Atmosphere of a Planet around a White Dwarf: Hydrocarbons, Aerosols, and Nighttime Thermal Emission

Original: "Aerosols and hydrocarbons in the atmosphere of a white dwarf planet"

· Ryan J. MacDonald, Christopher E. O'Connor, Victoria A. Boehm, E. M. May, David K. Sing, Elijah Mullens, L. C. Mayorga, Trevor O. Foote, Simon Blouin, Logan A. Pearce, Nikole K. Lewis, Jeff Valenti, Natasha E. Batalha, Maura Lally, Joshua D. Lothringer, Mark S. Marley, Ishan Mishra, Susan E. Mullally

arXiv:2607.01316v1 · 2026-07-01 · CC BY 4.0 · ⏱ 3 min · Exoplanets Stellar

JWST has for the first time studied the atmosphere of a giant planet that survived its star's death, finding methane, aerosols, and thermal glow from the night side.

Abstract

Transmission spectroscopy of exoplanet WD 1856 b at a white dwarf was performed with the JWST's NIRSpec PRISM instrument in the 0.5–5.0 μm range. The spectrum reveals the presence of hydrocarbons (odds ratio in favor of CH₄ 17:1–30:1), aerosols (odds ratio 2×10⁵:1–2×10⁶:1), and thermal emission from the nightside (2×10⁶³:1–2×10⁷³:1). The analysis constrains the object’s mass to 4.3–10.9 Jupiter masses, reveals a carbon-enriched atmosphere with a methane abundance of about 7% and an effective temperature of 390–412 K, significantly higher than the equilibrium value (~160 K). Cooling models point to a heating event associated with migration that occurred 3.0–5.5 billion years ago, already in the white dwarf stage, consistent with tidal evolution to the current circular orbit of radius 0.02 AU. These results open up the possibility of studying the ultimate fate of giant planets around solar-type stars.

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Context

After a Sun-like star becomes a red giant and then a white dwarf, the fate of its planets has long been a mystery. White dwarfs, predicted by Subrahmanyan Chandrasekhar, are remnants of stellar evolution. Recent discoveries of exoplanets orbiting them have confirmed that planets can survive this cataclysmic phase. However, the composition and physical conditions of their atmospheres remained unknown. The study of WD 1856 b — the first well-characterized planet around a white dwarf — sheds light on how giant planets retain and alter their gaseous envelopes after the star's death.

Methods

Observations with the James Webb Space Telescope (JWST) in NIRSpec PRISM mode yielded a transmission spectrum of planet WD 1856 b using the transit photometry method. The data were processed with two independent pipelines accounting for a grazing transit, where the planet only partially covers the white dwarf's disk. Modeling used the Bayesian code POSEIDON, adapted to account for thermal emission from the night side and incomplete overlap. The atmosphere was assumed to be rich in hydrogen with a dash of helium and spectral contributions of possible molecules. The analysis also included aerosols — an opaque cloud deck and a haze that scatters short-wavelength radiation.

Results

The resulting spectrum (0.5–5.0 µm) showed significant signatures of at least one hydrocarbon (odds ratios 167:1–5377:1), with methane CH4 (17:1–30:1) being the most likely, along with some ethane C2H6. Thermal emission from the night side (Bayes factor ln B ~ 146–169) and aerosols (ln B ~ 12–14) were also robustly detected. The atmosphere is carbon-enriched: about 7% CH4, corresponding to a metallicity ~100 times solar. The planet's mass is constrained for the first time to 4.3–10.9 M_J, and its effective emission temperature is 390–412 K, significantly above equilibrium (160 K). Thermal evolution models indicate that the planet's heating occurred 3.0–5.5 billion years after the asymptotic giant branch phase, ruling out common envelope evolution and consistent with high-eccentricity migration. Upper limits on water and ammonia showed no significant detections.

Implications

These results provide the first detailed picture of a planet's atmosphere after a star's death. The carbon enrichment links WD 1856 b to Neptune and points to accretion of volatile-rich material before or after migration. The presence of aerosols is consistent with high metallicity, which enhances haze formation. Detection of night-side thermal emission opens up new possibilities for directly measuring the thermal properties of planets around white dwarfs.

Future development

Further observations at longer wavelengths (JWST MIRI) will refine the effective temperature and aerosol composition. As more planets around white dwarfs are discovered, statistical studies of the consequences of stellar evolution will become possible. Theoretical work should improve cooling models for objects with high metallicity and masses around 7 M_J, to reconcile the temperature and radius of WD 1856 b.

Impact

The work impacts several fields: exoplanet atmospheres (first data on chemical composition after the main sequence), stellar evolution (connection to white dwarfs), planetary system dynamics (migration via tides and Lidov–Kozai effects).

Next steps

As part of the JWST Cycle 4 program, mid-infrared spectra (GO-9033, GO-9157) are planned for WD 1856 b. In parallel, the search for new transiting planets around white dwarfs continues to build up statistics.

Key open problems

The study directly addresses unsolved problems in physics: the fate of planetary systems after a star's death, the survival and migration mechanisms of planets through red giant and white dwarf stages, and carbon enrichment of atmospheres (including a possible link to prebiotic chemistry). It also provides a unique test of cooling models for substellar objects under conditions different from field objects.

🎯 The planet WD 1856 b is seven times larger than its white dwarf, so during transit it only partially covers the star — a phenomenon called a grazing transit. This is the first system with such geometry to have a detailed atmospheric spectrum obtained.

\frac{A_p}{A_*} = \frac{1}{\pi}\left[ \left(\frac{R_p}{R_*}\right)^2 \left(\theta - \frac{1}{2}\sin 2\theta\right) + \left(\phi - \frac{1}{2}\sin 2\phi\right) \right]

Dependence of the observed overlap area on the planet radius R_p, star radius R_*, and impact parameter b (included in angles θ and ϕ).

T_{\rm eff} = \left( \frac{1}{\sigma_{\rm SB}} \int F_{\rm p,surf,\lambda} \, d\lambda \right)^{1/4}

Temperature of a black body emitting the same integrated power per unit surface area.

\Delta_\lambda = \frac{A_{\rm p}^{(\rm top)} - \int_{A_{\rm p}} T_\lambda \, dA}{\pi R_*^2} \times \left( \frac{1}{1 + \frac{R_{\rm p,(night),\lambda}^2}{R_*^2} \frac{F_{\rm p (night),surf,\lambda}}{F_{*,\rm surf,\lambda}}} \right)

Transmission coefficient diluted by photons from the planet's night side.

Key numbers

mass of WD 1856 b: 4.3–10.9 M_J
effective temperature: 390–412 K
equilibrium temperature: 160 K
odds ratios for hydrocarbons: 167:1 – 5377:1
cooling age of the white dwarf: 5.4 ± 0.7 billion years

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