Low False Positive Rate of Kepler Candidates Estimated From A Combination Of Spitzer And Follow-Up Observations
NASA's Kepler mission has provided several thousand transiting planet candidates during the 4 yr of its nominal mission, yet only a small subset of these candidates have been confirmed as true planets. Therefore, the most fundamental question about these candidates is the fraction of bona fide planets. Estimating the rate of false positives of the overall Kepler sample is necessary to derive the planet occurrence rate. We present the results from two large observational campaigns that were conducted with the Spitzer Space Telescope during the the Kepler mission. These observations are dedicated to estimating the false positive rate (FPR) among the Kepler candidates. We select a sub-sample of 51 candidates, spanning wide ranges in stellar, orbital, and planetary parameter space, and we observe their transits with Spitzer at 4.5 μm. We use these observations to measures the candidate's transit depths and infrared magnitudes. An authentic planet produces an achromatic transit depth (neglecting the modest effect of limb darkening). Conversely a bandpass-dependent depth alerts us to the potential presence of a blending star that could be the source of the observed eclipse: a false positive scenario. For most of the candidates (85%), the transit depths measured with Kepler are consistent with the transit depths measured with Spitzer as expected for planetary objects, while we find that the most discrepant measurements are due to the presence of unresolved stars that dilute the photometry. The Spitzer constraints on their own yield FPRs between 5% and depending on the Kepler Objects of Interest. By considering the population of the Kepler field stars, and by combining follow-up observations (imaging) when available, we find that the overall FPR of our sample is low. The measured upper limit on the FPR of our sample is 8.8% at a confidence level of 3σ. This observational result, which uses the achromatic property of planetary transit signals that is not investigated by the Kepler observations, provides an independent indication that Kepler's FPR is low.
Additional Information© 2015. The American Astronomical Society. Received 26 June 2014, accepted for publication 7 February 2015; Published 30 April 2015. We thank the anonymous reviewer for the careful reading of our manuscript and the valuable comments. This work is based on observations made with Kepler, which was competitively selected as the 10th Discovery mission. Funding for this mission is provided by NASA's Science Mission Directorate. The authors would like to thank the many people who generously gave so much of their time to make this mission a success. This work is also based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. D.C. acknowledges support for this work from grants NNX09AB53G and NNX12AC77G, and G.T. acknowledges support from grants NNX12AC75G and NNX14AB83G, each from the NASA Kepler Mission Participating Scientist Program. D.C. acknowledges the support of a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. We would like to thank the Spitzer staff at IPAC and in particular Nancy Silbermann for scheduling the Spitzer observations of this program. J.-M.D. and S.B. acknowledge the Sagan Exoplanet Fellowship program supported by the National Aeronautics and Space Administration and administered by the NASA Exoplanet Science Institute (NExScI). We thank Samaya Nissanke for careful reading of the manuscript.
Published - 0004-637X_804_1_59.pdf
Submitted - 1503.03173v1.pdf