Sedna is one of the last locations you unlock through the star chart. It's enemies are grineer at level 30-40, the exception being the arena game mode and any endless nodes that allow for a higher enemy scaling. It's connections are Pluto and Tier 4 Void.
Here is the star chart (basically)
Mars - Phobos - Void
Jupiter - Europa - Void
Neptune - Void
Pluto - Eris
Sedna - Void
There are 4 branches to the void, they do not intersect with each other.
And somewhere in the system is the Orokin Derelict, floating in space, requiring clan researched derelict keys to visit.