Which Radial Velocity Exoplanets Have Undetected Outer Companions?
Abstract
The observed radial velocity (RV) eccentricity distribution for extrasolar planets in singleplanet systems shows that a significant fraction of planets are eccentric (e > 0.1). However, an RV planet's eccentricity, which comes from the Keplerian fitting, can be biased by low signaltonoise ratio and poor sampling. Here, we investigate the effects on 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 bias of known RV planets' eccentricities produced by undetected zeroeccentricity wideseparation companions and show that this effect alone cannot explain the observed distribution. We then modify the simulations to consist of two populations, one of zeroeccentricity planets in doubleplanet 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% zeroeccentricity double planets and 55% single eccentric planets. Assuming that our two simulated populations of planets are a good approximation for the true RV population, matching the observed distribution allows us to determine the probability that a known RV planet's orbital eccentricity has been biased by an undetected wideseparation companion. Averaged over eccentricity we calculate this probability to be ~4%, suggesting that a small fraction of systems may have a yet to be discovered outer companion. Our simulations show that moderately eccentric planets, with 0.1 < e < 0.3 and 0.1 < e < 0.2, have a ~13% and ~19% probability, respectively, of having an undetected outer companion. We encourage both highcontrast direct imaging and RV followup surveys of known RV planets with moderate eccentricities to test our predictions and look for previously undetected outer companions.
 Publication:

The Astrophysical Journal
 Pub Date:
 September 2009
 DOI:
 10.1088/0004637X/702/1/716
 arXiv:
 arXiv:0907.0020
 Bibcode:
 2009ApJ...702..716R
 Keywords:

 planetary systems;
 techniques: radial velocities;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 23 pages (12 text, 2 tables, 9 figures). Accepted to the Astrophysical Journal 30 June 2009