Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs

Tracing the compositional link between terrestrial super-Earths and their host stars provides clues about 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 planetary CMF derived from their masses and radii can reveal past formation processes, such as mantle stripping and water-rich formation plus sequestration in the planet core. We present the first results from the Near Infrared Planet Searcher (NIRPS) GTO CMF subprogram: an intensive radial velocity campaign to refine masses and compute host stellar abundances of three hot super-Earths around M dwarfs (GJ1132 b, GJ1252 b, and LTT 3780 b). We calculated masses of 1.69 ± 0.15 M ⊕ , 1.54 ± 0.18 M ⊕ , and 2.34 ± 0.10 M ⊕ respectively. We measured the CMFs of these and six further hot super-Earths with masses already available in the literature to a precision of 10–15%. We compared them to CMF predictions made from measuring the Fe, Mg, and Si abundances of their host stars measured from the NIRPS spectra. 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, and while these planets are too hot to harbor surface water, they likely have interior water mass fractions of ~1%.