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

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 3 × 5 hr at 2.3 μm at a spectral resolution of R = 100,000, targeting potential signatures from carbon monoxide, water vapor, and methane in the planet's dayside spectrum. In the first 2 × 5 hr of data, we find a combined signal from carbon monoxide and water in absorption at a formal 5.9σ 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, 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 five 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.