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

Creators: MacDougall, 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. M.; 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; Van Zandt, Judah; 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, Avi

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 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 >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.

Additional Information

© 2021. The American Astronomical Society. Received 2021 August 5; revised 2021 September 9; accepted 2021 September 17; published 2021 November 26. The authors thank the anonymous reviewer and data editor for constructive comments on the manuscript. M.M. acknowledges support from the UCLA Cota-Robles Graduate Fellowship. E.P. acknowledges support from the NASA Exoplanets Research Program (80NSSC21K0598). D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC21K0652), and the National Science Foundation (AST-1717000). P.D. is supported by a National Science Foundation (NSF) Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1903811. The authors also thank Daniel Foreman-Mackey for discussions regarding the parameterization of the transit models used in this work. This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. Data presented herein were obtained at the W. M. Keck Observatory from telescope time allocated to the National Aeronautics and Space Administration through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank the time assignment committees of the University of California, the California Institute of Technology, NASA, and the University of Hawaii for supporting the TESS–Keck Survey with observing time at Keck Observatory and on the Automated Planet Finder. We thank NASA for funding associated with our Key Strategic Mission Support project. We gratefully acknowledge the efforts and dedication of the Keck Observatory staff for support of HIRES and remote observing. We recognize and acknowledge the cultural role and reverence that the summit of Maunakea has within the indigenous Hawaiian community. We are deeply grateful to have the opportunity to conduct observations from this mountain. We thank Ken and Gloria Levy, who supported the construction of the Levy Spectrometer on the Automated Planet Finder. We thank the University of California and Google for supporting Lick Observatory and the UCO staff for their dedicated work scheduling and operating the telescopes of Lick Observatory. This paper is based on data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. This paper also includes data that are publicly available from the Mikulski Archive for Space Telescopes (MAST). We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This work also used computational and storage services associated with the Hoffman2 Shared Cluster provided by UCLA Institute for Digital Research and Education's Research Technology Group. Part of this work is based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. On behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). Data were collected under program GN-2019B-LP-101. Facilities: TESS - , Keck/HIRES - , APF/Levy - , Gaia - , Gemini/NIRI - . Software: We made use of the following publicly available Python modules: exoplanet (Foreman-Mackey et al. 2021), PyMC3 (Salvatier et al. 2016), theano (The Theano Development Team et al. 2016), LDTK (Parviainen & Aigrain 2015), RadVel (Fulton et al. 2018), RVSearch (Rosenthal et al. 2021), astropy (Astropy Collaboration et al. 2013, 2018), isoclassify (Huber et al. 2017), lightkurve (Lightkurve Collaboration et al. 2018), chainconsumer (Hinton 2016), matplotlib (Hunter 2007), numpy (Harris et al. 2020), scipy (Virtanen et al. 2020), and pandas (McKinney 2010).

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