Gemini/GMOS Transmission Spectroscopy of the Grazing Planet Candidate WD 1856+534 b

TL;DR

This study presents a transmission spectrum of the candidate planet WD 1856+534 b transiting a white dwarf, introducing a new modeling approach for grazing transits and constraining its mass despite observational challenges.

Contribution

We introduce a limb darkening corrected, time-averaged transmission spectrum and a modified radiative transfer method for analyzing grazing transits, providing new tools for exoplanet atmospheric characterization.

Findings

No prominent absorption features detected in the spectrum.

Mass of WD 1856+534 b constrained to > 0.84 M_J with 2σ confidence.

The system's unique transit geometry challenges standard modeling assumptions.

Abstract

WD 1856+534 b is a Jupiter-sized, cool giant planet candidate transiting the white dwarf WD 1856+534. Here, we report an optical transmission spectrum of WD 1856+534 b obtained from ten transits using the Gemini Multi-Object Spectrograph. This system is challenging to observe due to the faintness of the host star and the short transit duration. Nevertheless, our phase-folded white light curve reached a precision of 0.12 %. WD 1856+534 b provides a unique transit configuration compared to other known exoplanets: the planet is $8\times$ larger than its star and occults over half of the stellar disc during mid-transit. Consequently, many standard modeling assumptions do not hold. We introduce the concept of a `limb darkening corrected, time-averaged transmission spectrum' and propose that this is more suitable than $(R_{\mathrm{p}, \lambda} / R_{\mathrm{s}})^2$ for comparisons to atmospheric models for planets with grazing transits. We also present a modified radiative transfer prescription. Though the transmission spectrum shows no prominent absorption features, it is sufficiently precise to constrain the mass of WD 1856+534 b to be > 0.84 M$_\mathrm{J}$ (to $2 \, \sigma$ confidence), assuming a clear atmosphere and a Jovian composition. High-altitude cloud decks can allow lower masses. WD 1856+534 b could have formed either as a result of common envelope evolution or migration under the Kozai-Lidov mechanism. Further studies of WD 1856+534 b, alongside new dedicated searches for substellar objects around white dwarfs, will shed further light on the mysteries of post-main sequence planetary systems.