THE SIGNATURE OF THE ICE LINE AND MODEST TYPE I MIGRATION IN THE OBSERVED EXOPLANET MASS-SEMIMAJOR AXIS DISTRIBUTION

Existing exoplanet radial velocity surveys are complete in the planetary mass-semimajor axis (M-p-a) plane over the range 0.1 AU a < 2.0 AU where M-p greater than or similar to 100M(circle plus). We marginalize over mass in this complete domain of parameter space, and demonstrate that the observed a distribution is inconsistent with models of planet formation that use the full Type I migration rate derived from a linear theory and that do not include the effect of the ice line on the disk surface density profile. However, the efficiency of Type I migration can be suppressed by both nonlinear feedback and the barriers introduced by local maxima in the disk pressure distribution, and we confirm that the synthesized Mp-a distribution is compatible with the observed data if we account for both retention of protoplanetary embryos near the ice line and an order-of-magnitude reduction in the efficiency of Type I migration. The validity of these assumptions can be checked because they also predict a population of short-period rocky planets with a range of masses comparable to that of the Earth as well as a desert in the Mp-a distribution centered around M-p similar to 30- 50 M-circle plus and a < 1 AU. We show that the expected desert in the Mp-a plane will be discernible by a radial velocity survey with 1 m s(-1) precision and n similar to 700 radial velocity observations of program stars.