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THE PREMISE:

Let's imagine a solar system with at least two rocky planets. One is Earth sized "A" (or smaller) and the second is a Super Giant "B".

Both are in the habitable zone of their sun. A is closer to the sun.

On planet A, life develops. The mass of B attracts most celestial bodies and avoid them to crash on A.

As time passes by the mass of B is getting greater and greater. Either it's starting to capture more gas (and might become an unstable Gas Giant) , or either its mass starts to have an impact on A's orbit.

In any case, the people living on A, a very developed intelligence, fear of a gravitational kick out of the system due to B's mass/change of orbit (if it becomes unstable due to the gas, I'm not sure what might be the best explanation yet).

Therefore the people of Planet A decide to do something before their planet gets kicked and life destroyed. As Planet B is still important for their survival (since it attracts celestial bodies), and they can't just leave their planet and find "a new one", they go to planet B, with the intention to reduce its mass.

THE QUESTION:

Given that a civilization has the technology and means to do so (unobtainium is okay in that scenario), can the mass of a Super Giant be reduced and how so ?

The idea is to have the people of A on planet B, for a long time, doing some sort of "terra forming" but in order to stop the increase of B's mass. Can they add more water ? Hollow the planet ? Change its core ?

Note that it is also important that life ends up developing on planet B and the people of A do not want to kill life on planet B but still be able to reduce the mass of the planet at the same time. (I also hope I am not mistaking "mass" with "density")

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    $\begingroup$ "can the mass of a Super Giant be reduced" and "the people of A do not want to destroy it as they reduce the mass of the planet". are contradictory statement. Please clarify. $\endgroup$
    – L.Dutch
    Mar 23, 2023 at 5:53
  • $\begingroup$ I made an edit! I was talking about life on the planet, not the planet itself. Sorry for the confusion I hope it's clearer $\endgroup$
    – Delkhii
    Mar 23, 2023 at 5:59
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    $\begingroup$ everything you do to significantly change the planet will seriously endanger any native life. you can't just reduce the mass of a planet and hope the animals and plants won't care. for one thing, reduced gravity means the atmosphere will get thinner. $\endgroup$
    – ths
    Mar 24, 2023 at 15:56
  • $\begingroup$ @ths I agree with that statement. Note that I added that life must develop eventually. From that I meant that maybe reducing the mass of the planet could cause a massive extinction of the life on the planet as it was (early signs of life) but life could grow once again? just like Earth went through massive extinctions itself. The people of planet A care about life on planet B but their survival is their priority. If their action caused a mass extinction, they could try to rebuild life with the new parameters of the mass reduced planet. Would that be possible with the thinner atmosphere? $\endgroup$
    – Delkhii
    Mar 24, 2023 at 16:32
  • $\begingroup$ What is "gaz", please? A misspelling of "gas" or some other substance? $\endgroup$ Mar 25, 2023 at 11:23

2 Answers 2

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The people on planet A don't really have any chance of reducing the size of planet B. The only way to do so would be to ship mass off of the planet and it simply would not be practical to ship enough material off planet. There efforts would be confounded by the continuous infall of material from space, the sheer bulk of an astronomical body like a planet and their grossly limited capabilities.

To make an impossible situation even worse, planet B is a super-giant so the sheer quantity of material that would need removing is even bigger. To cap it all due to the rocket equation and the size of planet Earth, it is extremely difficult to reach Earth orbit as it is. Depending on the exact parameters of the super-giant it is likely that it would not even be possible (or remotely practical) to leave its surface using chemical rockets.

Your only hope is some form of unobtainium "magic", but in that circumstance almost anything is possible.

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  • $\begingroup$ For the technology, I do not mind unobtainium to explain that the people from planet A have the means to make the journey from A to B several times without much difficulty. B needs to be a super giant to explain that: 1) it might gravitationally kick planet A from their orbit and the system 2) it protects planet A from celestial bodies (it's a ringed planet). I do not know what are the exact size parameters, but if B can be smaller to make everything above + the mass reducing possible, I wouldn't mind. I mainly wonder what might be the most plausible way to reduce mass other than mining. $\endgroup$
    – Delkhii
    Mar 24, 2023 at 16:54
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    $\begingroup$ I think you will have your work cut out to make this believable to the more scientifically literate. Mining will not reduce the mass of the planet unless what is mined is moved off of the planet. Getting most of the planet effectively "off of the planet" is not really a practical proposition in all honesty. $\endgroup$
    – Slarty
    Mar 25, 2023 at 13:32
  • $\begingroup$ Yes that's what I figured. I think I'll go for unobtianium unfortunately since the presence of people A on planet B is a crucial point story wise. I think I'll have to go with handwaving alien technology capable to make viable space elevators. Unobtianium will definitely be the material for the tethers towards geostationary stations where most of the mass would be brought and then sent away from the planet B's orbit. I know space elevators aren't feasible with our current materials, technology and Earth's parameters but I think this will have to do $\endgroup$
    – Delkhii
    Mar 26, 2023 at 2:03
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    $\begingroup$ OK, but what happens to the planetary material when its in orbit? It will need to be sent away somewhere, I wouldn't underestimate the volume requirements it wouldn't be anything like mining as we know it. At a million cubic km a day it would be a slow process. $\endgroup$
    – Slarty
    Mar 26, 2023 at 10:56
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    $\begingroup$ I think the real issue is scale and getting your head around the orders of magnitude thing. Total steel production on Earth is much less than 1 cubic kilometre per year. And that's a lot of stuff to be needing. What are they going to do with a million cubic km? Every day? $\endgroup$
    – Slarty
    Mar 28, 2023 at 13:07
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Mining

The only way to make it smaller is to mine the hell out of it.

Planet A sends self replicating robots to mine the planet and build space stations, Dyson satellites, ring worlds, ark ships, whatever.

enter image description here

This mass is gone from the planet and used to make useful stuff.

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  • $\begingroup$ Thanks, mining was my n°1 solution. But I was also wondering if that might result in a potential collapse of the crust ? I guess there should be "pillars" strong enough inside the mines to avoid such thing ? $\endgroup$
    – Delkhii
    Mar 23, 2023 at 6:15
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    $\begingroup$ Mind that the satellites have to be orbiting something else than the planet they are mined from in order to reduce its mass $\endgroup$
    – L.Dutch
    Mar 23, 2023 at 8:08
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    $\begingroup$ @Delkhii Why would you mine underground? Surface mining would be enough. Even gas mining would change the mass. $\endgroup$
    – Thorne
    Mar 24, 2023 at 0:11
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    $\begingroup$ @Delkhii Why, does mining now put life at risk on Earth? It's a hole in the ground. Life can move to the side. $\endgroup$
    – Thorne
    Mar 24, 2023 at 1:51
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    $\begingroup$ Any mass doesn't need to orbit the planet. You'd build habitats and place them much further out $\endgroup$
    – Thorne
    Mar 28, 2023 at 4:44

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