Exoplanet Characterization by Proxy: A Transiting 2.15 R_⊕ Planet near the Habitable Zone of the Late K Dwarf Kepler-61
We present the validation and characterization of Kepler-61b: a 2.15 R_⊕ planet orbiting near the inner edge of the habitable zone of a low-mass star. Our characterization of the host star Kepler-61 is based upon a comparison with a set of spectroscopically similar stars with directly measured radii and temperatures. We apply a stellar prior drawn from the weighted mean of these properties, in tandem with the Kepler photometry, to infer a planetary radius for Kepler-61b of 2.15 ± 0.13 R_⊕ and an equilibrium temperature of 273 ± 13 K (given its period of 59.87756 ± 0.00020 days and assuming a planetary albedo of 0.3). The technique of leveraging the physical properties of nearby "proxy" stars allows for an independent check on stellar characterization via the traditional measurements with stellar spectra and evolutionary models. In this case, such a check had implications for the putative habitability of Kepler-61b: the planet is 10% warmer and larger than inferred from K-band spectral characterization. From the Kepler photometry, we estimate a stellar rotation period of 36 days, which implies a stellar age of >1 Gyr. We summarize the evidence for the planetary nature of the Kepler-61 transit signal, which we conclude is 30,000 times more likely to be due to a planet than a blend scenario. Finally, we discuss possible compositions for Kepler-61b with a comparison to theoretical models as well as to known exoplanets with similar radii and dynamically measured masses.
Additional Information© 2013 American Astronomical Society. Received 2012 September 5; accepted 2013 April 22; published 2013 July 30. We thank Perry Berlind and Mike Calkins at the Fred Lawrence Whipple Observatory for gathering the FAST spectra of Kepler-61 and GJ 380. We thank Courtney Dressing for applying the methodology of Dressing & Charbonneau (2013) to deduce the physical properties of Kepler-61 and sharing these values with us. We thank Philip Muirhead, Katherine Hamren, Everett Schlawin, Bárbara Rojas-Ayala, Kevin Covey, and James Lloyd for gathering, reducing, and sharing the TripleSpec K-band spectrum of Kepler-61. We thank the Spitzer team at the Infrared Processing and Analysis Center in Pasadena, California, and in particular Nancy Silbermann for scheduling the Spitzer observations of this program. This work was performed in part under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program. It was conducted with 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. This work is also based on observations made with Kepler, which was competitively selected as the tenth 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 their time to make this Mission a success. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.
Published - 0004-637X_773_2_98.pdf
Submitted - 1304.6726v1.pdf