[Space] Gliese 581

[Fred Brackin]

Originally Posted by Sunrunners_Fire

I'd wait for information on atmospheric composition before attempting to guess at any ecology. If it has measurable atmospheric oxygen, well ...

If you have oxygen some sort of ecology becomes overwhelmingly likely. You'd have already moved beyond speculating on "if" and moved on to the specifics of "how".

[dataweaver]

Originally Posted by Sunrunners_Fire

I'd wait for information on atmospheric composition before attempting to guess at any ecology. If it has measurable atmospheric oxygen, well ...

You'd be in for a long wait. The planets of Gliese 581 do not transit their star from the angle we've got on the system; so the usual (and, to date, only known) method of identifying chemical compositions of exosolar planetary atmospheres is unavailable in this case. We are unlikely to learn more about these planets (beyond refining the details that we already know) until such time as we can send a probe - which won't be any time soon.

Originally Posted by Sunrunners_Fire

I would also note that we have multiple types of infrared-specific photosynthetic bacteria (and plankton) in the here and now. One type of which we've done up a full DNA map / sequencing for the purpose of transferring that capability into more complex plants to increase photosynthesis efficiency. GMOs are so fun. :)

Yeah; I was thinking along these lines myself. A case could be made that these infrared-specific versions of chlorophyll are in the minority on Earth precisely because the solar distribution favors their higher-energy counterparts; if the same biochemical stock were to end up on a planet of a red dwarf, I'd expect the IR versions to fare better.

[gjc8]

Originally Posted by Fred Brackin

Yeah, it isn't about gross energy. It's the specific form of that energy.

The spectrum of a red dwarf peaks at a much lower place than the spectrum of a G2 like out sun. A greater proportion of the light is red at the bottom of the visual spectrum and a lower proportion are yellow to blue photons with higher energy than red photons.

These higher energy photons cause photosynthesis with greater efficiency. Photosynthesis works on a one photon at a time basis.

Color me unconvinced. Each photon will not produce as much energy, but you'll have a lot more photons available. As others have noted, there are species on earth that photosynthesize in the infrared. If your star produces light peaked at a different wavelength, you're going to evolve a different set of photosynthesis pigments. There MIGHT be biochemical difficulties in making effective pigments that absorb longer wavelengths, but there might not; their absence on Earth is easily explained by the spectrum of Sol.

Originally Posted by Fred Brackin

Also look at the big quote on this page in the post by strefanj.....

http://forums.sjgames.com/showthread.php?t=73515&page=2

.....the scientist there says that the reddish light is better at transmitting simple warmth than a spectrum like the one we're used to. He calculated insolation (more or less total solar energy received) at 60% of Earth.

That may be true. It's hardly an overwhelming problem. That puts the local insolation at the equator about that of the central US on Earth (though with equatorial temperatures). You'll end up with less biomass, perhaps, but it hardly rules out life.

Originally Posted by Fred Brackin

I am also quite concerned about clouds, both water vapor clouds and dust ones. That could significantly reduce the amount of light reaching the surface and make bad matters worse for Earth-like life.

I am also concerned about the density and composition. Greater than Earth mass but similar density likely means more vulcanism. That would mean more CO2 in the atmosphere and even more chance for a runaway greenhouse.
...
What was said in the link above about greater likelihood of retaining primordial atmosphere and less chance of losing atmosphere in late collisions (see articles related to formation of Earth's Moon) add to the greenhouse worries.

These are more serious concerns, especially with the potential to retain the primordial atmosphere.

I agree with you that the odds of life on Gliese 581g are low, at least for an Earth-like biosphere.

[dataweaver]

True; but they aren't necessarily so low that we need to resort to handwavium to get a reasonably habitable world for the purpose of Space roleplaying.

Note that if the world in question is terrestrial, its atmosphere is more likely to hug the surface, with pressure dropping off more rapidly than on Earth. As such, solar radiation won't have to penetrate as many miles (or km, if you prefer) to get to the surface. This is a mixed bag: on the one hand, this could potentially mitigate the effects of cloud cover in terms of photosynthesis; OTOH, that also provides less atmospheric shielding against solar flares. (OTGH, I suspect that the planet's magnetosphere is the primary defense against charged-particle radiation; and while the planet's slower rotation (assuming tidal locking) works against a strong magnetosphere, a significant iron core and the planet's larger mass would tend to work in favor of one. So even if the atmosphere is thicker at the surface (of which I'm not convinced; too many variables, not enough information), that doesn't have to mean a super-Venus.

