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 (Pb = 10.9 days, Rp, b = 2.62 ± 0.11 R⊕) orbiting interior to that of the system’s smaller planet (Pc = 18.8 days, Rp, c = 2.13 ± 0.12 R⊕). 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 (Mp, b = 4.23 ± 0.69 M⊕, Mp, c = 2.88 ± 0.80 M⊕). Our analysis also reveals a third planet candidate (Pd = 32.3 days, $M_{p,d}\sin {i} = 4.59^{+0.96}_{-0.94}\, \mathrm{M}_{\oplus }$), 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 (Xenv, b = 5.5 ± 0.7%) and a water-rich world (WMFc = 59 ± 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.