The search for reflected light from 51 Peg b at high spectral resolution

 Exoplanets exhibit a greater range of properties than solar system worlds. Studying the diverse exoplanets produces a better understanding of planet formation and planetary processes writ large. For example, observing and modelling atmospheric dynamics and clouds on exoplanets may eventually help us to comprehensively understand winds and cloud formation here on Earth. However, clouds are a major observational challenge for exoplanets and are universally a theoretical challenge. Moreover, studying the atmospheres of exoplanets is a promising avenue to empirically determine the prevalence of life elsewhere in the Galaxy.

 

Exoplanets have been mostly studied in transmission or emission, and comparatively few studies have focused on reflected light. However, light reflected by a planet offers unique insights into its atmosphere and surface.  Reflected light from exoplanets has been detected using photometry, polarimetry, and low to medium-resolution spectroscopy. These results indicate that the connection between Bond and geometric albedo for hot Jupiters is highly non-trivial. Reflected light measurements could help to elucidate this relationship and better understand the nature of clouds on hot Jupiters. Furthermore, near-term searches for biosignatures through the 2030s will rely on detecting reflected light.  Next-generation instruments like VLT/RISTRETTO or ELT/ANDES will aim to detect biosignatures with near-infrared reflected light from temperate rocky planets orbiting M-dwarfs, and NASA’s Habitable Worlds Observatory will aim to detect visible reflected light from Earth-like planets orbiting Sun-like stars. 

 

There has been one highly contested detection of reflected light using high-resolution spectroscopy from the hot Jupiter 51 Peg b. With its bright host star and close-in orbit, 51 Peg b is among the very best targets for high-spectral-resolution reflected light studies. We have obtained observations of 51 Peg b with the high-resolution optical spectrograph Maroon-X at Gemini North. The large collecting area of this telescope greatly improves the signal-to-noise of our observations, allowing us to investigate the geometric albedo of 51 Peg b with greater sensitivity. Additionally, we have examined the reflected light detection capabilities of high-resolution cross-correlation spectroscopy methods adapted from transmission and emission studies. Our results suggest that direct detections of reflected light at high-spectral resolution may be more difficult than previously predicted. This may indicate that it will not be possible to detect the much fainter signals of reflected light, and by extension biosignatures, from Earth-like planets with the next generation of instruments and observatories.