Combining Kepler and HARPS Occurrence Rates to Infer the Super-Earth Period-Mass-Radius Distribution

Details Combining Kepler and HARPS Occurrence Rates to Infer the Super-Earth Period-Mass-Radius Distribution Combining Kepler and HARPS Occurrence Rates to Infer the Super-Earth Period-Mass-Radius Distribution

Sep 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011ess.....2.2703w&link_type=abstract

American Astronomical Society, ESS meeting #2, #27.03

Astronomy and Astrophysics

Astronomy

Scientific paper

The ongoing High Accuracy Radial velocity Planet Search (HARPS) has found that 30-50% of FGK stars in the solar neighborhood host planets with Mpl ≤ MNeptune in orbits of P ≤ 50 days. At first glance, this high overall occurrence rate seems at best to be marginally consistent with the planet frequency measured during Q0-Q2 of the Kepler Mission, whose 1235 detected planetary candidates imply that 15% of main sequence dwarfs harbor a short-period planet with Rpl < 4 R⊕. A rigorous comparison between the two surveys is difficult, however, as they observe different stellar populations, measure different planetary physical properties, and are subject to radically different sampling plans. Here we report the results of a Monte Carlo study which seeks to partially overcome this discrepancy by identifying plausible planetary population distributions that jointly conform to the results of the two surveys. 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 frocky = 90% at M = M⊕ to frocky = 10% at M = MNeptune. Our best fit population has a total occurrence rate of 40% for 2 ≤ P ≤ 50 days and 1 ≤ M ≤ 17 M⊕, and is characterized by simple power-law indices of the form N(M)dM MαdM and N(P)dP PβdP with α = -1.0 and β = 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".

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