TESS Giants Transiting Giants. IV. A Low-density Hot Neptune Orbiting a Red Giant Star

Creators: Grunblatt, Samuel K.; Saunders, Nicholas; Huber, Daniel; Thorngren, Daniel; Vissapragada, Shreyas; Yoshida, Stephanie; Schlaufman, Kevin C.; Giacalone, Steven; Macdougall, Mason; Chontos, Ashley; Turtelboom, Emma; Beard, Corey; Murphy, Joseph M. Akana; Rice, Malena; Isaacson, Howard; Angus, Ruth; Howard, Andrew W.¹

1. California Institute of Technology

Abstract

Hot Neptunes, gaseous planets smaller than Saturn (∼3–8 R_⊕) with orbital periods less than 10 days, are rare. Models predict this is due to high-energy stellar irradiation stripping planetary atmospheres over time, often leaving behind only rocky planetary cores. Using our TESS full-frame-image pipeline giants in conjunction with Keck/HIRES radial velocity measurements, we present the discovery of TIC365102760 b, a 6.2 R_⊕(0.55 R_J), 19.2 M_⊕(0.060 M_J) planet transiting a red giant star every 4.21285 days. The old age and high equilibrium temperature yet remarkably low density of this planet (𝜌_𝑝 = 0.58_(−0.20)^(+0.30) 𝜌_J) suggest that its gaseous envelope should have been stripped by high-energy stellar irradiation billions of years ago. The present-day planet mass and radius suggest the atmospheric stripping was slower than predicted. Unexpectedly low stellar activity and/or late-stage planet inflation could be responsible for the observed properties of this system. Further studies of this system with more precise photometry in multiple passbands will be capable of revealing more details of this planet's atmosphere.

Copyright and License

© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

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 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. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. S.G. acknowledges support by the National Aeronautics and Space Administration under grants 80NSSC23K0137 and 80NSSC23K0168. S.G., N.S., and D.H. acknowledge support by the National Aeronautics and Space Administration under grant 80NSSC19K0593 issued through the TESS Guest Investigator Program. S.G. and K.S. acknowledge support by the National Aeronautics and Space Administration under grant 80NSSC20K0059 issued through the TESS Guest Investigator Program. D.H. acknowledges support from the Alfred P. Sloan Foundation and the National Aeronautics and Space Administration (80NSSC21K0652) and from the National Science Foundation (80NSSC21K0652). N.S. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program under grants 1842402 and DGE-1752134. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Funding for the TESS mission is provided by NASA's Science Mission Directorate.

Software References

this work has relied heavily on open-source software tools, and we would like to thank the developers for their contributions to the astronomy community. For data access and detrending, this research made use of lightkurve, a Python package for Kepler and TESS data analysis (Lightkurve Collaboration et al. 2018); TESSCut, a MAST tool for extracting observations from TESS FFIs (Brasseur et al. 2019); and the MIT QLP, a pipeline for producing and detrending TESS FFI light curves (Kunimoto et al. 2021). The analysis portion of this research relied on astropy (Astropy Collaboration et al. 2013; Price-Whelan et al. 2018), as well as exoplanet (Foreman-Mackey et al. 2020) and its dependencies (Kipping 2013; Salvatier et al. 2016; Theano Development Team 2016; Agol et al. 2019; Luger et al. 2019; Foreman-Mackey et al. 2020).

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