Evidence for multiple molecular species in the hot Jupiter HD 209458b

TL;DR

This study uses high-resolution spectroscopy to confirm molecular species in HD 209458b's atmosphere, including potential detection of HCN, offering insights into its composition and formation history.

Contribution

First detection of HCN in HD 209458b's atmosphere using high-resolution spectroscopy, expanding knowledge of molecular diversity in hot Jupiter atmospheres.

Findings

Confirmed CO and H$_2$O in HD 209458b

Detected potential HCN signal with S/N of 4.7

Implications for C/O ratio and planetary origins

Abstract

Molecular species in planetary atmospheres provide key insights into their atmospheric processes and formation conditions. In recent years, high-resolution Doppler spectroscopy in the near-infrared has allowed detections of H$_2$O and CO in the atmospheres of several hot Jupiters. This method involves monitoring the spectral lines of the planetary thermal emission Doppler-shifted due to the radial velocity of the planet over its orbit. However, aside from CO and H$_2$O, which are the primary oxygen- and carbon-bearing species in hot H$_2$-rich atmospheres, little else is known about molecular compositions of hot Jupiters. Several recent studies have suggested the importance and detectability of nitrogen-bearing species in such atmospheres. In this Letter, we confirm potential detections of CO and H$_2$O in the hot Jupiter HD 209458b using high-resolution spectroscopy. We also report a cross-correlation peak with a signal-to-noise ratio of $4.7$ from a search for HCN. The results are obtained using high-resolution phase-resolved spectroscopy with the Very Large telescope CRyogenic high-resolution InfraRed Echelle Spectrograph (VLT CRIRES) and standard analysis methods reported in the literature. A more robust treatment of telluric contamination and other residuals would improve confidence and enable unambiguous molecular detections. The presence of HCN could provide constraints on the C/O ratio of HD~209458b and its potential origins.