A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266

Abstract

We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright ($K=8.8$), quiet, and nearby (37 pc) M3V dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Mártir (Mexico). We also include additional ground-based follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner, larger planet has a radius of $R=2.37_{-0.12}^{+0.16}$ R${}_{\oplus }$ and an orbital period of 10.9 days. The outer, smaller planet has a radius of $R=1.56_{-0.13}^{+0.15}$ R${}_{\oplus }$ on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of $M_{\mathrm{p}}$ = $13.5_{-9.0}^{+11.0}$ ${\mathrm{M}}_{\oplus }$ ($<36.8$ ${\mathrm{M}}_{\oplus }$ at 2-$\ddot{I}ƒ$) for TOI-1266 b and $2.2_{-1.5}^{+2.0}$ ${\mathrm{M}}_{\oplus }$ ($<5.7$ ${\mathrm{M}}_{\oplus }$ at 2-$\ddot{I}ƒ$) for TOI-1266 c. We find small but non-zero orbital eccentricities of $0.09_{-0.05}^{+0.06}$ ($<0.21$ at 2-$\ddot{I}ƒ$) for TOI-1266 b and $0.04\pm 0.03$ ($<0.10$ at 2-$\ddot{I}ƒ$) for TOI-1266 c. The equilibrium temperatures of both planets are of $413\pm 20$ K and $344\pm 16$ K, respectively, assuming a null Bond albedo and uniform heat redistribution from the day-side to the night-side hemisphere. The host brightness and negligible activity combined with the planetary system architecture and favourable planet-to-star radii ratios makes TOI-1266 an exquisite system for a detailed characterisation.

In press. Accepted for publication in Astronomy and Astrophysics on 20 July 2020