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

Abstract

Abstract 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 $$\sim$$ ∼ 10 d. We measure a mass of $$7.67 \pm 0.75$$ 7.67 ± 0.75 $$M_{\oplus }$$ M ⊕ and a radius of $$1.57 \pm 0.08$$ 1.57 ± 0.08 $$R_{\oplus }$$ R ⊕ for the transiting planet, yielding a high mean density of $$11 \pm 2$$ 11 ± 2 g $$\hbox {cm}^{-3}$$ 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 ± 1.8 $$M_{\oplus }$$ M ⊕ for the outer planet, and explore a migration formation pathway via the von Zeipel–Kozai–Lidov (ZKL) mechanism and tidal dissipation.