Which Radial Velocity Exoplanets Have Undetected Outer Companions?

TL;DR

This study uses simulations to assess how undetected outer companions influence the eccentricity distribution of radial velocity exoplanets, finding that a significant fraction of moderate eccentricities may be due to unseen companions.

Contribution

The paper introduces a novel simulation approach combining two populations to explain observed eccentricity distributions and estimates the probability of undetected companions affecting specific exoplanets.

Findings

Moderately eccentric planets have a 13-19% chance of hosting undetected outer companions.

A mixture of 45% zero-eccentricity double-planet systems and 55% single eccentric planets best fits observations.

Undetected outer companions can bias eccentricity measurements but do not fully explain the observed distribution.

Abstract

(Abridged) The observed radial velocity (RV) eccentricity distribution for extrasolar planets in single-planet systems shows that a significant fraction of planets are eccentric ($e > 0.1$). Here we investigate the effects on an RV planet's eccentricity produced by undetected outer companions. We have carried out Monte Carlo simulations of mock RV data to understand this effect and predict its impact on the observed distribution. We first quantify the statistical effect of undetected outer companions and show that this alone cannot explain the observed distribution. We then modify the simulations to consist of two populations, one of zero-eccentricity planets in double-planet systems and the other of single planets drawn from an eccentric distribution. Our simulations show that a good fit to the observed distribution is obtained with 45% zero-eccentricity double-planets and 55% single eccentric planets. Matching the observed distribution allows us to determine the probability that a known RV planet's orbital eccentricity has been biased by an undetected wide-separation companion. Our simulations show that moderately-eccentric planets, with $0.1 < e < 0.3$ and $0.1 < e < 0.2$, have a $\sim 13%$ and $\sim 19%$ probability, respectively, of having an undetected outer companion. We encourage both high-contrast direct imaging and RV follow-up surveys of known RV planets with moderate eccentricities to test our predictions and look for previously undetected outer companions.