OK, this was weird. Having finished Coruscant I went to Ord Mantell. When I was on the overworld the villains were extremely weak so apparently I was supposed to have done this earlier in the game. But when I went into the base the villains were at my level and the boss killed me five times before I finished to half-ass my way to victory. Then I go back to the space station, get attacked by five Sith and immediately die. I have no idea how I'm expected to survive that encounter that's forced upon me.
Ord Mantell is the starting world for the Smuggler and Trooper classes, just like Tython is for Knights and Consulars, hence all the weak enemies in the overworld. It's the same on the Imperial side too with Sith Warriors and Inquisitors staring on Korriban while Bounty Hunters and Imperial Agents start on Hutta.
In the course of the main class story you will occasionally get small missions like this between the "major" planet quests. You know the enemies are going to be properly levelled when you pass through an instance barrier.