TOI-4552 b: A new ultra-short-period rocky world revealed by NIRPS and TESS

Context . A particularly intriguing subclass of rocky exoplanets are ultra-short-period worlds, which orbit their host stars in less than a day. These planets are particularly rare around M dwarf stars, and only ten of them have a constrained mass and radius so far. Aims . We present the validation and characterisation of the ultra-short-period (0.3 days) Earth-sized planet TOI-4552 b orbiting a nearby (27.26 pc away) M4.5V dwarf. Methods . We complemented TESS photometry ground-based transit observations from LCO, ExTrA, and SPECULOOS to validate the planetary radius, and we cleared the field of any contaminants. Speckle imaging with Zorro (Gemini-S) rules out false-positive scenarios caused by eclipsing binary sources. Spectroscopic observations with NIRPS and HARPS were used to obtain stellar abundances, constrain the planetary mass, and estimate the orbital parameters in conjunction with the transit observations. Results . TOI-4552 is a quiet star. It lacks short-term stellar variations in photometric or radial velocity data that might be associated with stellar rotation. Long-term photometric data from ASAS-SN also suggest a lack of activity signals. TOI-4552 b ( M p = 1.83 ± 0.47 M ⊕ , R p = 1.11 ± 0.04 R ⊕ ) lies between the Earth-like and iron-rich composition tracks on the mass-radius diagram. The exopie interior structure model, without constraints from refractory abundance ratio, yields a core mass fraction of 0.54 −0.25 +0.17 and a bulk density of 7.74±2.14g/cm 3 . Since the core mass fraction spans a wide range because the uncertainty on the mass is high, the definitive interior composition cannot be determined with the current dataset. Conclusions . TOI-4552 b probably is marginally richer in iron than the Earth, but confirmation of its status requires additional precise radial velocity measurements. Combined with its high emission spectroscopic metric (ESM = 19.5), negligible stellar activity, and short orbital period, TOI-4552 b emerges as a compelling target for atmospheric and surface composition studies with JWST.