I didn't read your link all the way through, but I suspect that bip300 could solve this problem by allowing other planets to branch off into their own independent blockchains but still permit sats to flow between the main earth blockchain and the other planet's blockchain. A layer 2 could be implemented on top of bip300 to allow off chain transactions like lightning, where the transaction between chains would use locked up escrow wallets so you can effectively "email" a UXTO without needing network-wide consensus, and the interplanetary inter-chain channels would only need to be settled occasionally