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

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 2.7 R-circle plus . The RV data yields a 3-sigma mass upper limit of 30.5 M-circle plus 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 similar to 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.

Automated summary

The discovery and validation of a sub-Neptune orbiting HD 18599, a young star with an age of 300 Myr, presents an ideal opportunity to study planet formation and evolution. Multiple observations from different missions and ground-based telescopes support the existence of this planet, and its brightness allows for detailed characterization, providing valuable insights into the interior structure of young planets.