The TESS-Keck Survey. VII. A Superdense Sub-Neptune Orbiting TOI-1824* *Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of

Abstract

We confirm a massive sub-Neptune-sized planet on a P = 22.8 days orbit around the star TOI-1824 (T eff = 5200 K, V = 9.7 mag). TESS first identified TOI-1824 b (formerly TOI-1824.01) as an object of interest in 2020 April after two transits in Sector 22 were matched with a single transit in Sector 21. TOI-1824 was subsequently targeted for ground-based Doppler monitoring with Keck-HIRES and APF-Levy. Using a joint model of the TESS photometry, radial velocities, and Ca ii H and K emission measurements as an activity indicator, we find that TOI-1824 b is an unusually dense sub-Neptune. The planet has a radius R p = 2.63 ± 0.15 R ⊕ and mass M p = 18.5 ± 3.2 M ⊕, implying a bulk density of 5.6 ± 1.4 g cm−3. TOI-1824 b's mass and radius situate it near a small group of “superdense sub-Neptunes” (R p ≲ 3 R ⊕ and M p ≳ 20 M ⊕). While the formation mechanism of superdense sub-Neptunes is a mystery, one possible explanation is the constructive collision of primordial icy cores; such giant impacts would drive atmospheric escape and could help explain these planets' apparent lack of massive envelopes. We discuss TOI-1824 b in the context of these overdense planets, whose unique location in the exoplanet mass–radius plane make them a potentially valuable tracer of planet formation.