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How would one determine the Habitable Zone for planets that might have a methane based life-form rather than water? Titan, for example. In the equations I've looked at, I haven't found one conveniently labeled for water / solvent temperature. Methane melts at about 90K and boils at about 110K, so an average of 100K turns out to be a nice round number.

Would I just say that I need 31% of the stellar flux (100 / 320), and thus about 9 times the distance?

I guess it would depend on whether methane-based life is even possible, and what parameters you think it needs. But I suppose you could find the distance where the black body temperature is 100K and call that the methane habitable zone.

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A liquid range of 20 K sounds awfully narrow. A world would need very low seasonal and diurnal temperature variation for the methane to not regularly freeze or boil.

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Possibly relevant:

Titan under a red giant sun: A new kind of "habitable" moon

Proposes a water-ammonia ecosystem at 200K with methane as greenhouse gas.

Titan under a red dwarf star and as a rogue planet: requirements for liquid methane(pay wall)

Yes, it seems like it. However, mixed with ammonia and a splash of water, you might get to a 40 degree span. Maybe the atmosphere or being 20 AU from the Sun can help moderate the temp.

The idea does come from some articles about Titan. Such life forms might be rather delicate, or require an ammonia - methane solution to allow the cellular mechanisms to work.

Slight side track, if I may...

Are you thinking intelligent methane based life?

I think the best bets for moderating temperature variation would be small axial tilt (to reduce seasonal variation), fast rotation (to reduce variation during the day/night cycle) and a dense atmosphere (to increase heat retention and thus decrease temperature drop during night/winter).

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Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"

Well, as far as we know today, if life exists it will eventually become sapient. That is with an n=1, however.

But, right now I'm just thinking about what life would be like if it would be possible at that temperature. If methane is the universal solvent in that biosphere, there are some challenges as it is non-polar. Water being a polar molecule is more able to solvate proteins and other charged molecules. The methane may need to be a solution of methane and chloro-methane to improve solubility.

You still have access to CHNO with methane, ammonia and water. Energy can be extracted from acetylene, perhaps stored as a olefin polymer, reduced to ethylene. Ammonium hydrate could be a proton source for that. Sulfur and chlorine chemistry may be expressed more. You can still have carbon chains for macromolecules, but anything like DNA may have to be individually smaller or shorter. Maybe substituting aluminum for calcium, so that critters have berlinite bones rather than hydroxyapatite.

I think that musing about a Titan situation would be more plausible than life developing in a gas giant atmosphere. As was mentioned elsewhere, having a "world ocean" cuts down on the chances of life since there is no means of concentrating base chemicals for life to spontaneously arise. I think that would apply to a gaseous ocean as well as a liquid one. On Titan, we have seen the lakes and streams.

I recently read Uller Uprising, by H. Beam Piper. Between the novel and the journal articles I've been thinking of non-standard life forms.



I agree. It's a very narrow range. Freezing point depression and boiling point elevation only get you so far. The lakes freeze from the bottom up, though. That makes them lose heat quicker, but the liquid phase is accessible longer.

Sorry, I don't know how I missed this before.

Nine times the distances gives 1/81 times the insolation: about 1.23%.

A black body in space reaches equilibrium where the insolation is equal to the thermal radiation, and according to the Stefan-Boltzman Law thermal radiation is proportional to the fourth power of absolute temperature. The insolation of Earth maintains a surface temperature here that averages 290 K, so ignoring albedo and greenhouse effects you want an insolation that is (100/290)4 times Earth's. That's 0.01414 times as the solar flux, and arrives at a distance 8.4 times as far.

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