197 Candidates and 104 Validated Planets in K2's First Five Fields

Abstract

We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), 30 false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of R_P = 2.3 R_E, P=8.6 d, Tef = 5300 K, and Kp=12.7 mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4 R_E, Kp=9-13 mag). Of particular interest are 37 planets smaller than 2 R_E, 15 orbiting stars brighter than Kp=11.5, five receiving Earth-like irradiation levels, and several multi-planet systems -- including four planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15-30%, with rates substantially lower for small candidates (< 2 R_E) and larger for candidates with radii > 8 R_E and/or with P < 3 d. Extrapolation of the current planetary yield suggests that K2 will discover between 500-1000 planets in its planned four-year mission -- assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, is essential to maximize the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.

ApJS accepted. 66 pages, 12 figures, 10 tables, 64 new planets. Data uploaded to https://exofop.ipac.caltech.edu/k2/ . v2: Now note 3 known planet names, and corrected a citation. v3: arXiv refs now display correctly

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• Transit Shapes and Self Organising Maps as a Tool for Ranking Planetary Candidates: Application to Kepler and K2
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• K2-66b and K2-106b: Two extremely hot sub-Neptune-size planets with high densities
• Measuring the Galactic Distribution of Transiting Planets with WFIRST
• Disproval of the validated planets K2-78b, K2-82b, and K2-92b
• A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at Thirty Parsecs
• Validation of small Kepler transiting planet candidates in or near the habitable zone
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• Mass Constraints of the WASP-47 Planetary System from Radial Velocities
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• Light Curve Modulation of Low Mass Stars in K2. I. Identification of 508 Fast Rotators in the Solar Neighborhood