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 d, R_p,b = 2.62 ± 0.11 R⊕) orbiting interior to that of the system’s smaller planet (P_c = 18.8 d, R_p,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 (M_p,b = 4.23 ± 0.69 M⊕, M_p,c = 2.88 ± 0.80 M⊕). Our analysis also reveals a third planet candidate (P_d = 32.3 d, M_p,d sin i = 4.59^+0.96_-0.94, M⊕), 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_env,b = 5.5 ± 0.7 per cent) and a water-rich world (WMF_c = 59 ± 14 per cent), 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.