K2-161b: a low-density super-Neptune on an eccentric orbit

Abstract

We report the discovery of EPIC 201498078b, which was first identified as a planetary candidate from Kepler K2 photometry of Campaign 14, and whose planetary nature and orbital parameters were then confirmed with precision radial velocities. EPIC 201498078b is half as massive as Saturn (M P = 0.179 ± 0.021 M J ), and has a radius of R P = 0.840 ± 0.011 R J , which translates into a bulk density of ρ P = 0.37 ± 0.05 g cm −3 . EPIC 201498078b transits its slightly evolved G-type host star (M = 1.105 ± 0.019 M , R = 1.669 ± 0.022 R ) every 11.63364 ± 0.00010 days and presents a significantly eccentric orbit (e = 0.420±0.034). We estimate a relatively short circularization timescale of 1.8 Gyr for the planet, but given the advanced age of the system we expect the planet to be engulfed by its evolving host star in ∼ 1 Gyr before the orbit circularizes. The low density of the planet coupled to the brightness of the host star (J = 9.4) makes this system one of the best candidates known to date in the super-Neptune regime for atmospheric characterization via transmission spectroscopy, and to further study the transition region between ice and gas giant planets.

WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration

“…WASP-107b is an extreme case even among the vastly diverse exoplanet interiors illustrated by the wide span in radius of the super-Neptunes, which highlights the great diversity of formation pathways within this population. WASP-107b, with its Jupiter-like composition, resembles K2-161b and Kelt-11b, which have large envelope mass fractions (>75%) associated with their low densities (Pepper et al 2017;Brahm et al 2019). Meanwhile, thermal evolution models predict for instance a Neptune-like structure for the 23 M ⊕ planet HAT-P-11b, with about 15% H/He by mass (Petigura et al 2017).…”

ACCESS: Tentative Detection of H2O in the Ground-based Optical Transmission Spectrum of the Low-density Hot Saturn HATS-5b

“…Using the methods outlined in Brahm et al (2018Brahm et al ( , 2019, we used all the available photometry for HATS-5, along with its spectra and Gaia parallaxes, in order to derive precise stellar properties. In particular, we obtain a precise estimate on the effective temperature of kg m −3 .…”

The Equilibrium Temperature of Planets on Eccentric Orbits: Time Scales and Averages

“…This suggestion has been followed by a number of authors (e.g. Brahm et al 2019;Schlecker et al 2020;Hobson et al 2021;Schanche et al 2022), but in most cases this is not appropriate because the ansatz inherent in Eqn. 5 neglects the thermal inertia of the planet.…”