Very intelligent people, the aliens quickly will conclude, after they will hear the heavenly message:

a) IF we had only one red haired people on the planet, he would know that, because he doesn't see another red haired people in central square and because he knows there's at least one red haired people there. And so, he would board, just on 1st day;

b) IF we had two red haired people on the planet, each of them would wait the other would board on 1st day. That wouldn't happen and both of them would conclude: "If the red haired people didn't board it's because he's seeing another red haired people. That's me". And both of them would board on 2nd day;

c) IF we had three red haired people on the planet, each of them would wait the other two would board on the 2nd day. That wouldn't happen and all of them would conclude: "If both of them didn't board it's because they're seeing another red haired people. That's me". And all of them would board on 3rd day;

d) And so on. On 6th day, six red haired people would board.

***

So, in blond people sight, the six red haired people will board on 6th day.

And, in red haired people sight, the five red haired people that each of them sees will board on 5th day. BUT they don't board. Each of them soon will conclude: "If they don't board it's because they're seeing another red haired people. That's me". And all of them will board on 6th day.

And so, the ship won't come back again.