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Peer-reviewedCC BY-NC-ND 4.0Proceedings of the National Academy of Sciences of the United States of America

The survival paradox of close-in planets orbiting red-clump giants

  • Wen-Xu Lin1
  • Sheng-Bang Qian1
  • Qi-Bin Sun2,3
  • Li-Ying Zhu2,3
  • Wen-Ping Liao2,3
  • Ting Li4
  • Fu-Xing Li1
  • Xiang-Dong Shi2,3
  • Tao Wang1
  • Er-Gang Zhao2,3
  • Hui-Ting Zhang1
  • Ping Li1
4 affiliations
  1. 1Department of Astronomy, School of Physics and Astronomy, Yunnan University, Kunming 650091, People’s Republic of China
  2. 2Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, People’s Republic of China
  3. 3University of Chinese Academy of Sciences, Beijing 101408, People’s Republic of China
  4. 4Yunnan Training Center for Ethnic Leadership, Yunnan Minzu University, Kunming 650504, People’s Republic of China
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Abstract

Close-in planets orbiting core-helium-burning red-clump giants directly challenge tidal-engulfment theory, which predicts that planets within about 1 au of Sun-like stars should be engulfed during red-giant-branch (RGB) expansion. The existence of such survivors would imply missing physical processes in the late stages of stellar and planetary evolution. Yet only one such “forbidden” planet had been identified to date. Here we report three systems—24 Boo b, HD 12648 b, and TYC 4282-605-1 b—whose host stars are confirmed red-clump giants through asteroseismology, showing that each has already undergone dramatic expansion and contraction following helium ignition. Their planets now reside at orbital separations smaller than the maximum stellar radii attained during the RGB phase, implying that they should not have survived. Among them, 24 Boo b orbits at only 0.183 au with a 30-d period, making it the closest planet known around a red-clump giant. These findings demonstrate that this paradox is not an isolated anomaly but signals the emergence of a class of evolved planetary systems. Their survival defies current models of stellar–planetary coevolution and points to unexplored physics shaping planetary architectures around post-RGB stars.

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Cite

BibTeXRIS
BibTeX
@article{lin2026survival,
  title = {{The survival paradox of close-in planets orbiting red-clump giants}},
  author = {Wen-Xu Lin and Sheng-Bang Qian and Qi-Bin Sun and Li-Ying Zhu and Wen-Ping Liao and Ting Li and Fu-Xing Li and Xiang-Dong Shi and Tao Wang and Er-Gang Zhao and Hui-Ting Zhang and Ping Li},
  journal = {Proceedings of the National Academy of Sciences of the United States of America},
  year = {2026},
  month = oct,
  doi = {10.1073/pnas.2603847123},
  url = {https://doi.org/10.1073/pnas.2603847123},
  note = {CC BY-NC-ND 4.0},
}
RIS
TY  - JOUR
TI  - The survival paradox of close-in planets orbiting red-clump giants
AU  - Wen-Xu Lin
AU  - Sheng-Bang Qian
AU  - Qi-Bin Sun
AU  - Li-Ying Zhu
AU  - Wen-Ping Liao
AU  - Ting Li
AU  - Fu-Xing Li
AU  - Xiang-Dong Shi
AU  - Tao Wang
AU  - Er-Gang Zhao
AU  - Hui-Ting Zhang
AU  - Ping Li
PY  - 2026
DA  - 2026/10/22
JO  - Proceedings of the National Academy of Sciences of the United States of America
DO  - 10.1073/pnas.2603847123
UR  - https://doi.org/10.1073/pnas.2603847123
AB  - Close-in planets orbiting core-helium-burning red-clump giants directly challenge tidal-engulfment theory, which predicts that planets within about 1 au of Sun-like stars should be engulfed during red-giant-branch (RGB) expansion. The existence of such survivors would imply missing physical processes in the late stages of stellar and planetary evolution. Yet only one such “forbidden” planet had been identified to date. Here we report three systems—24 Boo b, HD 12648 b, and TYC 4282-605-1 b—whose host stars are confirmed red-clump giants through asteroseismology, showing that each has already undergone dramatic expansion and contraction following helium ignition. Their planets now reside at orbital separations smaller than the maximum stellar radii attained during the RGB phase, implying that they should not have survived. Among them, 24 Boo b orbits at only 0.183 au with a 30-d period, making it the closest planet known around a red-clump giant. These findings demonstrate that this paradox is not an isolated anomaly but signals the emergence of a class of evolved planetary systems. Their survival defies current models of stellar–planetary coevolution and points to unexplored physics shaping planetary architectures around post-RGB stars.
SN  - 0027-8424
SN  - 1091-6490
ER  -