An ultra-short period rocky super-Earth with a secondary eclipse and a Neptune-like companion around K2-141

TL;DR

This paper characterizes an ultra-short period rocky super-Earth around K2-141, measuring its mass, radius, and possible surface properties, and also confirms a Neptune-like companion, providing insights into USP planet composition and formation.

Contribution

It presents the first detailed mass and density measurements of a USP planet with secondary eclipse detection, and confirms a Neptune-like outer planet in the system.

Findings

K2-141b is a rocky super-Earth with a radius of 1.51 R_⊕ and mass of 5.08 M_⊕.

Secondary eclipses and phase variations suggest possible surface thermal emission or high albedo.

The outer planet K2-141c is likely Neptune-like, but its density remains unconstrained.

Abstract

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 $1.51\pm0.05~R_\oplus$ and a mass of $5.08\pm0.41~M_\oplus$, 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 modeled 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.