NIRPS and TESS reveal a peculiar system around the M dwarf TOI-756: A transiting sub-Neptune and a cold eccentric giant

TL;DR

This study presents the first results from the NIRPS-GTO SP2 program, characterizing a unique two-planet system around the M1V star TOI-756: a transiting sub-Neptune TOI-756 b with $R_p = 2.81 \pm 0.10\,R_\oplus$ and $M_p = 9.8^{+1.8}_{-1.6}\,M_\oplus$, and a non-transiting eccentric cold giant TOI-756 c with $P_c = 149.6$ d and $M_p\sin i = 4.05 \pm 0.11\,M_J$, plus a radial-velocity trend suggesting a further companion. The joint RV analysis using HARPS and the infrared-capable NIRPS, complemented by TESS and ground-based photometry, yields precise orbital and physical parameters and enables interior modeling constrained by stellar refractory abundances; TOI-756 b’s density of $2.42 \pm 0.49$ g cm$^{-3}$ favors a volatile envelope with atmospheric mass fraction $AMF \approx 0.023$ and core-mass fraction $CMF \approx 0.27$, while TOI-756 c remains a distant eccentric giant ($e_c = 0.45 \pm 0.01$). This system is the first confirmed example of a transiting sub-Neptune with an outer giant around an M dwarf, offering crucial tests for planet formation theories in low-mass-star environments and a prime target for JWST transmission spectroscopy; Gaia DR4 will further refine the outer companion and the system’s architecture. The results underscore the value of combining NIRPS with HARPS for RV follow-up of faint M-dwarf hosts and contribute to understanding the Neptune desert and radius-cliff phenomena in the low-mass star regime.

Abstract

The Near InfraRed Planet Searcher (NIRPS) joined HARPS on the 3.6-m ESO telescope at La Silla Observatory in April 2023, dedicating part of its Guaranteed Time Observations (GTO) program to the radial velocity follow-up of TESS planet candidates to confirm and characterize transiting planets around M dwarfs. We report the first results of this program with the characterization of the TOI-756 system, which consists of TOI-756 b, a transiting sub-Neptune candidate detected by TESS, as well as TOI-756 c, an additional non-transiting planet discovered by NIRPS and HARPS. TOI-756 b is a 1.24-day period sub-Neptune with a radius of 2.81 $\pm$ 0.10 $R_\oplus$ and a mass of 9.8$^{+1.8}_{-1.6}$ $M_\oplus$. TOI-756 c is a cold eccentric (e$_c$ = 0.45 $\pm$ 0.01) giant planet orbiting with a period of 149.6 days around its star with a minimum mass of 4.05 $\pm$ 0.11 $M_\mathrm{jup}$. Additionally, a linear trend of 146$~\mathrm{m\,s}^{-1}\,\mathrm{yr}^{-1}$ is visible in the radial velocities, hinting at a third component, possibly in the planetary or brown dwarf regime. This system is unique in the exoplanet landscape, standing as the first confirmed example of such a planetary architecture around an M dwarf. With a density of 2.42 $\pm$ 0.49 g cm$^{-3}$, the inner planet, TOI-756 b, is a volatile-rich sub-Neptune. Assuming a pure H/He envelope, we inferred an atmospheric mass fraction of 0.023 and a core mass fraction of 0.27, which is well constrained by stellar refractory abundances derived from NIRPS spectra. It falls within the still poorly explored radius cliff and at the lower boundary of the Neptune desert, making it a prime target for a future atmospheric characterization with JWST to improve our understanding of this population.

Figures (15)

• Figure 1: TESS TPF of TOI-756 created with tpfplotter (Aller2020). The orange pixels define the aperture mask used for extracting the photometry. Additionally, the red circles indicate neighboring objects from the Gaia DR3 catalog, with the circle size corresponding to the brightness difference compared to the target (as indicated in the legend). Our target is marked with a white cross. Pixel scale is 21$\arcsec$/pixel. The co-moving companion of TOI-756 corresponds to the star labeled "2."
• Figure 2: SED of TOI-756, constructed using broadband photometric data from APASS (magenta), Gaia (green), 2MASS (black), and WISE (blue). The upper limit for the WISE W4 band is indicated by a red dot. The horizontal error bars represent the passband widths of the respective filters. Below the SED, the residuals are shown, normalized to the photometric errors. The SED was modeled using the BT Settl atmospheric model Allard2012 with $T_{\rm eff}$ = 3600 K, [M/H] = 0 dex, and $log\,g_\star$ = 4.5 $\mathrm{cm}\ \mathrm{s}^{-2}$.
• Figure 3: RV data from HARPS (blue dots) and NIRPS measurements. The NIRPS data is separated in unaffected datapoints (orange dots) and affected datapoints (red triangles) by the crossing of the V$_\text{syst}$ and BERV velocities during the observations. Yellow dots are NIRPS data with the correction explained in Sect. \ref{['sect:NIRPS_RVs_correction']}. The dotted red line together with the right y axis represent the total velocity of TOI-756, showing the crossing of the BERV with the V$_\mathrm{syst}$. The complete inferred model in Sect. \ref{['sect:joint_fit_juliet']}, which comprises signals from the two planets along with the linear model for the acceleration (in gray), is represented by a black solid line.
• Figure 4: NIRPS data residuals (upper panel) and LBL indicators D2V (middle panel), along with dTemp ($\Delta T$) calculated at 3500 K close to the effective temperature of TOI-756 (bottom panel). Orange dots are the indicators from unaffected NIRPS data, red triangles are the affected spectra by V$_\text{syst}$-BERV crossing and yellow dots are the indicators with the correction explained in Sect. \ref{['sect:NIRPS_RVs_correction']}.
• Figure 5: Top panel: Phase-folded TESS, ExTrA, and LCO-CTIO light curves of TOI-756 b (gray points). Dark red circles are data binned to 10 min. The black lines represent the median model of each instrument from the joint fit. Bottom panel: Residuals of the data compared to the model. An arbitrary offset has been added to the ground-based photometry for clarity.