Couldn't that be handled client-wise? So you don't put to much stress on the server. Each client calculates its own current value and sends it to the server.
Generally I'd say it's best to avoid trusting clients, but we already have people mining out 10,000x10,000 block areas and crashing servers so I suppose trust doesn't matter so much. Shifting sealed space detection to the client isn't going to save much server load, if any. In that vein, the way to improve performance would be to have the server ONLY simulate planets with two or more players on them, and leave simulating planets with only one player to their client. Since planet generation is deterministic the server only needs to send the client the differences between the planet's pristine state and the current state, and the client only needs to send the changes they make to it. The hard part is shifting the simulation back to the server if another player joins, but that's not too much of a problem.
