  We present the validation and characterization of Kepler-61b: a 2.15 R_Earth
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
the 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_Earth 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.
