K2-141 b: A 5-M${}_{\oplus }$ super-Earth transiting a K7 V star every 6.7 hours

We report on the discovery of K2-141 b (EPIC 246393474 b), an ultra-short-period super-Earth on a 6.7-hour orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planet’s existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. K2-141 b has a mass of $5.31 \pm 0.46 $ $M_{\oplus }$ and radius of $1.54_{-0.09}^{+0.10}$ $R_{\oplus }$, yielding a mean density of $8.00_{-1.45}^{+1.83}$ $\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ and suggesting a rocky-iron composition. Models indicate that iron cannot exceed $\sim$70 % of the total mass. With an orbital period of only 6.7 hours, K2-141 b is the shortest-period planet known to date with a precisely determined mass.