The TESS-Keck Survey. VI. Two Eccentric Sub-Neptunes Orbiting HIP-97166

Mason G. Macdougall, Erik A. Petigura, Isabel Angelo, Jack Lubin, Natalie M. Batalha, Corey Beard, Aida Behmard, Sarah Blunt, Casey Brinkman, Ashley Chontos, Ian J.M. Crossfield, Fei Dai, Paul A. Dalba, Courtney Dressing, Benjamin Fulton, Steven Giacalone, Michelle L. Hill, Andrew W. Howard, Daniel Huber, Howard IsaacsonStephen R. Kane, Andrew Mayo, Teo Močnik, Joseph M. Akana Murphy, Alex Polanski, Malena Rice, Paul Robertson, Lee J. Rosenthal, Arpita Roy, Ryan A. Rubenzahl, Nicholas Scarsdale, Emma Turtelboom, Judah Van Zandt, Lauren M. Weiss, Elisabeth Matthews, Jon M. Jenkins, David W. Latham, George R. Ricker, S. Seager, Roland K. Vanderspek, Joshua N. Winn, C. E. Brasseur, John Doty, Michael Fausnaugh, Natalia Guerrero, Chris Henze, Michael B. Lund, Avi Shporer

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


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.

Original languageEnglish (US)
Article number265
JournalAstronomical Journal
Issue number6
StatePublished - Dec 1 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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