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

TL;DR

This study provides the most detailed atmospheric analysis to date of the HR 8799 planets using decade-long Keck/OSIRIS observations, detecting key molecules and measuring radial velocities to inform formation theories.

Contribution

First direct detection of HR 8799 d at moderate resolution, with comprehensive analysis of atmospheric composition and orbital dynamics across four planets.

Findings

Detected water and CO in three planets

Measured radial velocities consistent with system stability

No significant deviation in C/O ratio from stellar value

Abstract

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~4,000). 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 8799b. 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 to 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 ratio 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.