Masses, Revised Radii, and a Third Planet Candidate in the "Inverted" Planetary System Around TOI-1266

Is the population of close-in planets orbiting M dwarfs sculpted by thermally driven escape or is it a direct outcome of the planet formation process? A number of recent empirical results strongly suggest the latter. However, the unique architecture of the TOI-1266 system presents a challenge to models of planet formation and atmospheric escape given its seemingly “inverted” architecture of a large sub-Neptune ($P_b=10.9$ days, $R_{p,b}=2.62\pm 0.11{\mathrm{R}}_{\oplus }$) orbiting interior to that of the system’s smaller planet ($P_c=18.8$ days, $R_{p,c}=2.13\pm 0.12{\mathrm{R}}_{\oplus }$). Here we present revised planetary radii based on new TESS and diffuser-assisted ground-based transit observations, and characterize both planetary masses using a set of 145 radial velocity measurements from HARPS-N ($M_{p,b}=4.23\pm 0.69{\mathrm{M}}_{\oplus },M_{p,c}=2.88\pm 0.80{\mathrm{M}}_{\oplus }$). Our analysis also reveals a third planet candidate ($P_d=32.3$ days, $M_{p,d}\sin i=4.59_{-0.94}^{+0.96}{\mathrm{M}}_{\oplus }$), which if real, would form a chain of near 5:3 period ratios, although the system is likely not in a mean motion resonance. Our results indicate that TOI-1266 b and c are among the lowest density sub-Neptunes around M dwarfs and likely exhibit distinct bulk compositions of a gas-enveloped terrestrial ($X_{\mathrm{e}\mathrm{n}\mathrm{v},b}=5.5\pm 0.7$%) and a water-rich world (WMF${}_c=59\pm 14$%), which is supported by hydrodynamic escape models. If distinct bulk compositions are confirmed through atmospheric characterization, the system’s unique architecture would represent an interesting test case of inside-out sub-Neptune formation at pebble traps.