Super-Earth Masses and Stellar Abundances from NIRPS Reveal Tentative Evidence for Water-Rich Formation around M Dwarfs

Abstract

Tracing the compositional link between terrestrial super-Earths and their host stars provides clues to their dominant formation pathway. By constraining the stellar abundances of refractory elements, we can predict the core mass fractions (CMFs) of their super-Earths. The level of agreement between this prediction and the planet’s true CMF from their masses and radii can teach us about past formation processes, like mantle stripping and water-rich formation plus sequestration in the planet’s core. Here, we present the first results from the Near Infrared Planet Searcher’s (NIRPS) GTO CMF subprogram: an intensive radial velocity campaign to refine masses of transiting super-Earths and compute their host stellar abundances. We retrieve precise masses for three hot super- Earths around M dwarfs (GJ 1132 b, GJ 1252 b, and LTT 3780 b), calculating masses of 1.69 ± 0.15M⊕, 1.54 ± 0.18M⊕, and 2.34 ± 0.10M⊕ respectively. We measure the CMFs of these and five further hot super-Earths from the literature to 10-15% precision. We compare these to CMF predictions from measuring the Fe, Mg, and Si abundances of their host stars measured from the NIRPS spectra. We find that the CMFs of these planets are smaller than expected from their host stellar abundances, to a statistically significant degree. This discrepancy is suggestive of significant reservoirs of water, generally consistent with water mass fractions of ∼ 1%, sequestered inside the interiors of these planets.