If a person is born in space how can they become a citizen ?

Nobody is sending pregnant women into space, but a baby born in orbit around the earth would get a nationality based on that of its parent(s).

NB lots of countries do not have "ius solis". Being born on their territory does not give you any special rights

Just like being born in the middle of the ocean, outside the borders of any country.

from where he came from is the standard rule , same as a ship birth , parents are chinese going to america , so the child is chinese .

Go ask in Immigration. Or, Law & Ethics.

In US law, one does not have to be born inside the US to be a US citizen.

tOnly if they came down to Earth

Which would probably be dangerous for them having never been in the Force of Gravity

From the State of Constant falling to being on Solid Ground

 The Force Change could actually kill them

They would have to remain in their own Alien World

The same way(s) they can if they aren't born in space.

They follow the parents' citizenship.

Citizenship isn't granted based only on where you were born.  I was born in Europe, my mom was a naturalized US citizen and my dad was a regular US citizen.  I was granted citizenship the minute I was born.  I suspect if a baby were born in space, then they would be granted citizenship based on the citizenship status of their parents. 

They can´t........