Validation of Small Kepler Transiting Planet Candidates in Or Near the Habitable Zone

The Astronomical Journal, 154:264 (19pp), 2017 December https://doi.org/10.3847/1538-3881/aa984b © 2017. The American Astronomical Society. All rights reserved. Validation of Small Kepler Transiting Planet Candidates in or near the Habitable Zone Guillermo Torres1 , Stephen R. Kane2 , Jason F. Rowe3 , Natalie M. Batalha4, Christopher E. Henze4, David R. Ciardi5, Thomas Barclay6 , William J. Borucki4, Lars A. Buchhave7 , Justin R. Crepp8 , Mark E. Everett9 , Elliott P. Horch10,11,14 , Andrew W. Howard12 , Steve B. Howell4 , Howard T. Isaacson13 , Jon M. Jenkins4 , David W. Latham1 , Erik A. Petigura12,15 , and Elisa V. Quintana6 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA; [email protected] 2 Department of Earth Sciences, University of California, Riverside, CA 92521, USA 3 Department of Physics and Astronomy, Bishop’s University, 2600 College Street, Sherbrooke, QC, J1M 1Z7, Canada 4 NASA Ames Research Center, Moffett Boulevard, Moffett Field, CA 94035, USA 5 NASA Exoplanet Science Institute/Caltech, Pasadena, CA 91125, USA 6 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 7 Centre for Star and Planet Formation, Natural History Museum of Denmark & Niels Bohr Institute, University of Copenhagen, DK-1350 Copenhagen K, Denmark 8 Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA 9 National Optical Astronomy Observatory, Tucson, AZ 85719, USA 10 Department of Physics, Southern Connecticut State University, New Haven, CT 06515, USA 11 Lowell Observatory, 1400 W Mars Hill Road, Flagstaff, AZ 86001, USA 12 California Institute of Technology, 1200 E California Boulevard, Pasadena, CA 91125, USA 13 Astronomy Department, University of California, Berkeley, CA 94720, USA Received 2017 August 11; revised 2017 November 1; accepted 2017 November 1; published 2017 December 1 Abstract ’ ( ) A main goal of NASA s Kepler Mission is to establish the frequency of potentially habitable Earth-size planets hÅ . Relatively few such candidates identified by the mission can be confirmed to be rocky via dynamical measurement of their mass. Here we report an effort to validate 18 of them statistically using the BLENDER technique, by showing that the likelihood they are true planets is far greater than that of a false positive. Our analysis incorporates follow-up observations including high-resolution optical and near-infrared spectroscopy, high-resolution imaging, and information from the analysis of the flux centroids of the Kepler observations themselves. Although many of these candidates have been previously validated by others, the confidence levels reported typically ignore the possibility that the planet may transit a star different from the target along the same line of sight. If that were the case, a planet that appears small enough to be rocky may actually be considerably larger and therefore less interesting from the point of view of habitability. We take this into consideration here and are able to validate 15 of our candidates at a 99.73% (3σ) significance level or higher, and the other three at a slightly lower confidence. We characterize the GKM host stars using available ground-based observations and provide updated parameters for the planets, with sizes between 0.8 and 2.9 R⊕. Seven of them (KOI-0438.02, 0463.01, 2418.01, 2626.01, 3282.01, 4036.01, and 5856.01) have a better than 50% chance of being smaller than 2 R⊕ and being in the habitable zone of their host stars. Key words: methods: statistical – planetary systems – stars: individual (KOI-0172.02 = Kepler-69c ...) – techniques: photometric 1. Introduction Many of the Kepler stars that appear to host small planets in the HZ are faint or have other properties such as significant The occurrence rate of terrestrial-size planets within the rotation or chromospheric activity that make it very difficult to habitable zone (HZ) of their host stars, referred to as “eta obtain the high-precision radial-velocity measurements needed Earth,” or h , is one of the fundamental quantities that the Å for a dynamical confirmation of the planetary nature of the exoplanet community is focusing their efforts on. The vast candidate. KOIs with long orbital periods (P) are even more numbers of transiting planet candidates from the Kepler challenging as the radial-velocity amplitudes fall off as P-13, ( ) Mission Borucki 2016 are the primary source for these resulting in Doppler signals that are often of the order of calculations, and there have been many efforts to estimate the 1ms−1 or less, which is at the limit of the detection capabilities ( value of hÅ from those data see the recent examples of of present instrumentation and techniques. Dressing & Charbonneau 2013; Kopparapu 2013; Burke et al. Kane et al. (2016) recently published a catalog of transiting 2015; Dressing & Charbonneau 2015; Mulders et al. 2015, and HZ candidates from Kepler based on the best available