Evidence for Spin-Orbit Alignment in the TRAPPIST-1 System

Teruyuki Hirano, Eric Gaidos, Joshua N. Winn, Fei Dai, Akihiko Fukui, Masayuki Kuzuhara, Takayuki Kotani, Motohide Tamura, Maria Hjorth, Simon Albrecht, Daniel Huber, Emeline Bolmont, Hiroki Harakawa, Klaus Hodapp, Masato Ishizuka, Shane Jacobson, Mihoko Konishi, Tomoyuki Kudo, Takashi Kurokawa, Jun NishikawaMasashi Omiya, Takuma Serizawa, Akitoshi Ueda, Lauren M. Weiss

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36 Scopus citations


In an effort to measure the Rossiter-McLaughlin effect for the TRAPPIST-1 system, we performed high-resolution spectroscopy during transits of planets e, f, and b. The spectra were obtained with the InfraRed Doppler spectrograph on the Subaru 8.2 m telescope, and were supplemented with simultaneous photometry obtained with a 1 m telescope of the Las Cumbres Observatory Global Telescope. By analyzing the anomalous radial velocities, we found the projected stellar obliquity to be λ = 1 ± 28° under the assumption that the three planets have coplanar orbits, although we caution that the radial-velocity data show correlated noise of unknown origin. We also sought evidence for the expected deformations of the stellar absorption lines, and thereby detected the "Doppler shadow" of planet b with a false-alarm probability of 1.7%. The joint analysis of the observed residual cross-correlation map including the three transits gave λ = 19± 13 15. These results indicate that the the TRAPPIST-1 star is not strongly misaligned with the common orbital plane of the planets, although further observations are encouraged to verify this conclusion.

Original languageEnglish (US)
Article numberL27
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Feb 20 2020

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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