A Planet's Atmosphere Around a White Dwarf: Hydrocarbons, Aerosols, and Thermal Emission ✦
Planets can survive the red giant stage and end up around white dwarfs. The atmosphere of planet WD 1856 b was studied using transmission spectroscopy with the NIRSpec PRISM instrument on JWST. The spectrum in the 0.5–5.0 μm range showed the presence of hydrocarbons (with a preference for CH4 at an odds ratio of 17:1–30:1), aerosols (2×10^5:1–2×10^6:1), and thermal emission from the nightside (2×10^63:1–2×10^73:1). The planet's mass is estimated at 4.3–10.9 Jupiter masses, the atmosphere is enriched in carbon (CH4 abundance ≈7%), and the effective temperature (390–412 K) significantly exceeds the equilibrium temperature (160 K). Cooling models point to a heating event associated with migration, 3.0–5.5 billion years after the star transitioned to the white dwarf stage, which is consistent with tidal evolution to the current circular orbit with a radius of 0.02 AU. These results offer a glimpse into the future of giant planets around solar-type stars.
Context
The fate of planets after a star's death is one of the key mysteries of astrophysics. Exoplanets around white dwarfs offer a unique opportunity to glimpse the finale of planetary system evolution. In recent years, several candidates have been found, but their atmospheres remained unstudied. Subrahmanyan Chandrasekhar laid the theoretical foundations of white dwarf physics, including the mass limit, while observational study of planets around them became possible only with the advent of modern telescopes. Planet WD 1856 b, orbiting at 0.02 AU around a white dwarf about 6 billion years old, became the first object for which a detailed spectrum was obtained.
Methods
The analysis used transmission spectroscopy with the NIRSpec prism on JWST in the 0.5–5.0 μm range, implementing the transit method, to which David Charbonneau made significant contributions. Observations were conducted under program GO-2358. Data were processed by two independent pipelines (FIREFLy and Juniper), and atmospheric parameters were retrieved using the Bayesian code POSEIDON. Due to the grazing transit geometry (the planet is 7 times larger than the star) and strong nightside thermal emission, a new transmission spectrum model accounting for partial disk overlap was required. To interpret the planet's thermal history, numerical simulations of cooling of substellar objects and evolutionary tracks of white dwarfs were used.
Results
In the planet's atmosphere, methane (CH₄) was reliably detected at a significance level of ~3σ, along with aerosols — both an optically thick cloud layer at a depth of ~100 mbar and an above-cloud haze creating a characteristic slope in the short-wavelength part of the spectrum. The methane abundance was about 7%, indicating an enrichment of the atmosphere in carbon by a factor of about 100 compared to the solar value. Strong thermal emission from the nightside was also recorded, allowing measurement of the planet's effective temperature: 390–412 K, much higher than the equilibrium value (160 K). The nightside photosphere analysis was based on Stefan–Boltzmann law, relating radiation flux to temperature. Additionally, the planet's mass was constrained for the first time: 4.3–10.9 M_J. Possible signs of ethane (C₂H₆) and phosphine (PH₃) were found, but their significance is currently insufficient. Thermal evolution modeling showed that the heating occurred 3.0–5.5 billion years after the AGB stage, ruling out the common envelope scenario in favor of migration with high eccentricity, which ended with tidal circularization of the orbit.
Implications
This first detailed study of a planet's atmosphere around a white dwarf confirms that giant planets can not only survive stellar evolution but also migrate to close orbits billions of years after the star becomes a white dwarf. The carbon enrichment points to accretion of comet or planetesimal material enriched in volatiles, consistent with models of white dwarf pollution by metals. The high metal content, in turn, promotes aerosol formation, explaining the strong scattering in the spectrum.
Future development
In the coming years, new observations with JWST (including Cycle 4) using the MIRI instrument will clarify the aerosol composition and confirm the presence of other hydrocarbons. Future missions such as Ariel and ground-based ELTs will be able to study entire populations of planets around white dwarfs, turning them into a laboratory for investigating the final stages of planetary system life.
Impact
The results impact understanding of planet evolution, dynamics of planetary systems after the main sequence, and atmospheric chemistry of cool giants.
Next steps
First of all, longer-wavelength spectra (MIRI LRS/MRS) are needed to unambiguously identify molecules and refine the temperature. Theoretical cooling models should be extended to the case of highly metal-enriched objects.
Key open problems
The study connects several unresolved problems: mechanisms of planet migration after the star leaves the main sequence (the role of tides and dynamical evolution), the origin of heavy elements in gas giants, and the ultimate fate of the Solar System.
🎯 Planet WD 1856 b is 7 times larger than its white dwarf — if placed in Earth's position, it would cover nearly the entire sky! However, the transit appears as a grazing touch because the orbit is inclined.
Key numbers
- mass: 4.3–10.9 Jupiter masses
- effective temperature: 390–412 K
- methane abundance: ~7%
- orbital period: 1.4 days
- white dwarf age: ~6 billion years