A sub-Neptune transiting the young field star HD 18599 at 40 pc

TL;DR

This paper reports the discovery and validation of a sub-Neptune exoplanet orbiting a young, nearby star, using multiple observational methods, providing a valuable case for studying planet formation and evolution.

Contribution

First validation of a transiting sub-Neptune around a young star using multi-instrument data, enabling future detailed characterization.

Findings

Planet has a 4.13-day orbit and 2.7 Earth radii.

RV data sets an upper mass limit of 30.5 Earth masses.

Host star's brightness allows for detailed follow-up studies.

Abstract

Transiting exoplanets orbiting young nearby stars are ideal laboratories for testing theories of planet formation and evolution. However, to date only a handful of stars with age <1 Gyr have been found to host transiting exoplanets. Here we present the discovery and validation of a sub-Neptune around HD 18599, a young (300 Myr), nearby (d=40 pc) K star. We validate the transiting planet candidate as a bona fide planet using data from the TESS, Spitzer, and Gaia missions, ground-based photometry from IRSF, LCO, PEST, and NGTS, speckle imaging from Gemini, and spectroscopy from CHIRON, NRES, FEROS, and Minerva-Australis. The planet has an orbital period of 4.13 d, and a radius of 2.7Rearth. The RV data yields a 3-sigma mass upper limit of 30.5Mearth which is explained by either a massive companion or the large observed jitter typical for a young star. The brightness of the host star (V~9 mag) makes it conducive to detailed characterization via Doppler mass measurement which will provide a rare view into the interior structure of young planets.