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Chthonian planets are gas giants that orbit their stars so closely, the intense heat from the star strips off the gas giant's atmosphere, leaving behind a tail of gas. This turns the gas giant into what is essentially a very big comet. Could the gas in the tail be harvested?

I was thinking that large metal rings could float suspended within the gas giant's tail and that those rings could use magnetic fields to channel the stripped gas to a central point, allowing it to be harvested.

Would the hot gas in the gas giant's tail just be too hot to capture? Is there even enough gas in the tail to make such a harvesting operation worth it? Or would the metal rings just get cooked?

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  • $\begingroup$ You are collecting ionized gas. More than temperature you should worry about electrostatic forces pushing the charged nuclei apart. And to neutralize them you need a large influx of electrons... $\endgroup$ – L.Dutch - Reinstate Monica Jan 11 '18 at 6:33
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    $\begingroup$ Why would someone want to wait until the gas is dispersed and only then start collecting it? These processes are sort of opposite. $\endgroup$ – ZuOverture Jan 11 '18 at 8:33
  • $\begingroup$ @ZuOverture, getting something out of a deep gravity well is not cheap $\endgroup$ – L.Dutch - Reinstate Monica Jan 11 '18 at 10:58
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    $\begingroup$ @L.Dutch, gathering gas from exosphere (typical densities are way below 1g/km^3) may be even more expensive. Worse than if we now try to extract radioisotopes from seawater. $\endgroup$ – ZuOverture Jan 11 '18 at 15:34
  • $\begingroup$ We've harvested less than a truckload of rocks from space, by hand. Hard science about harvesting millions of tons of gas might be hard to get. $\endgroup$ – user25818 Jan 11 '18 at 17:14
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It is hypothetically possible, given sufficiently advanced technology (as is anything that doesn't violate the laws of physics).

However, it's unlikely to be practical without very specific/weird technological assumptions.

The mass loss from hot Jupiters can certainly be significant - it's supposed to be thousands of tons per second for HD 189733 b. This is atomic hydrogen, so it may not be well-ionized enough for magnetic collection to be efficient.

However, a civilization that could do stuff like this could mine volatiles from small, icy outer-system bodies (Kuiper belt objects, centaurs, main-belt comets/volatile-rich asteroids etc.)

About the only gas that isn't likely to be available on those or easily made from ingredients on them would be helium, and it would probably be easier to scoop from the atmosphere of an Uranus/Neptune type planet. (And realistically, a civilization that can do this has probably solved fusion power, so they probably can make helium from hydrogen, which is easily extracted from water ice.)

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Let's consider a real-world candidate, the planet Osiris. https://en.wikipedia.org/wiki/HD_209458_b

Under the heading of Exosphere, we are told that the tail is about 200,000 km long, or the diameter of the planet. Thus a collection system would need to be placed close to the planet and probably held in position by an active thrust system.

Also Osiris is losing 100 million kilograms of hydrogen per second. The volume of hydrogen leaving the planet is bringing heavier elements, such as oxygen, with it.

If we use the surface temperature of Osiris at 1100K as a rough guide for temperature of the collection system then a well engineered system should survive the heat. Since the collection system is located on the dark side of the planet it will be in the shade, so the speak.

Because the gas is dispersing into space, it should cool rapidly. But as the solar wind works on the escaping gas, it will gain velocity. The collection system will have to balance the force of gravity of Osiris and the thrust of the hot gas coming off the planet. Too close to the planet and the gas velocity is low and gravity is high. Too far from the planet and the gravity is low, gas velocity is high, but gas density is low and thus thrust and gas collection is low.

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