  Recent years have seen increasing interest in the characterization of
sub-Neptune sized planets because of their prevalence in the Galaxy, contrasted
with their absence in our solar system. HD 97658 is one of the brightest stars
hosting a planet of this kind, and we present the transmission spectrum of this
planet by combining four HST transits, twelve Spitzer/IRAC transits, and eight
MOST transits of this system. Our transmission spectrum has higher signal to
noise ratio than that from previous works, and the result suggests that the
slight increase in transit depth from wavelength 1.1 to 1.7 microns reported in
previous works on the transmission spectrum of this planet is likely
systematic. Nonetheless, our atmospheric modeling results are not conclusive as
no model provides an excellent match to our data. Nonetheless we find that
atmospheres with high C/O ratios (C/O >~ 0.8) and metallicities of >~ 100x
solar metallicity are favored. We combine the mid-transit times from all the
new Spitzer and MOST observations and obtain an updated orbital period of
P=9.489295 +/- 0.000005 d, with a best-fit transit time center at T_0 =
2456361.80690 +/- 0.00038 (BJD). No transit timing variations are found in this
system. We also present new measurements of the stellar rotation period (34 +/-
2 d) and stellar activity cycle (9.6 yr) of the host star HD 97658. Finally, we
calculate and rank the Transmission Spectroscopy Metric of all confirmed
planets cooler than 1000 K and with sizes between 1 and 4 R_Earth. We find that
at least a third of small planets cooler than 1000 K can be well characterized
using JWST, and of those, HD 97658b is ranked fifth, meaning it remains a
high-priority target for atmospheric characterization.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics