Next day,
Alex already came back to earth because as you know the danger could appear on earth anytime and because of that he did not want to leave the earth at this critical moment. First of all he predicted most likely the world tower would try to attack everyone on that planet which wasn't that powerful and the warrior also wasn't that powerful to protect themselves.
Naturally compared to any of those other planets they just recently came into contact with the world tower and because of that they also did not have that much strong fighter among them. Naturally the world tower will use that as their advantage and they will try to destroy the earth so that it could absorb the earth as quickly as it can.