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Let's say it's sometime in the future, and humanity has expanded outwards into the solar system, and we've genetically engineered some really big plants that can for some unimportant reason, survive in the vacuum of space. These plants float around and preform photosynthesis, much like any other plant, absorbing sunlight and pumping oxygen out into the void. which they can do despite the complete and utter lack of any other usable atmospheric gasses or livable conditions, Because Reasons.

What happens to this oxygen? is it possible for this to lead to a breathable (if extremely thin) atmosphere in space? Would the oxygen pool up around celestial bodies, or the plants themselves? would any atmosphere get swept away by solar wind/the motion of the solar system as the galaxy rotates? Would this solve any of the other problems of floating exposed through space besides air? (radiation, freezing, etc)

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  • $\begingroup$ So just FYI, photosynthesis doesn't just manufacture pure oxygen straight from the aether. Carbon Dioxide and water are chemically transformed into a carbohydrate and the leftover oxygen is expelled. Photosynthesis is impressive, but definitely not miraculous. $\endgroup$ – MozerShmozer Jan 25 '17 at 19:30
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    $\begingroup$ Where is all the oxygen going to come from? There's simply not enough elemental oxygen in the solar system to fill a sphere with a radius of 200 million kilometres (or thereabouts), not by many orders of magnitude. In fact, there's not enough mass of any kind -- a back-of-the-envelope calculation suggests a sphere of gas at the density of Earth's sea-level atmosphere and a radius of 200,000,000km would mass about 10,000 times as much as the whole solar system. $\endgroup$ – Mike Scott Jan 25 '17 at 19:36
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    $\begingroup$ You might ask why the inner solar system isn't filled with hydrogen at around Earth atmospheric pressure. After all, there's a big clump of it right in the middle :-) $\endgroup$ – jamesqf Jan 25 '17 at 22:43
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I think you should probably look into something called an Ederworld or gravitational space balloons. The TLDR; is that once you get above a certain size, you can't get any bigger without shrinking or being a star.

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  • $\begingroup$ Even more fun, all that oxygen collapses into the sun hurtling it into the oxygen burning phase, which is soon followed by the silicon burning phase, which is quickly followed by a supernova. These plants sound like an awesome weapon. Maybe the Empire should genegineer these instead of Deathstars! :D $\endgroup$ – Tim Jan 25 '17 at 20:15
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Plants don't just take in sunlight and turn it into oxygen; that's not what photosynthesis does. Photosynthesis uses the energy in sunlight to break up carbon dioxide into carbon and oxygen. The carbon is combined with other molecules to form sugars, which the plant uses to power its growth, and the oxygen is discarded. The point being that you need carbon dioxide to start with - it's not going to magically appear out of nowhere.

So no, no matter how big you make your plants, you're not going to have them pumping oxygen out into the void.

If somehow you did have some kind of energy-to-matter machine that could take in literally just sunlight and convert it into atoms of oxygen, you'd still never get a breathable atmosphere in the inner solar system.

The gas would be released by the machine and diffuse rapidly away. Some would be captured by planets and added to their atmospheres. Some would escape into interstellar space with the solar wind. Most would probably just sink into the sun and be forgotten.

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  • $\begingroup$ some of the oxygen is discarded. At night plants need to take oxygen from the air. $\endgroup$ – JDługosz Jan 25 '17 at 23:00

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