Detection of molecular absorption in the dayside of exoplanet 51 Pegasi b?

TL;DR

This study used high-resolution ground-based spectroscopy to detect molecular absorption features from 51 Pegasi b's dayside atmosphere, revealing potential non-inverted atmospheric structure but with inconsistent signals across observation nights.

Contribution

First detection of molecular absorption in the dayside spectrum of 51 Pegasi b using high-resolution spectroscopy, highlighting atmospheric composition and structure.

Findings

Detection of CO and water absorption at 5.9σ confidence in initial data

Derived planet mass of 0.46 ± 0.02 M_Jup and high orbital inclination

Inconsistent signal presence across different observation nights

Abstract

In this paper we present ground-based high-resolution spectroscopy of 51 Pegasi using CRIRES at the Very Large Telescope. The system was observed for 3x5 hours at 2.3 {\mu}m at a spectral resolution of R = 100,000, targeting potential signatures from carbon monoxide, water vapour and methane in the planet's dayside spectrum. In the first 2x5 hours of data, we find a combined signal from carbon monoxide and water in absorption at a formal 5.9{\sigma} confidence level, indicating a non-inverted atmosphere. We derive a planet mass of M_P = (0.46 +- 0.02) M_Jup and an orbital inclination i between 79.6 and 82.2 degrees, with the upper limit set by the non-detection of the planet transit in previous photometric monitoring. However, there is no trace of the signal in the final 5 hours of data. A statistical analysis indicates that the signal from the first two nights is robust, but we find no compelling explanation for its absence in the final night. The latter suffers from stronger noise residuals and greater instrumental instability than the first two nights, but these cannot fully account for the missing signal. It is possible that the integrated dayside emission from 51 Peg b is instead strongly affected by weather. However, more data are required before we can claim any time variability in the planet's atmosphere.