set of references therein). A key aspect of determining the reliability planetary and stellar parameters available to them. Many of fi of these estimates is the con rmation of Kepler candidates these candidates are nominally smaller than 2 R⊕ and have (Kepler Objects of Interest, or KOIs), particularly for earlier- relatively long orbital periods up to several hundred days. type stars for which the orbital periods for the habitable zone These KOIs are therefore of great interest in connection with become increasingly longer and the data more prone to false efforts to establish hÅ, and yet most of them have remained positives (Burke et al. 2015; Coughlin et al. 2016). unconfirmed for one or more of the reasons mentioned above. An alternative to dynamical confirmation is statistical 14 Adjunct Astronomer. validation, in which the goal is to show that the likelihood 15 NASA Hubble Fellow. of a false positive is much smaller than that of a true planet. 1 The Astronomical Journal, 154:264 (19pp), 2017 December Torres et al. A number of the candidates presented by Kane et al. (2016) 1. Candidates in the conservative HZ with Rp 2 RÅ; have been validated by others and have subsequently received 2. Candidates in the optimistic HZ with Rp 2 RÅ; official Kepler planet designations. However, in most cases, 3. Candidates in the conservative HZ with any radius; those validation studies have only been concerned with 4. Candidates in the optimistic HZ with any radius. demonstrating the presence of a planet associated with the The present work began during the early stages of target but not necessarily orbiting it. In particular, in reporting a ( ) confidence level for the validation, they have usually not preparation of the catalog of Kane et al. 2016 with the selection of 19 candidates for validation from Categories 1 and accounted for the possibility that the planet may instead transit 2, which were ranked to be of high interest based on their small an unresolved star near the target, either physically bound to it, or a chance alignment (Lissauer et al. 2014; Morton et al. size and likelihood of being in the HZ. However, due in part to subsequent improvements in the stellar parameters (particularly 2016). Such situations can in fact be more common than the the stellar radii and temperatures) that led to revised planetary types of false positives normally considered in these validation studies, by one to three orders of magnitude (see, e.g., Fressin parameters, the KOIs considered for inclusion in the Kane et al. (2016) catalog evolved with time until its publication, and as a et al. 2013; Torres et al. 2015). If the planet orbits a different result not all of the targets we initially selected for validation star, the transit signal observed would not reflect the true size of ended up in the final version of the catalog. On the other hand, the planet. Instead, the true size could be considerably larger out of concern that some of the signals might be spurious, we (Ciardi et al. 2015; Furlan et al. 2017), possibly implying an icy had originally chosen to exclude candidates in the catalog with or gaseous composition rather than a rocky one. This would low or marginal signal-to-noise ratios (S/Ns) as represented by make the planet less interesting from the standpoint of ( ) ’ habitability and h . the Multiple Event Statistic MES listed on NASA s Å Exoplanet Archive.16 The MES measures the significance of The motivation for the present work is thus to examine each the observed transits in the detrended, whitened light curve of the most promising Kepler HZ candidates more closely and ( ) revisit the validations with attention to this issue, making use of Jenkins et al. 2002 . We rejected KOIs with MES values lower than about 10, based on the estimates from the Kepler data follow-up observations and other constraints not previously release current at the time (Q1–Q17 Data Release 24, or DR24; available. We use these observations to also provide updated Coughlin et al. 2016). The most recent and final data release parameters for the validated planets. (DR25; Thompson et al. 2017) did not alter that selection This paper is organized as follows. The target selection for except in the case of KOI-7235.01, which was accepted by this work is explained in Section 2, followed by a description Kane et al. (2016) but is now considered to be a false alarm. of the Kepler photometry we use. Section 4 presents the follow- We therefore dropped this candidate from the target list and up observations for the targets, which includes high-resolution retained the other 18. Six of these are in Category 1, three imaging, an analysis of the motion of the flux centroids, and additional ones are in Category 2, two more are in Category 3, high-resolution spectroscopy. Then, in Section 5, we describe our analysis of the spectroscopic material to determine the and one is in Category 4.

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