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A close-in giant planet escapes engulfment by its star

Nature volume 618, pages 917–920 (2023)Cite this article

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Abstract

When main-sequence stars expand into red giants, they are expected to engulf close-in planets1,2,3,4,5. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants6,7,8 has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars9. Here we present the discovery that the giant planet 8 Ursae Minoris b10 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 planet11. 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.

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Fig. 1: Radial velocity measurements of 8 UMi.
Fig. 2: Identification of the evolutionary state of 8 UMi as a core-helium-burning red giant.
Fig. 3: Canonical single-star evolutionary models for the 8 UMi.
Fig. 4: A possible history of host star 8 UMi in the form of a stellar merger.

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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/).

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Acknowledgements

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.

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Authors and Affiliations

  1. Institute for Astronomy, University of Hawaii, Honolulu, HI, USA

    Marc Hon, Daniel Huber, Daniel R. Hey, Jingwen Zhang, Benjamin J. Shappee, Jamie Tayar, Zachary R. Claytor, Casey Brinkman, Ashley Chontos & Joel Ong

  2. Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, Camperdown, New South Wales, Australia

    Daniel Huber, Timothy R. Bedding & Dennis Stello

  3. TAPIR, California Institute of Technology, Pasadena, CA, USA

    Nicholas Z. Rui & Jim Fuller

  4. Centre for Exoplanets and Habitability, University of Warwick, Coventry, UK

    Dimitri Veras

  5. Centre for Space Domain Awareness, University of Warwick, Coventry, UK

    Dimitri Veras

  6. Department of Physics, University of Warwick, Coventry, UK

    Dimitri Veras

  7. Center for Astronomy (Landessternwarte), Heidelberg University, Heidelberg, Germany

    James S. Kuszlewicz

  8. Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark

    James S. Kuszlewicz, Amalie Stokholm & Jakob Lysgaard Rørsted

  9. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden

    Oleg Kochukhov

  10. Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Bologna, Italy

    Amalie Stokholm

  11. INAF–Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Bologna, Italy

    Amalie Stokholm

  12. Department of Astronomy and Space Sciences, Science Faculty, Ege University, İzmir, Turkey

    Mutlu Yıldız, Zeynep Çelik Orhan & Sibel Örtel

  13. Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany

    Chen Jiang & Steven Giacalone

  14. Department of Astronomy, University of California Berkeley, Berkeley, CA, USA

    Howard Isaacson

  15. Department of Astronomy, The Ohio State University, Columbus, OH, USA

    Mathieu Vrard

  16. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, USA

    Keivan G. Stassun

  17. Department of Astronomy, Bryant Space Science Center, University of Florida, Gainesville, FL, USA

    Jamie Tayar, Zachary R. Claytor, Rae Holcomb & Jack Lubin

  18. Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, USA

    Corey Beard

  19. Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Porto, Portugal

    Tiago L. Campante

  20. Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Porto, Portugal

    Tiago L. Campante

  21. School of Physics and Astronomy, University of Birmingham, Birmingham, UK

    William J. Chaplin

  22. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA

    Ashley Chontos

  23. Department of Astronomy, California Institute of Technology, Pasadena, CA, USA

    Andrew W. Howard

  24. Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, CA, USA

    Mason MacDougall, Dakotah Tyler & Judah Van Zandt

  25. School of Physics, University of New South Wales, Sydney, New South Wales, Australia

    Benjamin T. Montet & Dennis Stello

  26. UNSW Data Science Hub, University of New South Wales, Sydney, New South Wales, Australia

    Benjamin T. Montet

  27. Australian Centre for Astrobiology, University of New South Wales, Sydney, New South Wales, Australia

    Benjamin T. Montet

  28. Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA, USA

    Joseph M. A. Murphy

  29. Department of Astronomy, Yale University, New Haven, CT, USA

    Joel Ong & Malena Rice

  30. Department of Earth and Planetary Sciences, University of California, Riverside, Riverside, CA, USA

    Daria Pidhorodetska

  31. Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA

    Alex S. Polanski

  32. Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA

    Malena Rice

  33. ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO-3D), Sydney, New South Wales, Australia

    Dennis Stello

  34. Department of Physics and Astronomy, University of Notre Dame, Notre Dame, IN, USA

    Lauren M. Weiss

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  1. Marc Hon
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  2. Daniel Huber
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  13. Chen Jiang
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  14. Daniel R. Hey
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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.

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Correspondence to Marc Hon.

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Extended data figures and tables

Extended Data Fig. 1 Posterior probability distribution of the fit to the combined BOES/HIRES radial velocity data.

The 16th, 50th, and 84th percentile values of each fitted parameter are indicated with dashed lines.

Extended Data Fig. 2 Constraints to the outer companion in the 8 UMi planetary system.

