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

Abstract

A particularly intriguing subclass of rocky exoplanets are the ultra-short period (USP) worlds that orbit their host stars in less than a day. These planets are particularly rare around M dwarf stars, with so far only ten that have a constrained mass and radius. We present the validation and characterization of the ultra-short period (0.3-days), Earth-sized planet TOI-4552b orbiting a nearby (27.26-pc away) M4.5V dwarf. Complementing the TESS photometry, ground-based transit observations from LCO, ExTrA and SPECULOOS validated the planetary radius and 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, in conjunction with the transit observations, estimate the orbital parameters. TOI-4552 is a quiet star exhibiting no short-term stellar variations seen in photometric or radial velocity data that can be associated to stellar rotation. TOI-4552b ($M_p=1.83\pm0.47\,M_e$, $R_p=1.11\pm0.04\,R_e$) 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 (CMF) of 0.54 and a bulk density of 7.74g/cm$^3$. Since the CMF spans a wide range due to the large uncertainty on the mass, the definitive interior composition cannot be determined with the current dataset. TOI-4552b hints as being marginally more iron-rich compared to 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-4552b emerges as a compelling target for atmospheric and surface composition studies with JWST.

Figures (14)

• Figure 1: Target pixel file (TPF) of TOI-4552 for TESS sector 12, created using tpfplotterAller2020. The pixels (21$^{\prime\prime}$ pixel scale) outlined in orange correspond to the aperture mask used to extract the SAP flux for the lightcurve. All Gaia DR3 GaiaDR3 sources in the field are numbered, with "1" marking TOI-4552. The size of the red circles is representative of the TESS magnitude of the stars relative to TOI-4552. Since it is a crowded field, TESS SPOC provides lightcurves corrected for the dilution caused by the strongest contaminants. Fig. \ref{['tesstpf_dil']} shows the TPF of all three sectors with the primary contaminants.
• Figure 2: TESS transit lightcurves from sectors 12, 39 and 66. The normalised flux (blue points) as well as the gaussian process (GP) model (orange) used to detrend the lightcurve is plotted on the left. The phase-folded lightcurves centred on the transit of TOI-4552 b, transit model (black dashed line), binned flux (red points) and the residuals after fitting for the transit are plotted on the right, for each sector. Sectors 12 and 39 were observed by TESS at a 2 minute cadence, whereas Sector 66 was observed at a 20 second cadence. Since TOI-4552 b is a USP, we choose to keep the high cadence of Sector 66 to provide a better sampling at the expense of lower precision.
• Figure 3: High contrast image of TOI-4552 using Zorro at Gemini-S Scott2021Howell2025 in the two simultaneous channels (562-nm and 832-nm) as a function of on-sky separation. Four nights of observations were combined here. The 5-$\sigma$ contrast curve shows no evidence for neighbours or companions contaminating the signal.
• Figure 4: Spectral Energy Distribution (SED) of TOI-4552. The best-fitting BT-Settl model (Allard2012) for parameters $T_{\rm eff}$ = 3300 K, [M/H] = 0 dex, and $\log\, g_*$ = 5.0 dex is shown by the black curve. Photometric data from APASS (magenta), Gaia (green), 2MASS (red), SDSS (black), and WISE (blue) are plotted with horizontal error bars indicating the filter passbands.
• Figure 5: NIRPS RV timeseries of TOI-4552 (top) along with the corresponding periodograms (bottom). Due to the BERV overlap issue discussed in Section \ref{['sec:rv_analysis']}, we masked a small number of pixels from each frame that contained stellar lines severely affected by under-corrected tellurics. Outlier pixels were identified using a $\sigma$-clipping threshold, such that any deviations from the median stellar spectrum exceeding the threshold were masked. We present here the unmasked (Residual uncorrected) and masked (Residual corrected) time series at 4$\sigma$ and 2.5$\sigma$ thresholds, along with their corresponding periodograms. After applying the 2.5-$\sigma$ threshold, the peaks associated with harmonics of Earth’s rotation (365.24/n days) fall below the confidence level, indicating that the telluric signals have been effectively suppressed. The telluric residual corrected time series (green) is adopted for our final analysis.