But in the end what was the exact reason for the planet destroying itself? Shiller tried to say humans were responsible but Lakadema said something else and I am not sure what is the exact cause. And could you tell me about the other endings and if there is a "perfect" one when you have time? I am curious, but at the same too lazy to try to get them all

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it's never explicitly said what was the reason, but in my idea was human exploitation, a sort of warning about what we're doing with our own planet

About "perfect" ending not sure, all the endings are rather sweet I think, I made sure that no ending was too sad and despite everything and the dark moments, there was hope at the end of the story. My favorite is

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the one where they escape on a planet to hide and start a new life, or where they become space pirates (but good pirates helping the defenseless ships, etc).