  K2-55b is a Neptune-sized planet orbiting a K7 dwarf with a radius of
$0.715^{+0.043}_{-0.040}R_\odot$, a mass of $0.688\pm0.069 M_\odot$, and an
effective temperature of $4300^{+107}_{-100}$K. Having characterized the host
star using near-infrared spectra obtained at IRTF/SpeX, we observed a transit
of K2-55b with Spitzer/IRAC and confirmed the accuracy of the original K2
ephemeris for future follow-up transit observations. Performing a joint fit to
the Spitzer/IRAC and K2 photometry, we found a planet radius of
$4.41^{+0.32}_{-0.28} R_\oplus$, an orbital period of
$2.84927265_{-6.42\times10^{-6}}^{+6.87\times10^{-6}}$ days, and an equilibrium
temperature of roughly 900K. We then measured the planet mass by acquiring
twelve radial velocity (RV) measurements of the system using HIRES on the 10m
Keck I Telescope. Our RV data set precisely constrains the mass of K2-55b to
$43.13^{+5.98}_{-5.80} M_\oplus$, indicating that K2-55b has a bulk density of
$2.8_{-0.6}^{+0.8}$ g cm$^{-3}$ and can be modeled as a rocky planet capped by
a modest H/He envelope ($M_{\rm envelope} = 12\pm3\% M_p$). K2-55b is denser
than most similarly sized planets, raising the question of whether the high
planetary bulk density of K2-55b could be attributed to the high metallicity of
K2-55. The absence of a substantial volatile envelope despite the large mass of
K2-55b poses a challenge to current theories of gas giant formation. We posit
that K2-55b may have escaped runaway accretion by migration, late formation, or
inefficient core accretion or that K2-55b was stripped of its envelope by a
late giant impact.
