Combining Kepler and HARPS Occurrence Rates to Infer the   Period-Mass-Radius Distribution of Super-Earths/Sub-Neptunes
  The ongoing High Accuracy Radial velocity Planet Search (HARPS) has found that 30-50% of GK dwarfs in the solar neighborhood host planets with sub-Neptune masses in orbits of P < 50 days. At first glance, this overall occurrence rate seems inconsistent with the planet frequency measured during Q0-Q2 of the Kepler Mission, whose 1,235 detected planetary candidates imply that ~ 15% of main sequence dwarfs harbor short-period planets with R_pl < 4 R_Earth. A rigorous comparison between the two surveys is difficult, however, as they observe different stellar populations and measure different planetary properties. Here we report the results of a Monte Carlo study that can account for this discrepancy via plausible distributions of planetary compositions. We find that a population concurrently consisting of (1) dense silicate-iron planets and (2) low-density gas-dominated worlds provides a natural fit to the current data. In this scenario, the fraction of dense planets decreases with increasing mass, from f_rocky = 90% at M = 1 M_Earth to f_rocky = 10% at M = 17 M_Earth. Our best fit population has a total occurrence rate of 40% for 2 < P < 50 days and 1 < M < 17 M_Earth, and is characterized by simple power-law indices of the form N(M)dM ~ M^alpha dM and N(P)dP ~ P^beta dP with alpha = -1.0 and beta = 0.0. Our model population therefore contains four free parameters and is readily testable with future observations. Furthermore, our model's insistence that at least two distinct types of planets must exist in the survey data indicates that multiple formation mechanisms are at work to produce the population of planets commonly referred to as "super-Earths". 
A. Wolfgang and G. Laughlin
astro-ph.EP