Talk:Planetary habitability

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Space colonization Space and survival Space habitat Planetary habitability Colonization of the Moon Colonization of Mars Colonization of Venus Colonization of the asteroids Colonization of Ceres Colonization of the outer solar system Colonization of Europa Colonization of Titan Terraforming Terraforming of Mars Terraforming of Venus Interstellar travel Mars Society National Space Society

September 2005 - January 2006 I pretty sure that the earth isn't geologically active simply because of its large mass/surface area ratio. From what I have read, I believe that planets are probably heated mostly by radioactive decay, and/or tidal forces, with heat from formation being significant only very early in their lives. See http://pda.physorg.com/lofi-news-heat-marone-mantle_62952904.html for example. It may be that the earth has an unusually large quantitity of radioactive materials. Clearly, the final temperature reached by a planet will be affected by the mass/surface area ratio, but I imagine that a mars-sized planet with a higher proportion of radioactives and/or large satalite could remain geoligically active. For that matter, if the moon orbitted significantly closer to the earth, the tidal forces on the earth would be immense (tidal forces are inversly proportional to the cube of distance?). --Pog 16:10, 25 April 2006 (UTC)

Contents 1 Anons concerns cont'd 2 Mistake ? 3 Picture Size 4 rate of binaries 5 Habitable to Whom? 6 Reference markup 7 Suggestion 8 Cellular life

[edit] Anons concerns cont'd

On the grav-locked, red dwarf planet:

Note that a planet gravitationally locked to its star will have tidal heating like Io, so it's not as if everything will freeze on the night side. The great advantage of a grav-locked planet is its stability. I think it's fair to say that stability is easier for life to deal with than instability. If you look at the stability of Jupiter's great red spot (400 years and posited to be permanent), weather on a grav-locked planet should be relatively stable, I presume a convection rising at the "sun pole" and falling at the "dark pole", with terrain eddies, of course. Such a planet will have no axial tilt, no day/night cycle--I don't even think it can have an eccentric orbit, but I'm unsure. This greatly reduces the complexity of an organism's job: for example a plant doesn't need the night "mode" earth plants need; they would not have to drop leaves for winter. These seem like advantages to me.

Based on the best information I can find on the US Dept. of Energy site (solar information), what plants get on a clear equinox day is about 8 kWH/m^2, peaking at 1 kW/m^2. The vast majority of that power comes in the middle 8 hours, less than 1 kWH/m^2 is in morning and evening. The night is essentially 0 kWH/m^2--not absolutely, no, but to the 2 digit accuracy I was using, yes. (Compare visual magnitudes of sun vs. moon.) This varies by latitude and season, of course. Now let's plug this into the supposedly horrible 60-200% variation of a red dwarf. Instead of roughly 1/3 time at 1 kW (yes, it really is close to that), equivalent energy would be all the time at 1/3 kW. Thus, the variation would be 1/5 kW during minimums (=60% of 1/3 kW) and, of course, 200% is 2/3 kW. Thus, ignoring frequency, the red dwarf plant has an easier time. The lower power of red dwarfs just pull the HZ in. Considering Earth shade plants, which live on far less than 1 kW/m^2, the HZ is where the total solar and gravitational energy are right for water.

Organizationally, perhaps the best thing would be to treat Earth-like planets in the main flow of the article: then discuss tidally-locked red dwarf planets, with a note that thinking on the issue is in flux; and note that chemosynthsis biospheres would be possible even around brown dwarfs or outlying moons, like Titan and Europa. Perhaps (keeping original numbers):

1 Suitable star systems

• 1.1 Spectral class
• 3.1 Binary systems (since all but the closest binaries are ok, put it here)
• 1.2 A stable habitable zone

2 Planetary characteristics

• 2.1 Mass
• 1.3/2.2 Acceptable energy variation (as I hope I showed about, "low" isn't really necessary; include the case of Jovian moons in the HZ)
• 1.4 High metallicity (it comes from system formation but it's a planet property)
• 2.3 Geochemistry (maybe combine with metallicity?)

3 Alternative biologies

• 3.2 Red dwarf systems (tidally locked; low light?)
• NEW Chemosynthesis-based biology.

4 on, no changes.

I should add, this is a really good article. Maybe I'm trying to gild the lily.

[edit] Mistake ?

