Regarding the Accretion of 2003 VB12 (Sedna) and Like Bodies in Distant   Heliocentric Orbits
  Recently, Brown et al. (2004) reported the exciting discovery of an ~800 km radius object, (90377) Sedna, on a distant, eccentric orbit centered at ~490 AU from the Sun. Here we undertake a first look exploring the feasibility of accreting this object and its possible cohorts between 75 AU (Sedna's perihelion distance) and 500 AU (Sedna's semi-major axis distance) from the Sun. We find such accretion possible in a small fraction of the age of the solar system, if such objects were initially on nearly circular orbits in this region, and if the solar nebula extended outward to distances far beyond the Kuiper Belt. If Sedna did form in situ, it is likely to be accompanied by a cohort of other large bodies in this region of the solar system. 
S.A. Stern