How Habiable is Kepler-61B?

EXOPLANET CHARACTERIZATION BY PROXY: A TRANSITING 2.15 R ⊕ PLANET NEAR THE HABITABLE ZONE OF THE LATE K DWARF KEPLER-61

Authors:

1. Sarah Ballard (a,k)
2. David Charbonneau (a)
3. Francois Fressin (a)
4. Guillermo Torres (a)
5. Jonathan Irwin (a)
6. Jean-Michel Desert (b)
7. Elisabeth Newton (b)
8. Andrew W. Mann (c)
9. David R. Ciardi (d)
10. Justin R. Crepp (b,e)
11. Christopher E. Henze (f)
12. Stephen T. Bryson (f)
13. Steven B. Howell (f)
14. Elliott P. Horch (g)
15. Mark E. Everett (h)
16. Avi Shporer (b,i,j)

Affiliations:

a. University of Washington, Seattle, WA 98195, USA

b. California Institute of Technology, Pasadena, CA 91125, USA

c. Institute for Astronomy, University of Hawai'i, Honolulu, HI 96822, USA

d. NASA Exoplanet Science Institute/Caltech, Pasadena, CA 91125, USA

e. Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

f. NASA Ames Research Center, Moffett Field, CA 94035, USA

g. Southern Connecticut State University, New Haven, CT 06515, USA

h. National Optical Astronomy Observatory, Tucson, AZ 85719, USA

i. Las Cumbres Observatory Global Telescope Network, Santa Barbara, CA 93117, USA

j. Department of Physics, University of California, Santa Barbara, CA 93106, USA

k. NASA Carl Sagan Fellow.

Abstract:

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.