All someone needs is a character sheet to join.  You can have app sheets for whatever you want to be in this RP.  If you make a faction, you don't need to make a character sheet.  If you make a character, you don't need a faction sheet.  A spaceship sheet is totally optional, you can have a character with a personal spacecraft without a spaceship sheet.  

If you make a alien kind, if they are planet bound you don't need a faction sheet.  A character with that planet bound species is recommended, but not needed since some alien mindsets aren't really conductive to individual characters.  A character sheet for a collective minded group would be amusing regardless.

In some cases, you can be just a spaceship if it's a self-aware spaceship since that counts as a character.  This may also apply to a planet that's self aware (world minds) since that counts as a "aliens species".