one can imagine a bigger, wetter, cooler(?) Io with abundant chemosynthetic life.

Also useful would be a historical survey of the gradually reducing parochialism regarding habitable planets. First, Earth was the center of the universe. Later, Earth was considered the product of hugely unlikely circumstances. Photosynthesis parochialism has started to collapse from the discovery of chemosynths 2 km deep in rock, near volcanic vents, in ice, in hot springs, etc. The implication of this history (which I can only sketch) is that we're probably still suffering from parochialism.

Hi anon. The article never states that the Earth solely relies on solar radiation. The third sentence reads: "the only absolute requirement for life is an energy source (usually but not necessarily solar energy)." Absolutely a sentence making clear that sub-surface, ocean-vent life etc. relies on chemical synthesis would be fine in the appropriate spot. But I'd suggest we not throw the baby out with the bath water on this one. Radioactive decay, for instance, may "dominate" in powering geology but surficial temperature on the Earth more or less remains a product of stellar radiation received + atmospherics. The HZ concept has definite problems which the article makes clear (see fnote 3, for instance) but the basic premise remains sound and ought to be central here: the body can or cannot support surficial liquid water.

Re new sections...this article is obese! It's 45k now and I'd suggest the hard limit needs to 50k or it will be so long that those interested will skip it. So, on heat budgets I'd suggest sharpening up sentences we already have rather than a new section. On the "reducing parochialism" business you'll notice the section added yesterday at the end which might absorb some commentary. I'd suggest also that the pendulum in some respects is tilting away from the mediocrity principle toward, if not a rare Earth, at least a "rarish" one. This article has never been a probability analysis and I don't think it should be. Whether we're still suffering from parochialism may not be a question that this page ought to foreground. Check fnote 1 for some other pages that may deal with it better. Marskell 09:08, 27 January 2006 (UTC)

I think HZ are more concerned with surface water, so solar energy would probably be the main factor to consider. And HZ themselves are fairly "parochial." Let's look at Europa. Europa is beyoned the habitable zone, but astronomers are very confident that Europa has water beneath the surface. However, looking at Europa, it doesn't follow the idea of a habitable zone. Its water is under the surface. Its heat, like Io, is derived from mainly from gravitational interactions. But HZ really don't account for this. Any HZ diagram of our system doesn't include any of the Galilean moons, because the HZ doesn’t take things like this into account. I don’t even know if things like different atmospheres are taken into account.

Re to “parochialism.” Most of our ideas of ET life are parochial. Look at carbon chauvinism. We tend to assume ET life will have a carbon biochemistry and use water as a solvent. The very idea of a habitable zone relates to water existing as a liquid. The section in the article says that the idea of an HZ may be thrown out as useless if we find out other liquids could be used as solvents. Although many say we shouldn't limit our ideas on what other life may be like (and I agree with this), scientists seems to grudgingly admit they have these parochial views because they would be lost without them. If you want to put a section on ideas of parochialism, it would probably be more suitable in the Beliefs in Extraterrestrial Life section in the extraterrestrial life article or in the article on cosmic pluralism than it would be here though. Mred64 03:20, 28 January 2006 (UTC)

Thank you for the response, Marskell. Perhaps the essential issue here is that (I feel) the article over-emphasizes HZ instead of staying on topic, Planetary Habitability. For example, the opening states, "The only absolute requirement for life is an energy source (usually but not necessarily solar energy)..." yet while only Earth is proven, the best candidates for habitability include Titan, Europa, and perhaps Io--usually gravity-powered. And with the vast majority of stars M class (and even more L, T, and Jovian bodies), arguably gravity power may be more usual than solar power, so the "usually" in the opening is unwarranted (IMO).

Another example is the "Low stellar variation" section. I think this section should be rewritten as "Low energy variation". The day/night cycle of Earth is a huge challenge for life on earth: many (most?) organisms essentially pack up and go into stasis during night; a significant number do so during day. Plants run two different energy systems, O2-producing photosynthesis by day and O2-using stored-energy by night. Then there are axial considerations, with bears and others hibernating by winter. I realize some of these factors are discussed in other sections; I guess I'm advocating a consolodation that puts all the variation issues in one section so the reader understands they're one issue. This consolodation might actually shorten the article.

Now contrast that with, "Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time, while at other times they emit gigantic flares that can double their brightness in a matter of minutes." Ok, so energy varies from 60% to 200%. Obviously, Earth energy varies from 0% to 100% (of Sol), far more variation. In more practical terms, an common Earth plant needs to deal with 100% of Sol, but gets far less average energy because of the day/night cycle, shifting shadows of surrounding plants, and seasons. Radiation frequency aside, a plant has a far easier time on the tidally locked planet of a red dwarf; animals don't need to worry about night.

This is part of a pattern of parochial views and pessimism about any system other than Sol's. Another example is the discussion of, "If, for example, Jupiter had appeared in the region that is now between the orbits of Venus and Earth, the two smaller planets would almost certainly not have formed." This neglects that Jupiter's Moons would be in the HZ. IOW, the article tends to mention the downside of situations without adequately considering the upsides.

Thank you for your time (and for accepting most of the changes I felt competent to make).

Well, we're all in agreement the HZ has its problems. But I must say that a large part of staying on topic is staying with the literature. The HabCat utilizes the HZ criterion, James Kasting and the other primary sources utilize it, it is constantly referred to in the "interview with X scientist" secondary sources, etc. If this article exists in a century it may read "scientists once deployed a Habitable Zone criterion which has since been discarded as parochial, tautological, and insufficient in describing the 17 bodies currently known to harbour life." Let's hope so, but for the time being I don't see it our place to downgrade the HZ's centrality to the topic.

Also, I'd say better pessimism than optimism. For instance: "Radiation frequency aside, a plant has a far easier time on the tidally locked planet of a red dwarf; animals don't need to worry about night." Well, no. The elephant in the room on this topic is always going to be heat transfer to the night side and homeostasis in general; this is an enormous hurdle and if you look, for instance, at Heath there is a guarded suggestion that arborous life is possible on Red Dwarf planets but definitely no indication that it would be easier. And the numbers deployed are a touch misleading. "Earth varies from 0% to 100% of Sol." More accurately, any given point on Earth spends 1/2 of its time (12 hour avg) without direct solar energy, while retaining significant energy atmospherically and receiving constant circulation from points that are receiving energy directly. At no point is it "zero" and I see this regularized phenomenon as much much less of a hinderance than, for example, a sudden doubling of the primary's brightness.

To quickly respond to a few things.

• Io, Europa, and Titan DO get mentioned.
• I can live with changing "usually" to "often."
• Mred makes a good point about looking at Extraterrestrials and numerous other pages to see if some of these thoughts fit better there.
• A consolidated section on energy could work. Where would it flow and what would be cut in its place?
• Finally, anon can you source the bit about no shade, not having to move toward sunlight being an advantage?

Whew, this talk page is now longer than the article I think :). Marskell 08:40, 28 January 2006 (UTC)