> If earth is visited by Aliens some day, we should assume they play on larger board than us. It would be laughable that we get visited by 9*9 players.

This only makes any sense if you're also confident that, should we visit an alien world some day, we would have updated the rules of Go to require a larger board.

If we can play on a 19x19 board for historical and sentimental reasons, aliens can play on a 9x9 board for historical and sentimental reasons.