Combining Transit and RV: A Synthesized Population Model

Abstract

We present a framework for estimating exoplanet occurrence rates by synthesizing constraints from radial velocity and transit surveys simultaneously. We employ approximate Bayesian computation and various mass-radius (M-R) relations to explore the population models describing these surveys, both separately and in a joint fit. Using this approach, we fit a planet distribution function of the form $d^{2}N/d\log Pd\log M\propto P^{\beta }{M}^{\alpha }$, with a break in the power law in mass at $M_b$, to planets orbiting FGK stars with periods $P=[25,200]$ days and masses $M=[2,50]M_{\oplus }$. We find that the M-R relation from Otegi et al. (2020), which lets rocky and volatile-rich populations overlap in mass, allows us to find a model that is consistent with both types of surveys. Our joint fit gives $M_b=21.6_{-3.2}^{+2.5}{M}_{\oplus }$ (errors reflect 68.3% credible interval). This is nearly a factor of three higher than the break from transit-only considerations and an M-R relation without such an overlap. The corresponding planet-star mass ratio break $q_b\sim 7\times 10^{-5}$ may be consistent with microlensing studies ($q_b\sim 6\times 10^{-5}-2\times 10^{-4}$). The joint fit also requires that a fraction of $F_{\text{rocky}}=0.63_{-0.04}^{+0.04}$ planets in the overlap region belong to the rocky population. Our results strongly suggest that future M-R relations should account for a mixture of distinct types of planets in order to describe the observed planet population.

Accepted for publication in The Astronomical Journal

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