GURPS Space: I want larger star sizes.

[doctorevilbrain]

Why wouldn't tidal locking make a planet uninhabitable?

[ericbsmith]

Originally Posted by doctorevilbrain

Why wouldn't tidal locking make a planet uninhabitable?

Under the right conditions an atmosphere and ocean can move the heat around enough that there can be a large habitable zone around the twilight zone of the planet.

OTOH, resonance locking - as discussed above, probably makes the planet far more habitable.

[Agemegos]

Originally Posted by doctorevilbrain

Why wouldn't tidal locking make a planet uninhabitable?

That's subject to on-going discussion.

Forty years ago we thought that the atmosphere would all freeze out on the dark side. Then we got computer modelling of the atmosphere that showed that even a fairly tenuous 0.1 atmosphere of CO₂ would transport enough heat to the dark side to keep the temperature there above the sublimation point of CO₂ (and therefore, perforce, above the boiling points of nitrogen and oxygen. More recently, modelling of a synchronously-rotating planet with a wet surface and a humid atmosphere has showing interesting results. It looks as though you'd get a large but not extremely cold cold patch on the dark side, with a lobster-shaped warm humid patch under the subsolar point. That means a lot of ice on the dark side, and the sunny side robbed of water. But return of water to the sunny side by glacial flow has not been modelled, and you might get melting of the ice base by geothermal heat.And the whole planet doesn't have to be all that hot for the dark side to melt, in which case water isn't a problem, but finding somewhere with enough light to grow plants that is cool enough for human habitation is. That's up in the air.

See Yongyun Hu & Jun Yang "
Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars" in Proceedings of the National Academy of Sciences January 14, 2014 111 (2) 629-634

On the other hand, I have just read a paper that argues that the amount of heating by tidal kneading that must occur in the tidal locking of any planet that ended cool enough to retain water would necessarily involve millions of years at much higher temperatures, and desiccation. Barnes, R. Mullins, Goldblatt, C., Meadows, V.S.,Kasting, J.F., & Heller, R. "Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating", Astrobiology. 2013 Mar; 13(3): 225–250

And last year I listened in on a conversation among some astrophysicist and astronomers who are friends of mine, in which they discussed the impact of the fact that M stars approach the main sequence along hte Hayashi track, which involved slowly getting much less luminous at roughly constant surface temperature. They agreed that that means that the circumstellar disc of an eventual M star has a history of millions of years at a much higher temperature than it ends up at, which means that the ice line is a long way out and what ends up being the inner system being strongly de-volatilised. They didn't discuss material migrating inwards afterwards.

Further to that, there is an argument that even red dwarfs that aren't flare stars now are former flare stars, and would have stripped any atmosphere that their Earth-like planet did have.

These sorts of speculations, however strongly founded they seem to be, have a long history of getting overthrown by observation. But for what it's worth there are three separate processes that would each tend to strip the air and water from any planet close enough to an M-type or late K-type star to become tidally locked in the Goldilocks zone.

Nevertheless, I have one such inhabited planet in my setting.

[doctorevilbrain]

Wouldn't the wind be warm enough to make it habitable around more than in the twilight zone?

[Agemegos]

Originally Posted by doctorevilbrain

Wouldn't the wind be warm enough to make it habitable around more than in the twilight zone?

That depends on how thick the atmosphere is, how wet the surface is, and the average temperature of the planet. That paper by Yongyun Hu & Jun Yang [2014] I cited above argues that oceanic transport of heat can certainly be sufficient to de-glaciate the dark side.

But it doesn't matter, except inasmuch as it allows water to return to the sunny side instead of being trapped as an ice cap. Plants need light, and you don't get an oxygen atmosphere without plants. To be habitable a planet needs an area that has liquid water and is illuminated with photosynthetically-active light, i.e. visible light.

And that reminds me of another difficulty for habitable planets orbiting M and late K stars: the bolometric correction for such stars is large, meaning that their light is much more efficient at warming than at illuminating the surface. Which means slow photosynthesis and a sluggish ecosystem at habitable temperatures. Such planets might take ten or twenty billion years to develop an oxygen atmosphere instead of 2.2 billion as on Earth; also, agricultural productivity is probably very low.

[Anthony]

Originally Posted by Agemegos

Which means slow photosynthesis and a sluggish ecosystem at habitable temperatures.

Well, slow-ish. Plants can't generally make use of the full intensity of sunlight anyway, so well illuminated areas would be fine, just shadowed areas would have issues (particularly since there also won't be a lot of scattered skylight).

[ericbsmith]

Originally Posted by Agemegos

But it doesn't matter, except inasmuch as it allows water to return to the sunny side instead of being trapped as an ice cap. Plants need light, and you don't get an oxygen atmosphere without plants. To be habitable a planet needs an area that has liquid water and is illuminated with photosynthetically-active light, i.e. visible light.

Well, Earth plants need sunlight. There is some chemosynthetic life on Earth - that is, life which gets it's energy by synthesizing chemicals, such as those around thermal vents - however those lifeforms are fairly limited on Earth due to photosynthesis generally being more readily available and energetic. On a world where large tracts of land are uninhabitable by photosynthesizing lifeforms the chemosynthesizer will certainly get a larger foothold and may stumble into better methods of chemical synthesizing than the ones on Earth use.

On a planet which is tidally locked you will also likely get much faster oceanic currents than on Earth, moving the warmer water to the night side and returning the cold water to the day side. This will result in many nutrients and lifeforms being swept into the dark side and travelling back, and could lead to an ocean with a robust ecology on the dark side.

[Agemegos]

Originally Posted by ericbsmith

Well, Earth plants need sunlight.

There are reasons to believe that the limits of the photosynthetically active range of photon energies are fundamental, dictated by the nature of chemical bonds in organic molecules. But like all such speculations they can be handwaved away if you want to.

There is some chemosynthetic life on Earth - that is, life which gets it's energy by synthesizing chemicals, such as those around thermal vents

Yes, but none of it produces waste oxygen. And even if it did there is vastly less power available in the sulphides coming out of geothermal vents than there is in the insolation of the planet. That's not a plausible route to an oxygen atmosphere.

[Johnny1A.2]

Originally Posted by ericbsmith

Any race advanced enough to place a world around a star is going to choose a star that will last 100 Billion or more years, not 1 Million years (and, yes, that is how long a ~0.4 Solar Mass M-Class star will last compared to 100 Solar Mass supergiant).

:lol:

"Any race advanced enough to build electronically-run semi-automated luxurious multiroom mansions powered by energy sources hundreds of miles away would of course build it where it could last, not perched in a river flood plain or on a sandbar in hurricane waters."

Which is eminently logical, except of course that our society does precisely that, and things like it, routinely. Technological power does not imply anything about priorities or choices.

I don't have any trouble believing that a race capable of terraforming a world around a B-type star might decide to do it, for whatever reasons of their own.

[Anthony]

Originally Posted by Agemegos

There are reasons to believe that the limits of the photosynthetically active range of photon energies are fundamental, dictated by the nature of chemical bonds in organic molecules. But like all such speculations they can be handwaved away if you want to.

There's also the problem of the water absorption spectrum. An infrared absorber won't do much good if it's immersed in water that stops IR.