"

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 cycleI 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^2not 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.

 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.  

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.   
Luna, Earth's Moon has volcanoes, and it's accepted as geologically dead. The presence of volcanoes doesn't mean ACTIVE volcanoes. 
That's basically what I was trying to say, I guess I just never made it really clear.  

 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. 

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...  

Earth's geological activity
I am 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 for