A Minimum-Mass Extrasolar Nebula
  By analogy with the minimum-mass solar nebula, we construct a surface-density profile using the orbits of the 26 precise-Doppler planets found in multiple planet systems: Sigma = 2200 grams per square centimeter (a/1 AU)^- beta, where a is the circumstellar radius, and beta = 2.0 plus or minus 0.5. The minimum-mass solar nebula is consistent with this model, but the uniform-alpha accretion disk model is not. In a nebula with beta > 2, the center of the disk is the likely cradle of planet formation. 
Marc J. Kuchner