Dynamical mass constraints on K2-33 b: a 5 Myr-old possible precursor to sub-Neptunes

K2-33b is the second youngest known transiting planet, orbiting a pre-main-sequence M dwarf member of the Upper Scorpius association at an age of 5-10 Myr. With a radius of about 5 Rearth and an orbital period of 5.42 days (David et al. 2016, Mann et al. 2016), it represents a unique opportunity to probe the physical state of a close-in planet near the epoch at which migration, atmospheric contraction, and volatile loss are expected to be most active.

We aim to place the first dynamical constraint on the mass of K2-33b, which is essential for interpreting its bulk density, atmospheric scale height, and the competing physical explanations for its chromatic transit depth. We collected 113 high-resolution spectra with the ESPRESSO spectrograph on the ESO VLT across two observing campaigns spanning 2023 and 2025, complemented by photometry from K2, TESS, and LCO. We jointly modelled the transit photometry and the radial velocity time series using Gaussian processes to account for the large activity-induced variability of this rapidly rotating young M dwarf.

We constrain the mass of K2-33b to an upper limit of 12 Mearth at the 3-sigma level. Notably, for a planet with a radius as large as ~5 Rearth, a higher mass would have been expected from the mass–radius relations of the older exoplanet population (Chen & Kipping 2017), which would in principle have rendered the Keplerian signal more accessible to detection. The fact that the mass falls below this threshold is therefore itself significant: it implies that K2-33b is less massive than its radius would suggest, confirming that the planet is genuinely inflated relative to its older counterparts of comparable mass. This result is consistent with recent findings for other very young close-in planets, including AU Mic b (~20 Myr, Mallorquín et al. 2024), HIP67522b (~17 Myr, Thao et al. 2024), and V1298 Tau b and e (~20 Myr; Livingston et al. 2026). In the context of the young planet population, the extended radius of K2-33b at only 5 Myr support a scenario in which sub-Neptunes descend from initially larger, low-density progenitors whose atmospheres are eroded on short timescales by the intense high-energy irradiation of their host stars.