Two Small Temperate Planets Transiting Nearby M Dwarfs in K2 Campaigns 0 and 1

Creators: Schlieder, Joshua E.; Crossfield, Ian J. M.; Petigura, Erik A.; Howard, Andrew W.; Aller, Kimberly M.; Sinukoff, Evan; Isaacson, Howard T.; Fulton, Benjamin J.; Ciardi, David R.; Bonnefoy, Mickaël; Ziegler, Carl; Morton, Timothy D.; Lépine, Sébastien; Obermeier, Christian; Liu, Michael C.; Bailey, Vanessa P.; Baranec, Christoph; Beichman, Charles A.; Defrère, Denis; Henning, Thomas; Hinz, Philip; Law, Nicholas M.; Riddle, Reed; Skemer, Andrew

Abstract

The prime Kepler mission revealed that small planets (<4 R_⊕) are common, especially around low-mass M dwarfs. K2, the repurposed Kepler mission, continues this exploration of small planets around small stars. Here we combine K2 photometry with spectroscopy, adaptive optics imaging, and archival survey images to analyze two small planets orbiting the nearby field-age M dwarfs, K2-26 (EPIC 202083828) and K2-9. K2-26 is an M 1.0 ± 0.5 dwarf at 93 ± 7 pc from K2 Campaign 0. We validate its planet with a day period of 14.5665 and estimate a radius of 2.67_(-0.42)^(+0.46)R_⊕. K2-9 is an M2.5 ± 0.5 dwarf at 110 ± 12 pc from K2 Campaign 1. K2-9b was first identified by Montet et al.; here we present spectra and adaptive optics imaging of the host star and independently validate and characterize the planet. Our analyses indicate K2-9b is a 2.25_(-0.96)^(+0.53)R_⊕ planet with a 18.4498 day period. K2-26b exhibits a transit duration that is too long to be consistent with a circular orbit given its measured stellar radius. Thus, the long transits are likely due to the photoeccentric effect and our transit fits hint at an eccentric orbit. Both planets receive low incident flux from their host stars and have estimated equilibrium temperatures <500 K. K2-9b may receive approximately Earth-like insolation. However, its host star exhibits strong GALEX UV emission which could affect any atmosphere it harbors. K2-26b and K2-9b are representatives of a poorly studied class of small planets with cool temperatures that have radii intermediate to Earth and Neptune. Future study of these systems can provide key insight into trends in bulk composition and atmospheric properties at the transition from silicate dominated to volatile rich bodies.

Additional Information

© 2016 American Astronomical Society. Received 2015 October 12; accepted 2015 December 28; published 2016 February 9. We thank the referee for their prompt, constructive report that has improved the quality of this manuscript. We thank the LBTI/LMIRcam instrument team for providing support during LBT observations. J.E.S. thanks Tom Greene and Mike Werner for helpful discussions. The research of J.E.S. was supported by an appointment to the NASA Postdoctoral Program at NASA Ames Research Center, administered by Oak Ridge Associated Universities through a contract with NASA. Support for E.A.P. and A.J.S. was provided by the National Aeronautics and Space Administration through Hubble Fellowship grant HST-HF2-51349 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. A.W.H. acknowledges NASA grant No. NNX12AJ23G and S.L. acknowledges NSF grant No. AST 09-08419. C.A.B. is grateful to Davy Kirkpatrick for his assistance with planning and reduction of the Palomar Double Spectrograph observations. The Large Binocular Telescope Interferometer is funded by NASA as part of its Exoplanet Exploration program. LMIRcam is funded by the National Science Foundation through grant NSF AST-0705296. The Robo-AO system was developed by collaborating partner institutions, the California Institute of Technology and the Inter-University Centre for Astronomy and Astrophysics, and supported by the National Science Foundation under Grant Nos. AST-0906060, AST-0960343, and AST-1207891, the Mt. Cuba Astronomical Foundation, and by a gift from Samuel Oschin. C.B. acknowledges support from the Alfred P. Sloan Foundation. This work made use of the SIMBAD database (operated at CDS, Strasbourg, France); NASA's Astrophysics Data System Bibliographic Services; NASA's Exoplanet Archive and Infrared Science Archive; data products from the Two Micron All Sky Survey (2MASS); the APASS database; the SDSS-III project; the Digitized Sky Survey; and the Wide-Field Infrared Survey Explorer. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Kepler - The Kepler Mission, K2 - , IRTF - Infrared Telescope Facility (SpeX - ), NTT - New Techology Telescope (EFOSC2 - ), LBT - Large Binocular Telescope Observatory (LBTI/LMIRcam - ), Keck:I - KECK I Telescope (HIRES - ), Keck:II - KECK II Telescope (NIRC2 - ), PO:1.5 m - (Robo-AO - ), PO:5.0 m - (Double Spectrograph - ). Based in part on data obtained at the LBT. The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, La Silla Observatory, Chile during program ID 194.C-0443. IRAF is distributed by the National Optical Astronomy Observatories which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

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