Sedna was the home planet of the Siccati race. It was located in the Solar System. (PROSE: Sedna)
In Steven Taylor's native time period, there was a human city on Sedna. (AUDIO: The First Wave)
A song in the 41st century was known as "Just Arrived on Sedna". (PROSE: The Best Joke I Ever Told)
Behind the scenes
In the real world, Sedna is considered to be a dwarf planet, but has not been referred to as such in-universe.