Multicolour Validation of Two Temperate Mini-Neptunes Around M-dwarf Habitable Zones

TL;DR

This paper develops and applies a multicolour transit validation framework to confirm two temperate mini-Neptunes, TOI-2094 b and TOI-7166 b, orbiting M-dwarf stars in or near their habitable zones. By combining TESS photometry with ground-based multicolour follow-up (notably GTC/HiPERCAM and MuSCAT2/3) and performing Bayesian model comparison among planet, white-dwarf eclipse, and brown-dwarf scenarios, the authors robustly rule out false positives and derive precise transit and planetary parameters. Statistical mass-radius inference suggests TOI-2094 b has a bimodal mass distribution (potential water world/gaseous envelope vs rocky) while TOI-7166 b is likely volatile-rich; transmission spectroscopy metrics indicate TOI-7166 b as an especially favorable JWST target, with atmospheric features detectable under reasonable observing commitments. The study demonstrates the effectiveness of multicolour validation with 8–10 m class facilities, enabling a scalable path to expanding the sample of habitable-zone exoplanets suitable for atmospheric characterization with next-generation observatories.

Abstract

For small planets orbiting within the habitable zones of their host stars, multicolour validation via photometric transit observations offers an efficient alternative to prioritize targets before intensive radial-velocity follow-up, thereby expanding the sample of habitable-zone exoplanets amenable for atmospheric characterisation. In this study, we validate two exceptional habitable-zone TESS candidates, orbiting around M-dwarfs, as genuine planets, precisely determining their transit and physical parameters. We perform Bayesian model comparison by jointly fitting multicolour light curves from TESS and ground-based follow-up, including observations with HiPERCAM at the 10.4-m GTC. Our approach uses wavelength-dependent transit depth variations and precise transit geometry to reject false positives. We validate TOI-2094 b and TOI-7166 b as two new benchmark temperate mini-Neptunes. TOI-2094 b (1.90 $R_{\oplus}$) orbits its M3V star with a period of $\sim$18.79 days, well within the habitable zone ($\sim$0.98 Earth insolation). TOI-7166 b (2.39 $R_{\oplus}$) orbits its M4.5V host star with a period of $\sim$12.92 days, placing it near the inner edge of the habitable zone ($\sim$1.93 Earth insolation). Statistical mass and density estimates suggest that TOI-2094 b may be a volatile-rich planet, such as a water world or a gaseous planet, and is less likely to be rocky, while TOI-7166 b is likely to be volatile-rich. Both planets are of great interest for detailed atmospheric characterisation with the JWST and future ELTs, which requires further precise mass measurements.

Figures (14)

• Figure 1: Toy models of transit light curves (left) and corresponding apparent radii in the HiPERCAM passbands (right), using different model hypotheses ($\mathcal{H}_1$ to $\mathcal{H}_3$ from top to bottom). The dashed line and gray shaded area are the input true radius and 1-$\sigma$ uncertainty. In all cases, we assume an edge-on circular orbit with a period of 20 days and a semi-major axis of 120 $R_\star$, and the primary star has $T_{\rm eff}=3500$ K, $\lg g=5.0$, and $\rm [Fe/H]=0$. Panel (a) assumes a transiting planet with 2 Earth radii. Panel (b) assumes an eclipsing white dwarf of Earth radius with $T_{\rm eff}=4500$ K. Panel (c) assumes a brown dwarf transiting on an unresolved, faint, background star with $T_{\rm eff}=3000$ K.
• Figure 2: Infrared AO imaging of TOI-2094 and corresponding magnitude sensitivity curves in the $J$ and $K_s$ bands observed using ShaneAO/ShARCS instrument, obtained from ExoFOP archival data (PI: C. Dressing). No close stellar companion is noted.
• Figure 3: Optical speckle imaging of TOI-7166 and corresponding 5-$\sigma$ magnitude sensitivity curves observed with Gemini/Zorro instrument using filters EO 562 and EO 832, obtained from ExoFOP archival data (PI: S. B. Howell). No close stellar companion is noted.
• Figure 4: Best SED models for TOI-2094 and TOI-7166. The blue solid lines are the grid models of BT-NextGen AGSS2009 2011ASPC..448...91A2012RSPTA.370.2765A. The lower panels show the normalized residuals.
• Figure 5: TESS target pixel file images of TOI-2094 ( top) and TOI-7166 ( bottom). The red circles are sources identified by Gaia DR3 catalogue 2016AA...595A...1G2023AA...674A...1G, labelled with numbers sorted by angular distance to the target source. The size of the circles indicates the magnitude contrast to the target source. The mosaic of orange pixels shows the aperture of TESS photometry, where the pixel scale is 20$"$. The plots were produced using tpfplotter2020AA...635A.128A.