Contours indicate 67% highest density intervals of permissible mass and separation values of the outer companion as estimated from the residuals of radial velocity measurements in Main Text Fig. 2 (purple), and from measurements of 8 UMi’s Gaia DR3-Hipparcos astrometric acceleration (green). These two measurements jointly constrain the outer companion’s mass and separation, which corresponds to the locus indicated by the shaded region between the contours.

Extended Data Fig. 3 Stellar activity of the host star 8 UMi and active red giant TYC 3542-1885-1.

The chromospheric activity of both stars are estimated using Ca II H and K indices (SHK) computed from Keck/HIRES spectra, with error bars indicating 1σ (standard deviation) uncertainties. (a-b) Variations of SHK with radial velocity from each star. Included for each are the Spearman correlation factors (R) and two-sided p-values (p) for the test whose null hypothesis is that SHK and radial velocity are uncorrelated. (c-d) Generalized Lomb Scargle (GLS) periodograms of radial velocity measurements and SHK. The vertical dashed line indicates 8 UMi b’s orbital period, and the horizontal lines indicate the periodogram’s False Alarm Probability (FAP).

Extended Data Fig. 4 Spectral features of 8 UMi and active red giant TYC 3542-1885-1.

Comparisons are additionally made with the inactive, Li-normal giant µ Pegasi. (a-b) The Ca II H and K absorption lines. (c) The 6707.8 Å Li I absorption line.

Extended Data Fig. 5 ESPaDOnS spectropolarimetry of the host star 8 UMi and active red giant TYC 3542-1885-1.

The least-squares deconvolution profiles in each panel, from top to bottom, are that of Stokes V, null polarisation N, and Stokes I, respectively. Error bars indicate 1σ (standard deviation) uncertainties for the profiles. Included are the Stokes V mean longitudinal magnetic field strength (BZ) and its corresponding 1σ (standard deviation) uncertainty, polarimetric signal-to-noise ratio (SNR), and observation times (t) in BJD - 2459000. Panels (a–d) correspond to observations of 8 UMi, whilst panels (e–h) correspond to observations of TYC 3542-1885-1.

Extended Data Fig. 6 Observed photometric variations of 8 UMi.

Time series photometry from (a) Hipparcos, (b) ASAS-SN, and (c) systematics-corrected TESS Simple Aperture Photometry. The standard deviation uncertainty for each photometric measurement is shown with error bars. These are visible for the Hipparcos data, but smaller than the symbol sizes for ASAS-SN and TESS data. The dispersion of the Hipparcos and ASAS-SN time series, σdisp, are quantified as a fraction of the star’s apparent magnitude. Generalized Lomb Scargle (GLS) periodograms for the (d) Hipparcos, (e) ASAS-SN, and (f) systematics-corrected TESS Simple Aperture Photometry light curves. The vertical dashed line indicates 8 UMi b’s orbital period, and the horizontal lines indicate the periodogram’s False Alarm Probability (FAP).

Extended Data Fig. 7 Spectral energy distribution of 8 UMi.

The distribution was estimated using BTVT magnitudes from Tycho-2, the JHKS magnitudes from 2MASS, the W1–W4 magnitudes from WISE, the GGBPGRP magnitudes from Gaia, and the NUV magnitude from GALEX. Red symbols represent the observed photometric measurements, where the horizontal error bars represent the effective width of the passband while the vertical error bars are 1σ (standard deviation) photometric uncertainties. Blue symbols are the model fluxes from the best-fit Kurucz atmosphere model (black), which have a reduced χ2 of 1.3, with extinction Av = 0.06 ± 0.02 mag, Teff = 4,900 ± 75 K, surface gravity log (g) = 2.5 ± 0.5 dex, and [Fe/H] = −0.5 ± 0.3 dex. Integration of the (unreddened) model SED gives the bolometric flux at Earth, Fbol = 6.48 ± 0.22 × 10−8 erg s−1cm−2.

Extended Data Fig. 8 Simulation of a stellar binary history for 8 UMi leading up to a stellar merger.

This fiducial model is simulated using β = 0.6, q = 0.7, and Pinit = 2 d, with the stellar merger occuring at the onset of unstable mass transfer at t ≈ 8.6 Gyr. (a) Binary separation (purple solid line) and orbital period (green dashed line) versus time for the simulated binary model. (b) Primary total and core masses versus time. (c) Secondary total/core mass versus time.

Extended Data Fig. 9 Simulated white dwarf–red giant binaries that successfully merge to produce a core-helium burning giant like 8 UMi.

These models are simulated using β = 0.6 and q = 0.7, with M1 = 1.23 M☉ and M2 = 0.86 M☉. (a) Final total mass Mtot,f and final helium core mass Mc,f of the merger remnant. (b) Binary separation and orbital period before (ai) and after (af) the common-envelope event. The orange region represents af values that result in a stellar merger.

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Hon, M., Huber, D., Rui, N.Z. et al. A close-in giant planet escapes engulfment by its star. Nature 618, 917–920 (2023). https://doi.org/10.1038/s41586-023-06029-0

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