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Abstract Among Neptunian mass exoplanets (20−50M⊕), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density (ρ< 0.3 g cm−3). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107b, TOI-1420b, and WASP-193b. In this paper, we report the discovery of TOI-1173 Ab, a low-density ( g cm−3) super-Neptune withP= 7.06 days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X and HIRES spectrographs and transit photometry from TESS, we determine a planet mass ofMp= 27.4 ± 1.7M⊕and radius ofRp= 9.19 ± 0.18R⊕. TOI-1173 Abis the first puffy super-Neptune planet detected in a wide binary system (projected separation ∼11,400 au). We explore several mechanisms to understand the puffy nature of TOI-1173 Aband show that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 Ablikely has maintained its orbital stability over time and may have undergone von-Zeipel–Lidov–Kozai migration followed by tidal circularization, given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 Abwill shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin–orbit analyses may elucidate the dynamical evolution of the system.
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This content will become publicly available on February 3, 2026
Revised Masses for Low-density Planets Orbiting the Disordered M-dwarf System TOI-1266
Abstract We present an analysis of 126 new radial velocity measurements from the MAROON-X spectrograph to investigate the TOI-1266 system, which hosts two known transiting sub-Neptunes at 10.8 and 18.8 days. We integrated our measurements with existing HARPS-N measurements for this system and derived revised masses for TOI-1266 b and c ofMb= 4.09 ± 0.45M⊕andMc= 2.64 ± 0.52M⊕, respectively. The Keplerian fit from the combined datasets enabled an ≈35% and ≈41% improvement in mass precision for planet b and c, respectively, compared to the previously published values. With bulk densities ofρb= 1.25 ± 0.21 g cm−3andρc= 1.51 ± 0.39 g cm−3, the planets are among the lowest density sub-Neptunes orbiting an M dwarf. They are both consistent with rocky cores surrounded by hydrogen helium envelopes. TOI-1266 c may also be consistent with a water-rich composition, but we disfavor that interpretation from an Occam's razor perspective.
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- Award ID(s):
- 2108465
- PAR ID:
- 10647700
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 169
- Issue:
- 2
- ISSN:
- 0004-6256
- Page Range / eLocation ID:
- 109
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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