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Published May 4, 2023 | Supplemental Material
Journal Article Open

An infrared transient from a star engulfing a planet


Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun. Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star. However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae—a class of eruptions now confirmed to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 1035 erg s⁻¹) and radiated energy (approximately 6.5 × 10⁴¹ erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.

Additional Information

© The Author(s), under exclusive licence to Springer Nature Limited 2023. K.D.'s work was supported by NASA through NASA Hubble Fellowship grant no. HST-HF2-51477.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. M.M.'s contributions were supported by the National Science Foundation under grant no. 1909203. A.-C.E. acknowledges support by NASA through NASA Hubble Fellowship grant no. HF2-51434 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. S.R.K. thanks the Heising-Simons Foundation for supporting his research. We thank B. Metzger, T. Matsumoto, M. Soares-Furtado and J. van Roestel for discussions. The discovery of the optical transient was based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under grant nos. AST-1440341 and AST-2034437, and by a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, Trinity College Dublin, the University of Wisconsin at Milwaukee, IN2P3 France, Lawrence Livermore National Laboratories and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC and UW. The ZTF forced-photometry service was funded under Heising-Simons Foundation grant no. 12540303 (PI: Graham). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership including the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and Academia Sinica. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have had the opportunity to conduct observations from this mountain. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Contributions. K.D. identified the object, initiated follow-up observations, carried out the analysis and wrote the manuscript. M.M. and A.L. led the theoretical interpretation of the transient, created the models presented in this work and wrote the manuscript. V.K., J.E.J., A.-C.E., L.A.H., M.M.K. and R.M.L. assisted with optical/IR follow-up observations, data interpretation and analysis. D.C., C.C., E.K., S.R.K., R.S. and A.V. assisted with interpretation of the data. R.D., M.J.G., F.M., M.S.M., R.L.R. and B.R. are builders of the ZTF observing system and contributed to survey operations during the observations presented here. A.M.M. assisted with analysis of NEOWISE data. N.P. and R.T. assisted with acquisition of SMA data and carried out SMA data analysis. L.H.Q.-N. and L.O.S. assisted with acquistion of VLA data and carried out VLA data analysis. All authors contributed to scientific interpretation. Data availability. All the data used in this work are provided in Extended data. Code availability. K.D. will provide python code used to analyse the observations, and any data used to generate figures, on request. The RLOF code used to model the pre-outburst light curve is publicly available. The authors declare no competing interests.

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Additional details

August 22, 2023
August 22, 2023