Sensitivity bias in the mass-radius distribution from Transit Timing Variations and Radial Velocity measurements

TL;DR

This study investigates biases in exoplanet mass-radius measurements from RV and TTV methods, revealing a sensitivity bias that affects the interpretation of planetary compositions and emphasizing the need for combined approaches.

Contribution

It demonstrates that RV and TTV methods have different sensitivity biases, influencing the observed mass-radius distribution and suggesting combined measurements improve understanding of exoplanet properties.

Findings

RV tends to find higher mass planets for a given size

TTV sensitivity is more uniform across different planets

Combined methods can mitigate bias in mass-radius estimates

Abstract

Motivated by recent discussions, both in private and in the literature, we use a Monte Carlo simulation of planetary systems to investigate sources of bias in determining the mass-radius distribution of exoplanets for the two primary techniques used to measure planetary masses---Radial Velocities (RVs) and Transit Timing Variations (TTVs). We assert that mass measurements derived from these two methods are comparably reliable---as the physics underlying their respective signals is well understood. Nevertheless, their sensitivity to planet mass varies with the properties of the planets themselves. We find that for a given planet size, the RV method tends to find planets with higher mass while the sensitivity of TTVs is more uniform. This ``sensitivity bias'' implies that a complete census of TTV systems is likely to yield a more robust estimate of the mass-radius distribution provided there are not important physical differences between planets near and far from mean-motion resonance. We discuss differences in the sensitivity of the two methods with orbital period and system architecture, which may compound the discrepancies between them (e.g., short period planets detectable by RVs may be more dense due to atmospheric loss). We advocate for continued mass measurements using both approaches as a means both to measure the masses of more planets and to identify potential differences in planet structure that may result from their dynamical and environmental histories.