I am interested in a character or race of humanoids similar to the green man in the tent in Wolfe's Book of the New Sun.

What qualities would the Sun need to have in order to support a race of human-like beings that rely mostly on photosynthesis for energy? Let's say they eat some sugar, some minerals, but subsist mostly on water and sunlight. They still move around (although could do so at a much slower pace if need be), manipulate things with their hands, and communicate with others in some way. Maybe they're slightly smaller than a modern human...call a 5'6" man tall for the species.

For our sun specifically, is there a stage in its life such that it would output enough energy for this to be possible? I remember a little bit of photosynthesis from school...plants get NADH from it or something. Is there a plausible way for photosynthesis to generate ATP? What would this sun mean for the environment and mammalian forms of life?

I guess this is a question about both the sun and photosynthesis.

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    $\begingroup$ Photosynthesis does produce ATP, as well as NADPH. $\endgroup$ – ApproachingDarknessFish May 27 '16 at 5:50
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    $\begingroup$ Very much related: Efficiency required of a photosynthetic system to support human level activities $\endgroup$ – user May 27 '16 at 7:22
  • $\begingroup$ See also: Hydrogenenic Photosynthesis: Strategies for animals $\endgroup$ – user May 27 '16 at 7:23
  • $\begingroup$ Thanks! I like those articles (the 2nd is perhaps too technical for me). However, I am also interested specifically in the behavior of the sun as the power plant here. If the sun currently provides 163 W/sq ft to the earth's surface, what if it suddenly produced 1630? Is this a feasible scenario within the lifetime of our sun, and would that also kill off most mammals? $\endgroup$ – zzu May 27 '16 at 15:43
  • $\begingroup$ human photosynthesis? You need to work backwards from that. Handwaving the sun would be the easy part. $\endgroup$ – Mazura Jan 30 '19 at 19:54

In a comment the OP asked...

If the sun currently provides 163 W/sq ft to the earth's surface, what if it suddenly produced 1630?

Bad things happen.

What you're asking for is a 10x increase in luminosity, the total energy the Earth receives from the Sun. As climate change has taught us, the Earth's climate is remarkably fragile to small increases in energy absorption and retention. Increasing it by 10x will do... bad things.

Mercury gets the 10x increase in solar power you're looking for. Even without an atmosphere to hold heat the temperature at the equator in daytime is hot enough to melt lead.

Venus gets a 2x increase in solar energy. It's temperature is complicated by its thick atmosphere, but let's just say it wouldn't be fun for the Earth to get even 2x more solar power.

Is this a feasible scenario within the lifetime of our sun?

Yes, for certain expansive values of "suddenly". Not in your lifetime. Not in humanity's lifetime. Give it another 6 or 7 billion years.

These two images sums up what will happen to the Sun over the next few billion years nicely.

enter image description here


enter image description here


And would that also kill off most mammals?

Oh, most certainly.

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Exactly the same. Photosynthesis uses chlorophyll, which is essentially a complex organic molecule which can be excited by the photons of the Sun. So, the photons of the Sun can switch it into a higher energy state, which can later be converted to chemical energy.

The optimal excitation energy of the chlorophyll is exactly the same as the solar photons have.

If the Sun radiated differently - for example, it was a red dwarf - then a different molecule would have been evolved for the task.

Whether animals can usefully use photosynthesis is another matter.

While near future genetic engineering technology could add chlorophyll cells to human skin and adapt our liver and metabolism, the real problem is the low amount of energy this generates. For example, consider how a cow gets its energy by eating grass compared to the time and energy required to create that grass through photosynthesis:

  1. Mean cow body size is around 600 kg.
  2. It eats around 20-30 kg grass per day.
  3. So, it eats the grass of its size in around a month.
  4. But, this grass requires months to grow and it has the surface of around 100 m^2, while the cow has only some m^2 of surface area.

So for an animal to use photosynthesis as its primary energy source:

  • either its energy requirements must be reduced by a factor of 200-300
  • or it requires a radically different body shape and lifestyle, such as a greatly-expanded surface area to maximize photosynthetic surface (this is what leaves on trees are for). Perhaps it might have an extremely large wing-like organ, which it could spread in the sun while sleeping.

I would also mention that cold-blooded animals require much less energy to survive.

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  • $\begingroup$ Even assuming the photosynthetic animals are ectothermic like reptiles or fish and have a high surface area, they still cannot exceed 30 centimeters in width (Source). That's the hard limit I'm afraid. $\endgroup$ – Anonymous May 27 '16 at 13:36
  • $\begingroup$ So there wouldn't be a situation where increased luminosity from our sun could make photosynthesis a more viable process for an active animal? Like say the "volume" of photons output by the sun suddenly increased 100-fold... is there a ratio of light energy to energy stored by a chlorophyll, and under what circumstances could that ratio be maximized? $\endgroup$ – zzu May 27 '16 at 15:36
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    $\begingroup$ @zzu It would cause an around 3-times absolute temperature growth on the Earth. With simpler words: the temperature would be around 1000 C. But anyways, it is so a soft earth science question. $\endgroup$ – Gray Sheep May 27 '16 at 15:52
  • $\begingroup$ @zzu There is another possibility: if the animal is photosynthetising some months long, and then it is active for a day. If its photosynthesis is somehow much more efficient, and it is cold-blooded, maybe a week of photosynth is enough to support a day of activity. But, so is it a biological question. $\endgroup$ – Gray Sheep May 27 '16 at 15:56
  • $\begingroup$ "The optimal excitation energy of the chlorophyll is exactly the same as the solar photons have." Not remotely true. Chlorophyll absorbs mostly in red and blue, which is why plants are green. Solar radiation peaks at about 500 nm (green). So chlorophyll is very badly matched to the solar spectrum. $\endgroup$ – WhatRoughBeast Jan 30 '19 at 16:45

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