JWST detects hydrocarbons, aerosols and excess heat in atmosphere of white-dwarf planet WD 1856 b

A peer-reviewed study published Wednesday in Nature says NASA’s James Webb Space Telescope has detected an atmosphere on WD 1856 b, a giant planet transiting a white dwarf, with evidence for hydrocarbons, aerosols and unexpected excess heat. The researchers say it is the first atmospheric characterization of a transiting white-dwarf planet by transmission spectroscopy, a method that studies starlight filtering through a planet’s atmosphere during transit.

The result matters because white dwarfs are the compact remnants left behind after Sun-like stars exhaust their fuel and pass through a red-giant phase. Astronomers have previously found other kinds of evidence for planets or planetary debris around white dwarfs, but this study points to something more direct: a surviving planet whose atmosphere can now be probed in detail by JWST.

The planet, WD 1856 b, also known as WD 1856+534 b, orbits the white dwarf WD 1856+534 about 25 parsecs, or roughly 80 light-years, from Earth. The system’s white dwarf has a reported cooling age of about 6 billion years. WD 1856 b was first reported in 2020 from TESS and Spitzer observations. It circles extremely close to its star, at about 0.02 astronomical units, completing an orbit in about 1.4 days.

That geometry is unusual. The planet’s transit is grazing and lasts only about eight minutes, making the signal harder to model. To address that, the team said it processed JWST data from an April 27, 2023, transit using two independent pipelines and found consistent results. The observations came from JWST’s NIRSpec PRISM instrument, which covered wavelengths from about 0.5 to 5.0 microns.

The paper, “Aerosols and hydrocarbons in the atmosphere of a white dwarf planet,” lists Ryan J. MacDonald as corresponding author. It reports strong evidence for hydrocarbons in the atmosphere, with methane favored in some model comparisons, along with aerosols and thermal emission from the planet’s nightside. The authors retrieved a carbon-enriched atmosphere with a methane abundance around 7%, within a broader estimated range of roughly 2% to 20%.

Their analysis also constrained the planet’s mass to about 4.3 to 10.9 times that of Jupiter. The atmosphere’s effective temperature was retrieved at about 390 to 412 kelvins, substantially warmer than the roughly 160 kelvins expected if the planet were heated only by the white dwarf.

The authors interpret that temperature gap as evidence that WD 1856 b was reheated during a later migration inward, long after its star had already become a white dwarf. In their scenario, the reheating event happened about 3 billion to 5.5 billion years into the white-dwarf phase, consistent with the idea that the planet moved inward after the star’s red-giant stage and later settled into its current tight orbit.

That interpretation is not the only possible framing of the planet’s history, and some inferred properties remain sensitive to the data and models used. A separate 2025 JWST study using the MIRI instrument reported mid-infrared thermal emission from the same planet but found a cooler temperature estimate of about 186 kelvins and a mass upper limit of about 6 Jupiter masses. The contrast does not erase the new result, but it does underscore that key properties are still being refined.

“The planet is about the size of Jupiter, but the white dwarf it orbits is the size of Earth, so the planet is seven times larger than its star,” MacDonald said in a NASA Science summary of the paper.

Co-author Victoria Boehm of Cornell University said in the same summary: “We saw the telltale signatures of small cloud particles and hydrocarbons, most likely methane, which is the first time we have seen an atmosphere on a planet transiting a dead star.”

If that result holds up, WD 1856 b will stand as a notable first: not the first evidence of planets around white dwarfs, but the first atmospheric detection by transmission spectroscopy for a planet transiting one.