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Published June 29, 2023 | Supplemental Material
Journal Article Open

A close-in giant planet escapes engulfment by its star


When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris b orbits a core-helium-burning red giant. At a distance of only 0.5 AU from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.

Additional Information

© The Author(s), under exclusive licence to Springer Nature Limited 2023. We recognize and acknowledge the cultural role and reverence that the summit of Maunakea has within the indigenous Hawaiian community. We are grateful for the opportunity to conduct observations from this mountain. The data in this study were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership between the California Institute of Technology, the University of California and NASA. The observatory was made possible by the financial support of the W. M. Keck Foundation. Additional observations were obtained at the CFHT, which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France and the University of Hawaii. M.H. acknowledges support from NASA through the NASA Hubble Fellowship grant HST-HF2-51459.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA contract NAS5-26555. D.H. acknowledges support from the Alfred P. Sloan Foundation, NASA (80NSSC21K0652, 80NSSC20K0593) and the Australian Research Council (FT200100871). N.Z.R. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant no. DGE‐1745301. O.K. acknowledges support from the Swedish Research Council under the project grant 2019-03548. A.S. acknowledges support from the European Research Council Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, grant agreement no. 772293). M.V. acknowledges support from NASA grant 80NSSC18K1582. This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through research grants UIDB/04434/2020 and UIDP/04434/2020. T.L.C. is supported by FCT in the form of a work contract (CEECIND/00476/2018). T.R.B. acknowledges support from the Australian Research Council through Discovery Project DP210103119 and Laureate Fellowship FL220100117. Contributions. M.H. identified the oscillations of 8 UMi, led the observing programme and data analysis and wrote most of the paper. D.H. organized observations, interpreted the asteroseismic and radial velocity data and contributed to writing the paper. N.Z.R. and J.F. conducted binary simulations for the host star, performed numerical calculations for planet survival scenarios and contributed to writing the paper. J.F. and D.V. interpreted formation scenarios for the host star. J.S.K. and M.V. extracted oscillation parameters from the TESS data. O.K. performed the spectropolarimetric analysis of the host star and the control target. A.S., J.L.R., M.Y., Z.Ç.O., S.Ö., C.J. and J.O. conducted grid-based modelling for 8 UMi. D.R.H., D.H. and M.H. fitted the radial velocity data. H.I. measured chromospheric activity indices from the HIRES data. J.Z. constrained the properties of the outer companion. K.G.S. performed the SED analysis for the host star. B.J.S. extracted ASAS-SN photometry for 8 UMi. J.T. and Z.R.C. provided interpolatable grids of isochrones. T.R.B. and D.S. analysed the asteroseismic data and helped to guide the strategy of the paper. B.T.M. identified and analysed the lithium richness of the control target. W.J.C., D.H. and T.L.C. are key architects of TASC working groups on exoplanet hosts, including evolved stars. H.I. and A.W.H. oversaw the California Planet Search observing programme. A.C., S.G., C. Beard, J.L., R.H., J.M.A.M., J.V.Z., D.T., D.P., C. Brinkman, M.M., A.S.P., M.R. and L.W. conducted Keck I/HIRES observations of 8 UMi and the control star. All authors reviewed the paper. Data availability. TESS light curves processed by the TESS Science Operations Center pipeline are available from MAST (https://archive.stsci.edu/). The spectra for µ Pegasi are accessible at http://polarbase.irap.omp.eu/. Astrometric measurements for 8 UMi are openly available from the Gaia archive (https://gea.esac.esa.int/archive/). The HIRES radial velocity measurements, ESPaDOnS spectra and spectropolarimetric data products, ASAS-SN time series, traces of the MCMC sampling from the radial velocity fits, MESA binary simulation inlists and SED data are available at https://zenodo.org/record/7668534. Code availability. The radial velocity fitting was performed using the exoplanet code (https://docs.exoplanet.codes/). The Generalized Lomb–Scargle periodogram implementation is available at https://github.com/mzechmeister/GLS. TESS-SIP for correcting TESS systematics is provided at https://github.com/christinahedges/TESS-SIP. The asteroseismic modelling was performed using BASTA (https://github.com/BASTAcode/BASTA), the PARAM web tool (http://stev.oapd.inaf.it/cgi-bin/param) and MESA (https://docs.mesastar.org). The binary module of MESA was used for binary simulations. Calibrated asteroseismic scaling relations used asfgrid (http://www.physics.usyd.edu.au/k2gap/Asfgrid/). Grids of isochrones publicly available are MIST (https://waps.cfa.harvard.edu/MIST/), PARSEC (https://github.com/philrosenfield/padova_tracks/releases/tag/v2.0), Dartmouth and GARSTEC (https://zenodo.org/record/6597404) and BASTI (http://albione.oa-teramo.inaf.it/). The authors declare no competing interests.

Attached Files

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Additional details

August 22, 2023
October 23, 2023