Masses, revised radii, and a third planet candidate in the ‘Inverted’ planetary system around TOI-1266

Ryan Cloutier*, Michael Greklek-McKeon, Serena Wurmser, Collin Cherubim, Erik Gillis, Andrew Vanderburg, Sam Hadden, Charles Cadieux, Étienne Artigau, Shreyas Vissapragada, Annelies Mortier, Mercedes López-Morales, David W Latham, Heather Knutson, Raphaëlle D Haywood, Enric Pallé, René Doyon, Neil Cook, Gloria Andreuzzi, Massimo CecconiRosario Cosentino, Adriano Ghedina, Avet Harutyunyan, Matteo Pinamonti, Manu Stalport, Mario Damasso, Federica Rescigno, Thomas G Wilson, Lars A Buchhave, David Charbonneau, Andrew Collier Cameron, Xavier Dumusque, Christophe Lovis, Michel Mayor, Emilio Molinari, Francesco Pepe, Giampaolo Piotto, Ken Rice, Dimitar Sasselov, Damien Ségransan, Alessandro Sozzetti, Stéphane Udry, Chris A Watson

*Corresponding author for this work

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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, Rp,b = 2.62 ± 0.11 R⊕) orbiting interior to that of the system’s smaller planet (Pc = 18.8 d, 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 d, Mp,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 (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.
Original languageEnglish
Pages (from-to)5464-5483
Number of pages20
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date8 Nov 2023
Publication statusPublished - 1 Jan 2024


  • Techniques: photometric
  • Techniques: radial velocities
  • Planets and satellites: composition
  • Planets and satellites: formation
  • Stars: individual: TOI-1266
  • Stars: low-mass


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