Deep exploration of the planets HR 8799 b, c, and d with moderate-resolution spectroscopy

Jean-Baptiste Ruffio*, Quinn M. Konopacky, Travis Barman, Bruce Macintosh, Kielan K. W. Hoch, Robert J. De Rosa, Jason J. Wang, Ian Czekala, Christian Marois

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


The four directly imaged planets orbiting the star HR 8799 are an ideal laboratory to probe atmospheric physics and formation models. We present more than a decade's worth of Keck/OSIRIS observations of these planets, which represent the most detailed look at their atmospheres to date by its resolution and signal-to-noise ratio. We present the first direct detection of HR 8799 d, the second-closest known planet to the star, at moderate spectral resolution with Keck/OSIRIS (K band; R ≈ 4000). Additionally, we uniformly analyze new and archival OSIRIS data (H and K band) of HR 8799 b, c, and d. First, we show detections of water (H2O) and carbon monoxide (CO) in the three planets and discuss the ambiguous case of methane (CH4) in the atmosphere of HR 8799 b. Then, we report radial-velocity (RV) measurements for each of the three planets. The RV measurement of HR 8799 d is consistent with predictions made assuming coplanarity and orbital stability of the HR 8799 planetary system. Finally, we perform a uniform atmospheric analysis on the OSIRIS data, published photometric points, and low-resolution spectra. We do not infer any significant deviation from the stellar value of the carbon-to-oxygen ratio (C/O) of the three planets, which therefore does not yet yield definitive information about the location or method of formation. However, constraining the C/O for all the HR 8799 planets is a milestone for any multiplanet system, and particularly important for large, widely separated gas giants with uncertain formation processes.
Original languageEnglish
Article number290
Number of pages27
JournalAstronomical Journal
Issue number6
Publication statusPublished - 6 Dec 2021


  • Extrasolar gaseous giant planets
  • Exoplanet atmospheric composition
  • Direct imaging
  • Exoplanet formation


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