An Earth-mass planet and a brown dwarf in orbit around a white dwarf

Creators: Zhang, Keming^{1, 2}; Zang, Weicheng^{3, 4}; El-Badry, Kareem⁵; Lu, Jessica R.²; Bloom, Joshua Simon²; Agol, Eric⁶; Gaudi, B. Scott⁷; Konopacky, Quinn¹; LeBaron, Natalie²; Mao, Shude³; Terry, Sean⁸

1. University of California, San Diego
2. University of California, Berkeley
3. Tsinghua University
4. Harvard-Smithsonian Center for Astrophysics
5. California Institute of Technology
6. University of Washington
7. The Ohio State University
8. Goddard Space Flight Center

Abstract

It has been theorized that terrestrial planets born beyond 1–3 au could avoid being engulfed during the red-giant phases of their host stars. Nevertheless, only a few gas-giant planets have been observed around white dwarfs (WDs), the end product left behind by a red giant. Here we report on evidence that the lens system that produced the microlensing event KMT-2020-BLG-0414 is composed of a WD orbited by an Earth-mass planet and a brown dwarf companion, as shown by the non-detection of the lens flux using Keck adaptive optics. From microlensing orbital motion constraints, we determine the planet to be a 1.9 ± 0.2 Earth-mass (M_⊕) planet at a physical separation of 2.1 ± 0.2 au from the WD during the event. By considering the system’s evolutionary history, we determine the brown dwarf companion to have a projected separation of 22 au from the WD and reject a degenerate model that places the brown dwarf at 0.2 au. Given the planetary orbital expansion during the final evolutionary stages of the host star, this Earth-mass planet may have existed in an initial orbit close to 1 au, thereby offering a glimpse into the possible survival of planet Earth in the distant future.

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Acknowledgement

K.Z. is supported by the Eric and Wendy Schmidt AI in Science Postdoctoral Fellowship, a Schmidt Futures programme. K.Z. and J.S.B. were partially supported by the Gordon and Betty Moore Foundation and a grant from the National Science Foundation (Award No. 2206744). W.Z. acknowledges support from the Harvard-Smithsonian Center for Astrophysics through a fellowship. W.Z. and S.M. acknowledge support from the National Natural Science Foundation of China (Grant No. 12133005). J.R.L. acknowledges support from the National Science Foundation (Grant No. 1909641) and the Heising-Simons Foundation (Grant No. 2022-3542). K.Z. thanks M. He for assistance on an initial version of the cover art proposal. W.Z. thanks H. Wang for fruitful discussions on the Keck proposal. This research has made use of the KMTNet system operated by the Korea Astronomy and Space Science Institute. The data were obtained at three host sites, namely the Cerro Tololo Inter-American Observatory in Chile, the South African Astronomical Observatory in South Africa and the Siding Spring Observatory in Australia. Data transfer from a host site to the Korea Astronomy and Space Science Institute was supported by the Korea Research Environment Open Network. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration (NASA). The observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the Native Hawaiian community. We are most fortunate to have had the opportunity to conduct observations from this mountain. This research uses data obtained through the Telescope Access Program, which was funded by member institutes and was partly based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and DAPNIA, which is a part of the French Alternative Energies and Atomic Energy Commission, at the CFHT, which is operated by the National Research Council of Canada, the Institut National des Science de l’Univers of the French National Centre for Scientific Research and the University of Hawaii. The observations at the CFHT were performed with care and respect at the summit of Mauna Kea, which is a significant cultural and historic site.

Data Availability

The reduced Keck images are available via Zenodo at https://doi.org/10.5281/zenodo.13128167 (ref. ⁶¹). The raw data will be available on the Keck Observatory Archive (https://koa.ipac.caltech.edu/) after the 18-month proprietary period.

Supplemental Material

Supplementary Figs. 1–3 and Table 1.

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