K2-140b and K2-180b - Characterization of a hot Jupiter and a mini-Neptune from the K2 mission

Abstract

We report the independent discovery and characterization of two K2 planets: K2-180b, a mini-Neptune-size planet in an 8.9-day orbit transiting a V = 12.6 mag, metal-poor ([Fe/H] =$-0.65\pm0.10$) K2V star in K2 campaign 5; K2-140b, a transiting hot Jupiter in a 6.6-day orbit around a V = 12.6 mag G6V ([Fe/H] = $+0.10\pm0.10$) star in K2 campaign 10. Our results are based on K2 time-series photometry combined with high-spatial resolution imaging and high-precision radial velocity measurements. We present the first mass measurement of K2-180b. K2-180b has a mass of $M_\mathrm{p}=11.3\pm1.9$ ${M_{\oplus}}$ and a radius of $R_\mathrm{p}=2.2\pm0.1$ ${R_{\oplus}}$ , yielding a mean density of $\rho_\mathrm{p}=5.6\pm1.9\,g\,cm^{-3}$, suggesting a rock composition. Given its radius, K2-180b is above the region of the so-called `planetary radius gap'. K2-180b is in addition not only one of the densest mini-Neptune-size planets, but also one of the few mini-Neptune-size planets known to transit a metal-poor star. We also constrain the planetary and orbital parameters of K2-140b and show that, given the currently available Doppler measurements, the eccentricity is consistent with zero, contrary to the results of a previous study.