TOI-7166 b: A Habitable Zone mini-Neptune planet around a nearby low-mass star

TL;DR

TOI-7166 b is a nearby M-dwarf sub-Neptune with $R_p = 2.01\,R_$ in a $P \approx 12.92$ d orbit, placing it near the inner edge of the star's Habitable Zone and receiving $S_p \approx 1.07\,S_$ with $T_{\rm eq} \approx 249$ K. The authors combine TESS photometry, an extensive ground-based, multi-band follow-up, high-resolution imaging, and spectroscopy to validate and precisely characterize the system, deriving robust stellar parameters ($T_{\rm eff} \approx 3100$ K, $M_* \approx 0.19\,M_\odot$, $R_* \approx 0.22\,R_\odot$) and planet parameters. A statistical validation (FPP $\approx 1.8\times10^{-3}$) coupled with the achromatic transit depths across multiple bands confirms the planetary nature. With a bright infrared host (e.g., $J \approx 11.4$, $K \approx 10.6$) and a sizable transmission signal, TOI-7166 b is an excellent target for future radial-velocity mass measurements (MAROON-X favored) and JWST transmission spectroscopy to probe atmospheric composition and bulk structure.

Abstract

We present the discovery and validation of TOI-7166b, a 2.01+/-0.05R_Earth planet orbiting a nearby low-mass star. We validated the planet by combining TESS and multi-color high-precision photometric observations from ground-based telescopes, together with spectroscopic data, high-contrast imaging, archival images, and statistical arguments. The host star is an M4-type dwarf at a distance of ~35 pc from the Sun. It has a mass and a radius of Ms=0.190+/-0.004M_Sun and Rs=0.222+/-0.005R_Sun, respectively. TOI-7166b has an orbital period of 12.9 days, which places it close to the inner edge of the Habitable Zone of its host star, receiving an insolation flux of Sp=1.07+/-0.08S_Earth and an equilibrium temperature of Teq=249+/-5K (assuming a null Bond albedo). The brightness of the host star makes TOI-7166 a suitable target for radial velocity follow-up to measure the planetary mass and bulk density. Moreover, the physical parameters of the system including the infrared brightness (Kmag = 10.6) of the star and the planet-to-star radius ratio (0.0823+/-0.0012) make TOI-7166b an exquisite target for transmission spectroscopic observations with the JWST, to constrain the exoplanet atmospheric compositions.

Figures (13)

• Figure 1: TESS target pixel file images of TOI-7166 observed in Sectors 82 made by https://github.com/jlillo/tpfplotterAller_2020AA. Red dots show the location of Gaia DR3 sources, and the yellow shaded regions show the photometric apertures used for photometric measurements extraction.
• Figure 2: TESS PDC-SAP flux measurements extracted from the 2-min cadence data of TOI-7166. The target was observed in sector 82. The blue points show the 2-min data, and the red points show the 20-min binned data. The transit locations of TOI-7166 b are shown with black arrows and zoom boxes.
• Figure 3: TESS and ground-based phase-folded transit light curves of TOI-7166 b. The colored data points show the relative flux, and the colored solid lines show the best-fitting transit model superimposed. The transit light curves are shifted along y-axis for visibility.
• Figure 4: The figure shows 5$\sigma$ magnitude contrast curves in both filters as a function of the angular separation out to 1.2 arcsec. The inset shows the reconstructed 832 nm image of TOI-7166 with a 1 arcsec scale bar. TOI-7166 was found to have no close companions from the diffraction limit (0.02") out to 1.2 arcsec to within the magnitude contrast levels achieved.
• Figure 5: Spectral energy distribution of TOI-7166. The colored points with error bars represent the observed photometric measurements. The black circles are the model fluxes from the best-fit PHOENIX atmosphere model. The absolute flux-calibrated Gaia spectrophotometry is shown as the gray swathe.