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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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.
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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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.
The authors declare no competing interests.
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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.
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.
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).
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.
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).
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