An ultra-short-period super-Earth with an extremely high density and an outer companion

We present the discovery and characterization of a new multi-planetary system around the Sun-like star K2-360 (EPIC 201595106). K2-360 was first identified in K2 photometry as the host of an ultra-short-period (USP) planet candidate with a period of 0.88 d. We obtained follow-up transit photometry, confirming the star as the host of the signal. High precision radial velocity measurements from HARPS and HARPS-N confirm the transiting USP planet and reveal the existence of an outer (non-transiting) planet with an orbital period of \(\sim\) $\sim$ 10 d. We measure a mass of \(7.67 \pm 0.75\) $7.67\pm 0.75$   \(M_{\oplus }\) $M_{\oplus }$ and a radius of \(1.57 \pm 0.08\) $1.57\pm 0.08$   \(R_{\oplus }\) $R_{\oplus }$ for the transiting planet, yielding a high mean density of \(11 \pm 2\) $11\pm 2$  g  \(\hbox {cm}^{-3}\) ${\text{cm}}^{-3}$ , making it the densest well-characterized USP super-Earth discovered to date. We measure a minimum mass of \(15.2 \pm 1.8\) $15.2\pm 1.8$   \(M_{\oplus }\) $M_{\oplus }$ for the outer planet, and explore a migration formation pathway via the von Zeipel–Kozai–Lidov (ZKL) mechanism and tidal dissipation.