Masses, revised radii, and a third planet candidate in the ‘Inverted’ planetary system around TOI-1266
Ryan Cloutier, Michael Greklek-McKeon, S. Wurmser, Collin Cherubim, Erik Gillis, Andrew Vanderburg, Sam Hadden, Charles Cadieux, Étienne Artigau, Shreyas Vissapragada, Annelies Mortier, Mercedes López‐Morales, David W. Latham, Heather A. Knutson, R. D. Haywood, Ε. Πάλλη, René Doyon, Neil J. Cook, G. Andreuzzi, M. Cecconi, R. Cosentino, A. Ghedina, A. Harutyunyan, M. Pinamonti, M. Stalport, M. Damasso, Federica Rescigno, T. G. Wilson, Lars A. Buchhave, David Charbonneau, A. Collier Cameron, X. Dumusque, C. Lovis, M. Mayor, E. Molinari, F. Pepe, G. Piotto, Ken Rice, Dimitar Sasselov, D. Ségransan, A. Sozzetti, S. Udry, Chris Watson
Abstract
ABSTRACT 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 d, $R_{p,b}=2.62\pm 0.11\, \mathrm{R}_{\oplus }$) orbiting interior to that of the system’s smaller planet (Pc = 18.8 d, $R_{p,c}=2.13\pm 0.12\, \mathrm{R}_{\oplus }$). 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 (Pd = 32.3 d, $M_{p,d}\sin {i} = 4.59^{+0.96}_{-0.94}\, \mathrm{M}_{\oplus }$), 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 per cent) and a water-rich world (WMFc = 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.