The TESS-Keck Survey. VI. Two Eccentric Sub-Neptunes Orbiting HIP-97166
Batalha, Natalie M.; Latham, David W.; Rice, Malena; Kane, Stephen R.; Huber, Daniel; Rosenthal, Lee J.; Howard, Andrew W.; Petigura, Erik A.; Behmard, Aida; Chontos, Ashley; Crossfield, Ian J. M.; Dalba, Paul A.; Rubenzahl, Ryan A.; Weiss, Lauren M.; Ricker, George R.; Winn, Joshua N.; Jenkins, Jon M.; Seager, S.; Shporer, Avi; Dai, Fei; Lund, Michael B.; Matthews, Elisabeth; Vanderspek, Roland K.; Fausnaugh, Michael; Guerrero, Natalia; Angelo, Isabel; Giacalone, Steven; Brasseur, C. E.; Fulton, Benjamin; Doty, John; Robertson, Paul; Roy, Arpita; Blunt, Sarah; Isaacson, Howard; Močnik, Teo; Beard, Corey; Lubin, Jack; Akana Murphy, Joseph M.; Dressing, Courtney; Hill, Michelle L.; MacDougall, Mason G.; Scarsdale, Nicholas; Brinkman, Casey; Mayo, Andrew; Zandt, Judah Van; Polanski, Alex; Turtelboom, Emma; Henze, Chris
United States, Australia, Switzerland
Abstract
We report the discovery of HIP-97166b (TOI-1255b), a transiting sub-Neptune on a 10.3 day orbit around a K0 dwarf 68 pc from Earth. This planet was identified in a systematic search of TESS Objects of Interest for planets with eccentric orbits, based on a mismatch between the observed transit duration and the expected duration for a circular orbit. We confirmed the planetary nature of HIP-97166b with ground-based radial-velocity measurements and measured a mass of M b = 20 ± 2 M ⊕ along with a radius of R b = 2.7 ± 0.1 R ⊕ from photometry. We detected an additional nontransiting planetary companion with M c sini = 10 ± 2 M ⊕ on a 16.8 day orbit. While the short transit duration of the inner planet initially suggested a high eccentricity, a joint RV-photometry analysis revealed a high impact parameter b = 0.84 ± 0.03 and a moderate eccentricity. Modeling the dynamics with the condition that the system remain stable over >105 orbits yielded eccentricity constraints e b = 0.16 ± 0.03 and e c < 0.25. The eccentricity we find for planet b is above average for the small population of sub-Neptunes with well-measured eccentricities. We explored the plausible formation pathways of this system, proposing an early instability and merger event to explain the high density of the inner planet at 5.3 ± 0.9 g cc-1 as well as its moderate eccentricity and proximity to a 5:3 mean-motion resonance.