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

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× 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 p,λ /R 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 J (to 2 σ 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.

TRIDENT: A Rapid 3D Radiative Transfer Model for Exoplanet Transmission Spectra

“…Another extreme example of a grazing transit is WD 1856+534b (Vanderburg et al 2020) -a planet transiting a white dwarf with R p /R * ≈ 8 -for which we show a model spectrum, assuming a Jupiterlike composition, in Figure 13 (bottom panel). Recently, we demonstrated the principles of TRIDENT's grazing transit modelling in an analysis of Gemini/GMOS observations of WD 1856+534b (Xu et al 2021). With the success of the Transiting Exoplanet Survey Satellite (TESS), the number of planets with grazing transits is steadily increasing.…”

Secular chaos in white-dwarf planetary systems: origins of metal pollution and short-period planetary companions

“…Another object of interest is WD 1856+534 b, an intact, Jupiter-sized body transiting its WD host with a period of 1.4 d (Vanderburg et al 2020). The mass of the transiting companion is not known precisely, but it is constrained within the range of ∼ 1-14MJ (Vanderburg et al 2020;Xu et al 2021). Various dynamical histories have been proposed for this system, notably high-eccentricity migration through the Lidov-Kozai mechanism driven by the WD's bound stellar companions (Muñoz & Petrovich 2020;O'Connor, Liu & Lai 2021;Stephan, Naoz & Gaudi 2020).…”