[doctorevilbrain]

Dataweaver, when I clicked on the link, the print box on this computer popped up. This is a Library computer, so I have no control of what pops up on the screen. When I asked the Librarian, he couldn't figure out what to do. I printed out what was on the page and discovered that is a link that I want to click on, and I couldn't access it. When I typed in the loooong Internet address on the page on Google, I still couldn't access it, either with, or without the part where it says format=print. I don't have my own computer at home , so I don't have anyway to see what it says. I can't make the print box disappear without making what pops up disappear. Does anybody have any idea of what to do? I couldn't even access it when I typed in "Icarus Academic Press".

[dataweaver]

Which link are you referring to? I've posted at least two.

[lwcamp]

Originally Posted by Fred Brackin

If you have oxygen some sort of ecology becomes overwhelmingly likely. You'd have already moved beyond speculating on "if" and moved on to the specifics of "how".

There are possible abiological methods of producing oxygen - although in this case my favorite, UV photolysis of water and subsequent escape of the hydrogen to space, is unavailable.

Luke

[lwcamp]

Originally Posted by dataweaver

You'd be in for a long wait. The planets of Gliese 581 do not transit their star from the angle we've got on the system; so the usual (and, to date, only known) method of identifying chemical compositions of exosolar planetary atmospheres is unavailable in this case. We are unlikely to learn more about these planets (beyond refining the details that we already know) until such time as we can send a probe - which won't be any time soon.

A large space telescope with a coronagraph would enable spectral measurement of exoplanets and would be orders of magnitude cheaper than a probe to another solar system.

Luke

[lwcamp]

Originally Posted by gjc8

Color me unconvinced. Each photon will not produce as much energy, but you'll have a lot more photons available. As others have noted, there are species on earth that photosynthesize in the infrared. If your star produces light peaked at a different wavelength, you're going to evolve a different set of photosynthesis pigments. There MIGHT be biochemical difficulties in making effective pigments that absorb longer wavelengths, but there might not; their absence on Earth is easily explained by the spectrum of Sol.

Thermodynamics does impose an ultimate limit on what fraction of the energy can be harvested from the sunlight, because any physical process can be driven in reverse, and the absorption mechanisms can be thermally activated to cause emission. The ultimate efficiency depends on the temperature of the star's surface and the temperature at which your photopigment operates. For life on earth, operating at about 300 K and harvesting light from a 6000 K surface, the maximum theoretical efficiency is (6000 K - 300 K)/(6000 K) = 95%. You can see that life on earth has a long way to go before the physical limit is reached. For life around a red dwarf sun, with a temperature of 3000 K, the maximum efficiency drops to (3000 K - 300 K)/(3000 K) = 90%. Again, this limit is probably well above the efficiency of any actual photopigments that would have evolved.

The astute reader may have noticed that the above calculations for efficiency use the same formula as the efficiency of a Carnot cycle heat engine. This is because the underlying thermodynamic mechanisms - the exchange of energy and entropy - are the same in both cases.

Luke

[lwcamp]

Originally Posted by dataweaver

Note that if the world in question is terrestrial, its atmosphere is more likely to hug the surface, with pressure dropping off more rapidly than on Earth. As such, solar radiation won't have to penetrate as many miles (or km, if you prefer) to get to the surface. This is a mixed bag: on the one hand, this could potentially mitigate the effects of cloud cover in terms of photosynthesis; OTOH, that also provides less atmospheric shielding against solar flares.

For a given mass of atmosphere, higher gravity will make it hug the planet tighter, but will also make it denser. The two effects cancel out. The useful parameter for radiation penetration is the areal density - mass per area - which only depends on how much atmosphere there is over a given area and not how close the atmosphere is squished down by gravity.

Originally Posted by dataweaver

(OTGH, I suspect that the planet's magnetosphere is the primary defense against charged-particle radiation;

For earth, our atmosphere is quite effective at screening out charged particle radiation. without our magnetosphere, we would see very little increase in the amount of cosmic background radiation we receive. Note, for instance, the the particles which do get through our magnetosphere are those which are the most energetic, and it is just those particles that produce radiation showers that penetrate farthest.

Originally Posted by dataweaver

and while the planet's slower rotation (assuming tidal locking) works against a strong magnetosphere, a significant iron core and the planet's larger mass would tend to work in favor of one.

There is an interesting effect going on deep in the planet. When you heat iron, it gets molten and can create a global magnetic field. However, when you compress iron, it tends to solidify. On earth, the outer core is under a low enough pressure that it can remain molten, giving us our magnetosphere. As a planet gets larger however, its interior pressure rises from the weight of the overlying material, which will tend to solidify the entire core. Consequently, "super-earths" are expected to lack a magnetic field (although there well be interesting surprises from unexpected dynamics).

Luke