I am not a specialist but "Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere [10]." (mass paragraph) seems doubtful to me. I mean, Mars still has volcanoes (Mount Olympus for example) so it sees strange to me that it's geologically dead, but once again I am no specialist. Poppypetty 19:28, 1 February 2006 (UTC)

Mars does have volcanoes, but as far as I know, they're not really active, so nothing is really "powering them" and geological activity isn't really showing up. Mred64 22:49, 2 February 2006 (UTC)

Luna, Earth's Moon has volcanoes, and it's accepted as geologically dead. The presence of volcanoes doesn't mean ACTIVE volcanoes.CFLeon 21:59, 21 March 2006 (UTC)

That's basically what I was trying to say, I guess I just never made it really clear. Mred64 03:08, 27 March 2006 (UTC)

[edit] Picture Size

Can a larger size by found for the Habitability Zone picture? I'd like to use it as a background, but it won't scale up.CFLeon 21:59, 21 March 2006 (UTC)

I downloaded it here: http://library.thinkquest.org though I can't remember the exact spot on the site. An image search for habitable zone turns it up first thing, but only at 19k... Marskell 21:42, 24 March 2006 (UTC)

[edit] rate of binaries

The article says:

"Typical estimates often suggest that 50% or more of all stars are in a binary system. This may be partly sample bias, as massive and bright stars tend to be in binaries and these are most easily observed and catalogued; a more precise analysis has suggested that more common, fainter, stars are usually singular and that up to two thirds of all stellar systems are therefore solitary"

I fail to see the opposition. Consider 4 stars, two of them in a binary system, two single:

**       *        *

Here 50% of all stars are in a binary system. At the same time, two thirds of the stellar systems are solitary.

Should it, perhaps, read:

"that up to two thirds of all stars are therefore solitary"?

SHagel 12:17, 8 May 2006 (UTC)

[edit] Habitable to Whom?

This article seems to have a very narrow definition of life. Who is to say that an exotic form a life could not exist in conditions that would be considered extreme by human standards? The assertion that water is a pre-requisite for life is based on a single case study: our planet and our solar system. It may be that life exists elsewhere in forms we are currently unable to recognise. Would it not be more accurate to define planetary habitability as a planet whose conditions are favourable to human life, or earth-like life? --70.82.50.67 18:13, 15 June 2006 (UTC)

The problem is that we don't really know what life is. Are viruses alive? Or computer viruses? Prions? Is fire? We only have one sample of a process that we can comfortably call life, and it is the one we happen to be in. Therefore it makes sense, when searching for life out there in the cosmos, to stick to the one example of life we know of as our determining factor. Either that or come up with a workable definition of life which answers all of your objections, and I don't see that happening any time soon. Serendipodous 16:48, 16 June 2006 (UTC)

It is not asserted that water is necessary for life. It is strongly suggested, then qualified: "After an energy source, liquid water is considered the most important ingredient for life, considering how integral it is to all life-systems on Earth. This may reflect the bias of a water-dependent species, and if life is discovered in the absence of water (for example, in a liquid-ammonia solution), the notion of an HZ may have to be greatly expanded or else discarded altogether as too restricting."

The fundamental point is to follow the literature rather than introduce utterly speculative language or original resources. Numerous academics have defined habitability in terms of water—so then must we. Marskell 17:02, 16 June 2006 (UTC)

[edit] Reference markup

I just noticed that Marskell reverted by conversion of this article to cite.php markup. It's not so much that I feel this article must use cite.php, though that happens to be the system I consider best for almost all footnoting situations, but that I feel it shouldn't be using {{mn}}. This is the Wikipedia:Footnote4 system, a proposal that was never even adopted in the first place and which is now used by only about a dozen articles (see Special:Whatlinkshere/Template:Mn). Marskell, if you feel strongly that cite.php is inappropriate for this article, perhaps you could convert it to some other system that's currently in use? Wikipedia talk:Footnote4#Absorbed in Footnote3 suggests that the same functionality exists in the Footnote3 system, which is at least much more standardized across Wikipedia even if I don't personally consider it superior. Bryan 06:32, 18 June 2006 (UTC)

The main reason I reverted was because some work had gone into seperating the primary and secondary references and cite.php mashed them together. I will look at changing to a more common system. Thanks for the note. Marskell 09:47, 18 June 2006 (UTC)

I suspected that might be the case. I don't see much purpose in separating primary and secondary references like that myself, but there are plenty of other articles in need of reference work so I'm not going to worry about it. :) Bryan 05:12, 19 June 2006 (UTC)

[edit] Suggestion

Isn't Phosphorus a required element for life, even in its simplest form? DNA or RNA couldn't exist without their Phosphorus backbone. Quote from Wikipedia (topic "Phosphorus"): "Phosphorus is a key element in all known forms of life. Inorganic phosphorus in the form of the phosphate PO43- plays a major role in biological molecules such as DNA and RNA where it forms part of the structural backbone of these molecules." http://en.wikipedia.org/wiki/Phosphorus Hugo Dufort 04:55, 28 September 2006 (UTC)

In the strictest sense, all we can say for sure is that life requires an energy source. Any other point speaks to "life as we know it" but not necessarily "life as we don't." Even "life requires water" may be parochial. That said, a sentence on phosphorus could be worked in, if you can find a non-wiki source. Marskell 08:19, 28 September 2006 (UTC)

[edit] Cellular life

The article is focused solely on cellular life, as far as I can see. I think we should talk about how we can mention the possibility of other forms of life in this article. All discussion can go here.