Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars III: A High Mass & Low Envelope Fraction for the Warm Neptune K2-55b

Abstract

K2-55b is a Neptune-sized planet orbiting a K7 dwarf with a radius of $0.715_{-0.040}^{+0.043}{R}_{\odot }$, a mass of $0.688\pm 0.069M_{\odot }$, and an effective temperature of $4300_{-100}^{+107}$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 $4.41_{-0.28}^{+0.32}{R}_{\oplus }$, an orbital period of $2.84927265_{-6.42\times 10^{-6}}^{+6.87\times 10^{-6}}$ 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 $43.13_{-5.80}^{+5.98}{M}_{\oplus }$, 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 ($M_{\mathrm{envelope}}=12\pm 3\%M_p$). 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.

arXiv comment22 pages, 7 figures, 4 tables. Accepted to the Astronomical Journal on June 25, 2018

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