Journal ArticleThe TESS–Keck Survey. VI. Two Eccentric Sub-Neptunes Orbiting HIP-97166MacDougall, Mason G.; Petigura, Erik A.; Angelo, Isabel; Lubin, Jack; Batalha, Natalie M.; Beard, Corey; Behmard, Aida; Blunt, Sarah; Brinkman, Casey; Chontos, Ashley; Crossfield, Ian J.; Dai, Fei; Dalba, Paul A.; Dressing, Courtney; Fulton, Benjamin; Giacalone, Steven; Hill, Michelle L.; Howard, Andrew W.; Huber, Daniel; Isaacson, Howard; Kane, Stephen R.; Mayo, Andrew; Močnik, Teo; Akana Murphy, Joseph M.; Polanski, Alex; Rice, Malena; Robertson, Paul; Rosenthal, Lee J.; Roy, Arpita; Rubenzahl, Ryan A.; Scarsdale, Nicholas; Turtelboom, Emma; Zandt, Judah Van; Weiss, Lauren M.; Matthews, Elisabeth; Jenkins, Jon M.; Latham, David W.; Ricker, George R.; Seager, S.; Vanderspek, Roland K.; Winn, Joshua N.; Brasseur, C. E.; Doty, John; Fausnaugh, Michael; Guerrero, Natalia; Henze, Chris; Lund, Michael B.; Shporer, AviAbstract 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 sin i = 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 >10 5 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.2021-11-2610378633The Astronomical Journal16262650004-6256https://doi.org/10.3847/1538-3881/ac295e1717000National Science Foundation