Ultra-short period ( USP ) planets are a class of low mass planets with periods shorter than one day .
Their origin is still unknown , with photo-evaporation of mini-Neptunes and in-situ formation being the most credited hypotheses .
Formation scenarios differ radically in the predicted composition of USP planets , it is therefore extremely important to increase the still limited sample of USP planets with precise and accurate mass and density measurements .
We report here the characterization of an USP planet with a period of 0.28 days around K2-141 ( EPIC 246393474 ) , and the validation of an outer planet with a period of 7.7 days in a grazing transit configuration .
We derived the radii of the planets from the K2 light curve and used high-precision radial velocities gathered with the HARPS-N spectrograph for mass measurements .
For K2-141b we thus inferred a radius of 1.51 \pm 0.05 R _ { \oplus } and a mass of 5.08 \pm 0.41 M _ { \oplus } , consistent with a rocky composition and lack of a thick atmosphere .
K2-141c is likely a Neptune-like planet , although due to the grazing transits and the non-detection in the RV dataset , we were not able to put a strong constraint on its density .
We also report the detection of secondary eclipses and phase curve variations for K2-141b .
The phase variation can be modelled either by a planet with a geometric albedo of 0.30 \pm 0.06 in the Kepler bandpass , or by thermal emission from the surface of the planet at \sim 3000K .
Only follow-up observations at longer wavelengths will allow us to distinguish between these two scenarios .